HVAC System meaning

ACTIVE CLIMATE CONTROL SYSTEMS are thermal comfort and indoor air quality modification systems that: (1) require the use of purchased energy, (2) involve numerous single-purpose components, (3) are typically only lightly integrated into the overall building fabric, and (4) are normally designed by a consultant other than the architect. They are, conceptually, the opposite of a passive systems approach to climate control. Although this book exhibits a preference for passive solutions, the use of active systems is very commonly dictated by realities of climate or building function. A designer of buildings virtually anywhere in the world should be familiar with active systems (for climate control, lighting, vertical circulation, fire protection, and sanitation)—they are a part of modern design.

The acronym HVAC is typically used to describe an active climate control system. HVAC stands for heating, ventilating, and air-conditioning. Heating is a process whereby the temperature of something (such as the air in a building) is increased. Ventilation is a process that moves or circulates air. It would be easy to assume that the AC part of HVAC refers to “and cooling.” This would be incorrect—AC stands for air conditioning, which has very specific process requirements. An air-conditioning system should be able to: (1) control air temperature (by heating and/or cooling as required), (2) control the humidity of air (perhaps bi-directionally), (3) control air distribution (speed and direction), and (4) control air quality (IAQ). If it sounds like the term AC generally covers the expectations of the term HVAC, this is true. AC is commonly used as a synonym for HVAC and vice versa.
The term HVAC will be used in this chapter for consistency with usage in the design professions. Occasionally, a project may demand only a heating system, or only a ventilation system, or only a cooling system—more typically, however, today’s projects demand (or at least expect) air-conditioning as provided by an HVAC system. Ideally, the capabilities of the passive systems discussed in Chapters 8 and 9 would be considered first. Even when a passive system cannot fully provide for project requirements, such an approach might be used during part of the year or in some part of a building to mitigate the energy and carbon impacts that come with active systems.

12.1  INTRODUCTION

The process of acquiring a viable HVAC system parallels the process of acquiring a building and generally involves decisions and actions taken during the predesign, design, construction, and occupancy phases of a project. The following list describes some of the key aspects of HVAC system acquisition in the order that would be typical for a new construction project. The list presumes that the commissioning process is in place (as would be expected for any high-performance project).

• Establish HVAC-related owner’s project requirements (design issues/intents/criteria; including code/standard compliance)
• Establish zoning requirements
• Make a preliminary system selection based on the above information
• Calculate design heating/cooling loads
• Select appropriate source equipment (to meet loads, intent, and context)
• Select appropriate distribution approach (to meet intents and fit context)
• Coordinate HVAC components with other building systems
• Rough-size equipment (fans, pumps, valves, dampers, pipes, ducts, condensers, air-handlers, tanks, …)
• Run energy analyses to optimize equipment selections and system assemblies
• Final-size equipment based upon optimization studies
• Coordinate final individual equipment selections into a cohesive whole
• Develop appropriate control logic and strategies
• Develop commissioning test protocols and checklists
• Witness systems installation and verifications
• Develop systems manuals for the owner
• Provide benchmark (new system) performance data for the owner
• Assist in initial operations to maintain the owner’s project requirements

The scope and extent of the actions described above will vary from project to project. They generally apply to small and simple (single-family residence) as well as large and complex (teaching hospital) projects; but specific actions will be adapted to fit the project scale, schedule, and budget. For smaller buildings, HVAC system selection/design may be done by the architect alone (or with assistance from a mechanical contractor). For larger, more complex buildings, consulting engineers will be involved (perhaps along with other specialists such as fire protection engineers or laboratory consultants). At whatever scale, a high-performance outcome will normally be most easily achieved through an integrated design process. Several of the earlier actions in this list (specifically, developing the owner’s project requirements and thermal zoning) are arguably best done by the project architect (who will likely spend the most time with the client). Other actions will be completed by a consulting engineer; yet others by a contractor; others by the owner’s operating personnel. A process that taps into the experiences and expertise of these diverse players in a timely manner simply makes sense. With inspired teamwork, the integration of HVAC services can enhance building form, as many examples in this chapter show.
Earlier chapters in this book deal with foundational considerations that feed into HVAC systems design. Chapter 1 addresses the design process and the many influences on that complex activity (such as design intent and codes/standards) that may impact HVAC system and equipment selection. Chapter 2 deals with environmental resources, which are a consideration in all aspects of building design. Chapter 3 addresses sites and their resources, which will affect building heating/cooling loads and the selection of appropriate systems solutions. Chapter 4 deals with comfort; the thermal aspects of which will be a major element of an owner’s project requirements for virtually all building types. Chapter 5 provides background information on indoor air quality. Along with comfort, acceptable IAQ will be an intended outcome for most projects. Chapters 6 and 7 present the fundamentals of building envelope design and performance—which will affect the loads that an HVAC system must mitigate. Chapters 9 and 10 present passive heating and cooling systems as preferred starting points for climate control. Chapter 11 discusses integration of passive heating, cooling, and lighting systems. xxxxx.
A potential problem faced by one attempting to understand HVAC systems is their seeming complexity. HVAC system types are numerous, their components are more numerous, and the expected system outcomes can vary widely. This is unavoidable as such diversity permits these systems to work in a wide range of contexts while providing some flexibility in selection. Fewer options would ease understanding—but would be unacceptable to the design professions who thrive on choice. An attempt has been made to chunk and lump the aspects of HVAC systems presented below to facilitate initial engagement with the systems. First, however, comes an introduction to some basic concepts.

At the onset of the twenty-first century, HVAC is showing several trends. One is the increasing willingness to let mechanical equipment share its tasks with natural ventilation and daylighting. Building automation has made this easier to manage. Another trend is toward an underfloor plenum air supply (related to, but not identical to, displacement ventilation approaches commonly used in critical-environment facilities) rather than using ducts to diffusers and return grilles, both on the ceiling. Concern about air quality indoors and the environment outdoors is producing a variety of approaches to increased ventilation that avoid refrigerants containing CFCs and HCFCs. Fuel cells and photovoltaics are promising increased energy autonomy to larger buildings. Move to trends

12.2  HISTORY AND CONTEXT

One of the most basic functions of a building is to provide shelter from weather. In the words of James Marston Fitch: “… to interpose itself between people and the natural environment … to remove the gross environmental load from their shoulders” (Fitch 1999). Historically, this role was accomplished through passive systems—for heating, cooling, and daylighting. In a carefully designed building, the roofs, walls, windows, and interior surfaces alone can maintain comfortable interior temperatures for most of the year in most North American climates. With appropriate scheduling, the most uncomfortable hours within buildings can often be avoided; for example, the siesta avoids the hottest afternoon hours within stores and office buildings. Several aspects of comfort and climate, however, pose difficult challenges for ordinary building forms and materials. Under these circumstances, when passive systems were the only available solutions, human comfort and wellbeing suffered. Active systems provide another option.
A building surface influences comfort primarily through its surface temperature; secondarily, a surface can modify air temperature (as when cool air moves across a warm surface). As important as these two determinants of comfort are, they are often not sufficient by themselves. In cooling situations, air motion and relative humidity are significant comfort determinants. For most building occupancies, air quality is an important issue under both heating and cooling conditions.
Building form can work with climate to produce air motion for cooling, although the higher air speeds that can extend the human comfort zone may be difficult to provide without mechanical assistance. Relative humidity is most readily controlled by mechanical or chemical means. Building form and materials may be able to keep spaces surprisingly cool, but without dehumidification, surfaces in many North American summer climates can become clammy and covered with mold. Further, it is difficult to filter air for IAQ purposes without a fan to force the air through the filtering medium.
Thus, whereas air and surface temperatures can often be successfully manipulated by passive means (a combination of building form, surface material, and informed user response) the comfort determinants of air motion and relative humidity and the health and comfort considerations of air quality often require mechanical systems. As the control of air properties—motion, moisture, pollutant content—becomes more critical to project success, the designer becomes more likely to respond with a sealed building, excluding outdoor air except through carefully controlled mechanical equipment intakes. In the recent past, this exclusion of outdoor air has often been accompanied by the exclusion of daylight, of view, of solar heat on cold days—in sum, by a general rejection of all aspects of the exterior environment. As designers come to terms with the role of mechanical systems in high-performance buildings, they should also clarify the role of these systems in reaching net-zero energy or carbon-neutral outcomes. Passive systems are historic; but have limitations. Active systems are more recent, seem to have few if any limits on capabilities, but do have serious environmental impacts.
Depending upon how one interprets the definition of an active system, it is reasonably fair to claim that active heating systems have been in use for a rather long time. Roman hypocaust systems exhibit basic characteristics that mirror today’s central hot air heating systems. Such sophistication was generally lost for almost two millennia until scientifically designed central heating systems reappeared in the 1800s. Rationally designed central ventilation systems also appeared in the early to mid 1800s. Design methods and component analyses were documented in early heating/ventilating texts (see, for example, the timeline from Haberl 2012). The cooling part of an active climate control system is a much more recent development. Early forays into central cooling occurred in the mid 1800s, but true diffusion of active cooling into the building sector came only after the Second World War.
As active climate control systems became commonplace and found increasing use in a range of building types, they also became more complex and more specialized. This has resulted in a specialization and compartmentation of design responsibilities that mirrors the situation in many other aspects of design and modern life in general. Current terminology describes this as working in separated silos or towers. As suggested in Chapter 1, a relay-race-like, non-integrated design process seldom serves the long-term interests of the owner and places roadblocks in the path to high-performance building outcomes. A fully integrated design process is strongly recommended. If this is not yet feasible on a specific project then the commissioning process is suggested as a bridging mechanism. The ongoing collaborative work of the commissioning team (comprised of diverse project participants) can provide some of the benefits of a fully integrated process—and is well-documented and well-accepted in many parts of the world. It is unreasonable that important aspects of building design remain a mystery or appear to be magic to other design team participants.
12.3  RELEVANT CODES AND STANDARDS
The role of codes, standards, and guidelines in the design of an HVAC system is quirky; but consistent with the pattern seen in other areas of building design. In the United States there is rarely a code mandate that requires that an HVAC be provided for a building. There may be requirements here and there that dictate that building occupants not be allowed to freeze in winter (we seem less concerned as a society about summer heatstroke), but there is no known code requirement that U.S. buildings be thermally comfortable. Increasing adoption of ASHRAE Standard 62.1 is moving the profession toward provision of code-mandated minimum air quality. Once a decision has been made to provide an HVAC system, however, all sorts of codes, standards, and guidelines come into play. These may address the design and installation of specific components or equipment (such as boilers, flue stacks, window air-conditioning units), the arrangement and installation of systems (primarily from the perspective of life-safety and energy), and the overall performance of a system (mainly its energy performance).
It would be unreasonable in this chapter to attempt to list all the design guidance documents that would apply (in the United States) to an HVAC system and its components. If the geographic perspective is widened to systems designed worldwide, the number of applicable standards and guidelines mushrooms to easily exceed a thousand documents. Chapter 39 of the 2013 ASHRAE Handbook—Fundamentals provides an extensive and up-to-date listing of HVAC-applicable codes and standards sorted by subject (ASHRAE 2013). Some of the more important and commonly encountered standards and guidelines include:

• ASHRAE  Guideline 0-2005: The Commissioning Process
• ASHRAE Guideline 1.1-2007: HVAC&R Technical Requirements for the Commissioning Process
• ASHRAE Standard 90.1-2010: Energy Standard for Buildings Except Low-Rise Residential Buildings
• ASHRAE Standard 62.1-2013: Ventilation for Acceptable Indoor Air Quality
• ASHRAE Standard 62.2-2013: Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings
• ASHRAE Standard 55: Thermal Environmental Conditions for Human Occupancy
• USGBC: LEED rating systems for various building types
• NFPA: numerous fire protection standards (see Part VI of this book)

12.4  FUNDAMENTALS

A number of fundamental principles underlie the selection and design of an HVAC system. The most basic of these considerations are summarized herein.
(a) Design Intent and Design Criteria
The design of any building system must start with a clear understanding of the outcomes required and/or expected by the client, by users/occupants, and by society. This is certainly true for HVAC systems design. Society’s expectations are typically expressed through building codes (in particular those dealing with energy and IAQ). The aspirations of the client and subsequent building users are best expressed through development of a comprehensive and cohesive statement of Owner’s Project Requirements—OPR; a formal document under the ASHRAE commissioning process. A good OPR will address: thermal comfort requirements for all occupied spaces, indoor air quality requirements for all occupied spaces, minimum acceptable energy performance, environmental preferences for constituent components, first cost, life-cycle costs, reliability, maintainability, building space and volume demands, aesthetics, and other considerations. The commissioning process will verify the likelihood that the HVAC system(s) proposed, designed, and installed will be able to deliver on the performance targets established in the OPR. Selecting an HVAC system in the absence of clear selection criteria is not designing—it is wishful thinking.
(b) Thermodynamics Laws
The Laws of Thermodynamics apply to the design of HVAC systems and are worthy of a quick summarization and interpretation.

• Zeroth law: Two systems that are in thermal equilibrium with a third system are in thermal equilibrium with each other. This law helps to define the notion of temperature. Heat will only flow from a higher to a lower temperature.

• First law: Heat and work are forms of energy transfer. Energy is always conserved; thus, all energy associated with a system must be accounted for. Perpetual motion is impossible and efficiencies greater than 100% are impossible. All macro energy forms within a building devolve to heat.

• Second law: A system attempts to move toward a state of thermodynamic equilibrium, which increases entropy (disorder). It takes energy to order (and maintain order within) a system, which will otherwise naturally move toward disorder.

• Third law: The entropy of a system approaches a constant value as the temperature approaches zero; not particularly germane to HVAC system design.

(c) Important “e” Terms
Several terms that start with the letter “e” are important to consider when designing an HVAC system. These terms include:

• Energy: energy is the ability to do work. In HVAC system design, the amount of energy required to accomplish a defined work task is often of substantial interest to the design team. Power is an instantaneous snapshot of work (over a very short time increment); energy is the integration of power over time (per hour, per day, per year). Energy requirements may be analyzed at the site boundary of a project (effectively at the electric meter for example) or they may be analyzed at the source of the particular energy supply (such as at the electrical power plant). These two system analysis boundaries will yield very different results. The appropriate system boundary is presently a matter of serious contention.
• Effectiveness: effectiveness is a qualitative descriptor that describes success in meeting defined objectives. An HVAC that delivers the Owner’s Project Requirements would be properly termed effective.
• Efficiency: efficiency is a defined ratio (output / input) that is a quantitative assessment of the ability of a system to provide some effect relative to the resource cost of doing so. An HVAC system that provides a lot of useful heat while requiring just slightly more heat as input would be termed efficient. A system can be efficient while being ineffective. Conversely, a system can be effective while being inefficient. A high-performance building seeks systems that are simultaneously effective and efficient.
• Enthalpy: enthalpy is a measure of the total (sensible and latent) energy content of a sample of air. See section 11.4 (d) for further information.
• Entropy: entropy is a measure of the disorder of a system. Without counterbalancing energy inputs, most building systems lose order over time (thus increasing in entropy). A hot water pipe loses energy when not in use. A layer of paint degrades over time. Water must be pumped to overcome the effects of gravity. Buildings continually require the input of resources for operation and maintenance.
• Exergy: exergy is a generally not-well-understood measure of the utility of an energy exchange. In the future we may evaluate the exergy of competing HVAC system solutions to judge their appropriateness. As an example, the exergy of using a nuclear power plant to heat water by 50 degrees is quite different from the exergy involved when using a passive solar thermal system to accomplish the same task.

In addition, one “c” term is also of interest—conservation. Conservation implies the use of less of something (energy, water, materials, cash). Conservation has both a noble history and detractors. In some circles conservation is seen as unnecessarily doing with less. Conservation is a necessary concept when efficiency makes no sense. For example, discussing the efficiency of a shower head is irrational (output equals input). A water-conserving shower head is a logical way to address this situation—where there is an attempt to provide equal effectiveness using less water.  
(d) Applied Psychrometrics
Psychrometrics (introduced in section 9.12) is the study and manipulation of the properties of moist air. Adjusting the properties of air (which, in a building, always contains water vapor) is precisely what the thermal side of an air conditioning system is intended to do. HVAC systems are psychrometric systems; and a fundamental understanding of moist air properties and their modification is critical to an understanding of climate control systems. Psychrometrics, which can be addressed through equations, data tables, and/or a diagram called the psychrometric chart, is a means of understanding the relationships among various properties of air. The psychrometric chart (Figure F.1, F.2) is commonly used as an HVAC system design tool.
As seen in Figure 9.36, there are eight distinct psychrometric processes. These are related to commonly encountered HVAC systems in the following discussion.
Sensible heating: under this process, room air temperature is increased with no change in absolute humidity. Relative humidity will decrease as the air is heated, but there is no change in air moisture content. This process is very common and represents the outcome of the typical building heating system, which is purely sensible and has no inherent capability to affect latent loads. Hot air heating, baseboard radiation, and solar heating systems are all sensible heating systems.
Heating and humidifying: this process, which moves up and to the right on the psychrometric chart, is often desired in smaller buildings located in cold climates. There is no single HVAC device that will produce this effect; sensible heating equipment must be paired with a humidification device. 
Heating and dehumidification: this process, which moves down and to the right on the psychrometric chart, is not a commonly desired or encountered process. No single HVAC device will produce this effect (although it is possible to accomplish).
Humidification: this process adds moisture to the air without intentionally changing air temperature. A device called a humidifier can accomplish this effect (although rarely will this alone be adequate to produce thermally comfortable conditions).
Dehumidification: this process removes moisture from the air without intentionally changing air temperature. A device called a dehumidifier can accomplish this effect. This may occasionally (in benign climates) be the only effect required to produce thermally comfortable conditions.
Sensible cooling: this process reduces air temperature without a change in absolute humidity. Some HVAC systems will produce sensible cooling through a part of their operating range. Other HVAC systems (such as a radiant cooling system) are specifically designed to operate as sensible cooling devices. Most active cooling systems, however, produce sensible and latent cooling effects.
Cooling and dehumidification: this combined sensible and latent cooling process is commonly desired in a wide range of building situations and is produced by the common vapor compression cooling process. A cooling coil is brought to a temperature below the dew point of the room air such that moisture condenses and leaves the air while the air is reduced in dry bulb temperature.
Evaporative cooling: this process, which involves simultaneous sensible cooling and latent heating (cooling and humidification), is an intriguing HVAC option. The evaporative cooling process occurs along a line of near-constant enthalpy; sensible heat is exchanged for latent heat at little net energy cost. Where climate conditions will support this process, direct evaporative cooling can be a very energy efficient space cooling option. Indirect evaporative cooling can be employed where high relative humidity from a direct evaporative cooling process would be objectionable.   
An example showing the use of the psychrometric chart to analyze the conditions that occur during typical HVAC system operation may be helpful. Review of Figs. 9.35 through 9.42 is suggested.

Fig. 12.1 Sizing cooling equipment using the psychrometric chart. (a) Finding the conditions for the supply air. SI values are: 12.8, 23.9, and 26.7oC DB. (b) Finding the conditions for the return air–outdoor air mixture. SI values are: 23.9 and 32.2oC DB; 23.9oC WB. (c) Points A, B, C, and D are representative conditions within the cooling cycle. SI values are: Point A (23.9/16.9oC); Point B (12.8/10.7oC); Point C (29.4/21.8oC); Point D (32.2/23.9oC); outdoor air (1321 L/s); exhaust air (1321 L/s); supply air (2010 L/s); return air (689 L/s); room (23.9oC).

(e) Metrics for HVAC System Performance
A wide variety of metrics (measures) for the expression of HVAC system and equipment performance will be encountered. In many cases there will be a mandated minimum performance threshold that is embedded in a building code. In the U.S., for example, minimum acceptable performance values for most common types of source equipment will be found in ASHRAE Standard 90.1. These values are usually the same minimum performance limits promulgated by the U.S. government. In almost all design situations higher performing equipment will be available from manufacturers. Such equipment will normally have a higher first cost than the code-minimum equipment—but the additional first cost may often be offset by energy savings and reduced life-cycle costs.
No matter the specific metric being considered, the performance of any given type of equipment will vary as its operating parameters vary. Listed equipment performance (and capacity) ratings are typically based upon a consensus standard that defines the parameters under which equipment will be rated. Such conditions are not necessarily the peak conditions that will be experienced in an actual building situation, so caution needs to be exercised when considering metrics such as efficiency or COP. In addition, most equipment operates less efficiently at partial load and such reduced performance will affect annual energy consumption and payback analyses.  
The following are some commonly encountered performance metrics that will affect HVAC system selection and design for buildings.

• Efficiency, as discussed in section 12.4(c), is the ratio of system output to systems input when both values are presented in consistent units; expressed as a decimal value or percentage. Unless otherwise noted, efficiency should be taken to mean instantaneous efficiency at some defined point in time and under some specific set of operating conditions.
• Annual fuel utilization efficiency (AFUE) is the ratio of annual fuel output energy to annual input energy, which includes any off-season pilot input loss.
• Coefficient of performance (COP) is defined slightly differently, depending upon the task. For cooling, it is the ratio of the rate of heat removal to the rate of energy input in consistent units, for a complete cooling system (or factory-assembled equipment), as tested under a nationally recognized standard or designated operating conditions. For heating (heat pump), it is the ratio of the rate of heat delivered to the rate of energy input in consistent units, for a complete heat pump system as tested under designated operating conditions. Supplemental heat is not included in this definition.
• Seasonal coefficient of performance for cooling or for heating (SCOPC, SCOPH) are formulations of COP that consider the total output of a device during its normal operating season (versus an instantaneous output value for COP)
• Energy efficiency ratio (EER) is the ratio of net equipment cooling capacity in Btu/h to the total rate of electric input in watts under designated operating conditions. (When consistent units are used, this ratio is the same as COP.)
• Integrated part load value (IPLV) is a single-number figure of merit based on part-load EER or COP expressing part-load efficiency for air-conditioning and heat pump equipment on the basis of weighted operation at various load capacities for the equipment.
• Seasonal energy efficiency ratio (SEER) is the total cooling output of an air conditioner during its normal annual usage period for cooling, in Btu, divided by the total electric energy input during the same period, in watt-hours.
• Heating seasonal performance factor (HSPF) is the total heating output of a heat pump during its normal annual usage period for heating, in Btu, divided by the total electric energy input during the same period, in watt-hours.
• Energy utilization index (EUI; also energy use intensity) is an indicator of total annual building energy usage normalized per unit floor area; Btu/ft2 yr (W/m2 yr); for a typical building, HVAC energy is a large part of EUI.  

(f) Thermal Zoning
A thermal zone is an area of a building that must be provided with separate control if thermal comfort expectations are to be met. Thermal zones are very often a key basis for HVAC system selection and zoning decisions can play a critical role in occupant comfort responses. Theoretically, the design team wants to provide just enough zones to meet the owner’s project requirements (with perhaps some flexibility for anticipated future needs). Fewer zones will result in some level of discomfort. More zones than necessary will increase the first cost of the project.
Thermal zoning must be established prior to the selection of a climate control system. The concept of thermal zones applies equally well to passive climate controls systems as to active climate control systems—and was discussed in connection with passive systems design in Section 9.2. A zone may be a room in a building, may consist of multiple rooms, or may be a portion of a room (a room may have more than one zone). The intent of zoning is to set up a control scenario that can respond to changing room loads and maintain thermally desirable conditions. Zone control in an HVAC system is most commonly initiated by a thermostat that senses room air temperature. As noted in Chapter 4, thermal comfort is affected by variables other than dry bulb air temperature—nevertheless, temperature by itself is a de facto control variable in the vast majority of buildings. Successful zoning allows the HVAC control system to provide conditions amenable to thermal comfort in the face of changing heating and cooling loads. In a passive system, zone control might be exercised by opening and closing windows or draperies.    
Thermal zoning decisions are typically driven by differences in the timing of loads from one room to another. Timing (scheduling) is the driver—not differences in the magnitude of loads. As an example, an east-facing office must be zoned separately from a west-facing office due to solar radiation patterns. If the two offices are on the same zone (with one thermostat) the occupants of the office without a thermostat (without control capability) will be consistently uncomfortable. The same would be true of a classroom placed on the same zone as an office. If the thermostat is in the office, the system cannot respond to changes in classroom occupancy (empty, half-full, full).
To repeat, thermal zoning must precede system selection—so that a system with adequate zoning capability for the project context is selected. The minimum number of thermal zones for a conventionally designed multipurpose, mid-rise building is shown in Fig. 12.2. More than the minimum 16 zones could be required due to differences in load scheduling within a zone, such as between offices and stores or between offices and conference rooms. The apartments are also shown with the minimum five zones (based on solar orientation); an emphasis, however, on individual ownership of units and the variation in usage patterns typical of residential occupancies often results in a decision to provide as many zones as there are apartment units. Otherwise, the hard decision of which apartment gets a thermostat and which does not will need to be addressed.
It may be premature to call it a trend, but there seems to be a lot of interest in Europe in personal control systems for ventilation and thermal comfort. Under this approach, each work station is set up as a separate thermal zone whose conditions are controlled by the occupant. This approach is logical—although likely expensive—not just for the control capability, but also for the distribution delivery elements. There are also limits upon building types where this would make sense (each classroom seat in a school would be hard to individually zone). On the other hand, comfort, energy efficiency (perhaps), and diversity of conditions would be fostered when this zoning arrangement is possible.
Fig. 12.2 A reasonable minimum number of thermal zones for a large multipurpose building.
Once the basics of system zone determination have been established, a design philosophy question is in order. How similar should the interior environments of buildings be? This question encompasses not only thermal experiences, but visual and acoustical ones as well.
The advantages of uniformity are most evident in ease of design and construction that, through mass production and speed, often brings lower first costs. In the case of an office, uniformity of ceiling heights, light fixture placement, grille locations, and so on promotes flexibility in varying arrangements that can extend a building’s usable life span. There are, however, at least four types of offices that may need to be interchangeable within such generic space. The typical enclosed office has the privacy of four walls and a door. The bullpen office has repeated, identical workstations, with low dividers at about the height of the desk surface. The uniform open plan office resembles the bullpen, but with higher divider partitions for added privacy. The free-form open plan office has some individually designed workstations with divider partitions of varying heights (sometimes reflecting the varying status of workers). In the bullpen and uniform open plan office, the resulting uniformity is not always attractive to users, and diversity is often encouraged at a more personal level—with office furnishings, for example. A more thorough approach to diversity can provide stimulus to the user who spends many hours away from the variability of the exterior climate.
If offices must be uniform with respect to ceilings, lighting, air distribution, and even size, the corridors that connect them and the lounges or other supporting service spaces can deliberately be made different. Diversity requires a complete and detailed design of places; it gives the builder a more complex and interesting task; and it can provide orientation and interest to the users. The attractiveness of diversity is evident in most collections of retail shops, in which light and sound—and sometimes heat and aroma—are used to distinguish one shop from the next.
Diversity in the thermal conditions to be maintained, such as warmer offices and cooler circulation spaces in the winter, can be used to enhance the comfort of the office users. Designers have long recognized that a space can be made to seem brighter and higher if it is preceded by a dark, low transition space. Thermal comfort impressions can be manipulated similarly. Less than comfortable conditions in circulation spaces or other less-critical zones not only make the key functional spaces seem more comfortable by contrast, but also save significant amounts of energy over the life of a building. Furthermore, allowing diversity in conditions can make passive strategies more feasible.

A large-scale demonstration of diversity in thermal zones is shown in Fig. 12.3. Passive solar heating can make a significant contribution, even through a shallow-sloped, single-glazed cover in cloudy Glasgow, Scotland, largely because the mall area and leisure areas are allowed a much wider thermal range than would be permitted in stores and offices. The overcast skies are quite suitable for daylight, and the addition of summer sunshading makes natural ventilation (through the stack effect, assisted by fans) possible during the cool summers. U.S. Pacific Northwest climate conditions are similar.
Fig. 12.3 St. Enoch’s Square, Glasgow, Scotland: a proposal to use passive solar heating, daylighting, and natural ventilation. Reiach & Hall and GMW Partnership, architects (joint venture); Cosentini Associates, energy consultants; Princeton Energy Group, daylighting consultants. (a) Schematic section showing winter operation; the mall temperature varies around 63ºF (17ºC) during operating hours, while offices are kept near 70ºF (21ºC). (b) Schematic section showing summer operation; the mall temperature varies from about 68 to 74ºF (20 to 23ºC) during operating hours. (c) Estimates of annual energy consumption for a conventional-design base case and several alternative configurations. Note the significantly lower heating energy requirements, resulting in part from the lower winter temperatures allowed for the less-critical zones such as the mall and the leisure areas in configurations A to E.
 (g) Preliminary Space Requirements for HVAC
Detailed information regarding the floor area and distribution volume requirements for an HVAC system will become available during the design development phase of a project. Unfortunately, architectural design decisions regarding these space requirements were made much earlier—probably during the conceptual design phase as floor plans were first developed and accepted by the owner. Providing too much space for HVAC artifacts is not architecturally desirable, but a boon to the HVAC designer. Providing too little space for HVAC equipment, distribution, and maintenance is a serious problem that will likely compromise system performance and longevity. Having HVAC expertise available to the design team early in the design process can go a long way toward right-sizing space allocations. This can be accomplished through an integrated design process or (somewhat less so) via a well-implemented commissioning process. In the absence of such early expertise, precedent buildings can be used to inform space decisions; or a design guidance tool (such as seen in Figs. 12.4 and 12.5) can be employed.
There is no single correct HVAC system for most building types, no single correct way to arrange an HVAC system, and no single correct way to place the system into a building. Fortunately, many decisions that need to be made to properly integrate an HVAC system into a building are contextual, rather than technical, issues. The project architect usually knows more about project context than any other design team member. Some of the issues to be considered early in the HVAC placement process are whether the system will be a local (requiring no central space allocation), central, or district (where source equipment is located remote from the building) system (see Section 11.x) and what the most logical distribution of major equipment might be.
An important decision is whether to spatially group or separate the HVAC source equipment and the HVAC distribution (air-handling) equipment (see Fig. 12.6). A single central mechanical room in a good location might serve an entire building, with area and height sufficient for both heating/cooling and air-handling equipment. If separated, a large space for heating/cooling equipment is typically located in the basement or the penthouse, with a fan room on each floor (or alternate floors).
A central mechanical (plant; source equipment) room should be located so as: to minimize distribution component runs to fan rooms, minimize electrical distribution runs from the building service, allow for easy installation and replacement of large and heavy equipment, allow for easy connection to cooling towers/air-cooled condensers, simplify noise control, minimize structural systems costs, and (as necessary) simplify provision of fuel storage. Mechanical rooms need relatively high ceilings; 12 ft (3.7 m) clear is a typical minimum.   
A fan room should be logically located relative to the areas being served and have easy access to the exterior for connection to an outdoor air intake. Central locations minimize the distribution tree size and length (100 ft (30 m) might be considered a maximum run for energy efficiency); access to the exterior facilitates the use of outdoor air for both IAQ and economizer cycle purposes and simplifies equipment installation (or removal during remodeling). Fan rooms do not require the same ceiling height as a central mechanical room; 10 ft (3.1 m) clear is often acceptable—although getting ductwork into and out of the room may be as volumetrically demanding as placing the air-handling unit. Although air-handling units are typically not nearly as noisy as chillers, placement of a fan room to facilitate ductborne noise control makes economic sense.
Tables 10.3 and 10.4 present approximate space requirements for conventional mechanical systems. For more detailed information on equipment room space requirements, see Figs. 12.7, 12.8, and 12.9.
Fig. 12.4 Approximate Space Requirements for Major Heating and Cooling Equipment

Fig. 12.5 Approximate Space Requirements for Air-Handling Equipment

The charts presented in Figures 12.4 and 12.5 are intended to give a quick approximation of areas; more detailed procedures for sizing are presented in Section 10.3 (for boilers, chillers, fan rooms) and Section 10.4 (for air ducts). The width of the “line” shown for each building type allows some opportunity for consideration of building energy efficiency. An efficient building will fall toward the left of a given building-type-line width. These estimates for equipment sizes are likely high for a substantially-high-performance building (such as one based upon the recommendations in the ASHRAE Advanced Energy Design Guides). For buildings with large heat gains or losses, however, these graphs may slightly undersize the areas needed.

The capacity of cooling equipment is commonly rated in tons of refrigeration. Rough estimates of cooling capacity versus floor area for several building types are provided in Table 10.5.
TABLE 10.5 Preliminary Estimates for Cooling Capacity

Note: 1 ton = 12,000 Btu/h (3.514 kW).
For a thermally well-designed detached residence, a design guideline of 1000 ft2 of floor area/ton (26 m2/kW) is suggested.

12.5  HVAC COMPONENTS
An HVAC system is comprised of a number of components—each with a specific function. Because of the potential complexity of many common HVAC arrangements, looking at conceptual building blocks for these systems may assist in demystifying the options.  Each of these categories of components will be looked at in turn prior to exploring the numerous HVAC systems that may be considered for building applications. Four main categories of functional elements will be encountered in the typical HVAC system:

• source components: these produce heating effect and/or cooling effect
• distribution components: these circulate the heating/cooling effects from the source(s) to the various conditioned zones (in a local system this component category is minor or non-existent)
• delivery components: these introduce the heating/cooling effect into the various spaces being conditioned
• control components: these provide for beneficial operation of a system, such as on-off functionality, temperature control, energy efficiency, freeze protection, fire response, etc.

  
(a) Source Components: Heat
An HVAC system that will  offset building heat losses must have a heat source. Selection of a heat source is a high-level decision that should generate some design reflection.  There are four conceptually (and physically) different means of introducing heat into a building. They have different architectural design impacts and environmental concerns/benefits. The four basic approaches mushroom into dozens of specific equipment options; and these options expand into more dozens of specific equipment offerings.
On-site combustion: a fuel of some type (natural gas, oil, propane, firewood, coal) is burned on site, usually within the building; heat is produced as an outcome of the combustion process; the fuel may be delivered to the building site upon demand (natural gas) or be delivered in bulk for on-site storage (fuel oil); combustion air will need to be provided at the combustion location and combustion gasses exhausted from the building; with the exception of biomass, heating fuels are non-renewable and all produce carbon emissions; efficiencies of combustion equipment can vary substantially, but modern equipment (such as a condensing boiler or furnace) tends to perform at or above 95% efficiency; hot water or hot air can be produced by combustion sources
Electric resistance:  electricity is passed through an electrical resistance element to produce heat; the electricity may come from off-site (via a utility) or from an on-site source (such as PV or wind); electric resistance heating is typically cited as being 100% efficient—but this is a site efficiency (similar to the 95%+ value noted for on-site combustion); electricity is generally delivered to a building on demand (with storage only required for some approaches to on-site generation); no combustion air or exhaust for products of combustion is necessary; hot air or hot water can be produced by electric resistance; the distinction between site and source efficiency is of particular interest for electric resistance (typically involving a 1:3 magnitude ratio)
Heat transfer: finding some heat already on site and moving this heat to a place where it is more useful is the essence of heat transfer; an air-to-air heat exchanger (whereby heat in exhaust air is transferred to incoming fresh air) is an example of this approach; a heat pump (discussed in more detail in section 11.x) is another example of this approach; a heat transfer system may use electricity as a driving force or be self-powered; heat transfer does not require the introduction of “new” energy and can be very energy effective; ground-source heat pump COPs can reach 5-6 and air-source units should be greater than 1.5 (denoting more than 100% “efficiency”); heat pumps and air-to-air heat exchangers are both commonly found in high-performance buildings; in general, hot water or hot air may be produced—but this choice is limited by the specific transfer equipment being considered
Energy capture: this approach is similar to heat transfer, but an energy form other than heat is tapped into and converted to heat for use in a building; solar energy is by far the most common such energy resource (although wind is also covered by this approach); energy capture sources tend to be carbon-free and also economically free for the taking; hot water or hot air may be readily produced
Several examples of heat source equipment and applications are provided by way of introduction to this part of an HVAC system. 
Boilers: boilers produce hot water or steam; the heat source may be electric resistance or on-site combustion. Boilers provide the heating effect required to elevate water temperature to a point where it can be used for building heating (or to evaporate water to steam). There are a number of different types of boilers on the market—attesting to a wide range of application contexts and system demands. The type of boiler selected for a project depends upon the size of the building heating load, the heating fuels available (and desired), the desired efficiency of operation, and whether single or modular boilers are the most reasonable. Boiler sizes are commonly expressed either in Btu/h of net output or in (gross) boiler horsepower.

            In I-P units,
boiler horsepower = heating load (Btu/h)\% boiler efficiency
´ 33,470 Btu/h per horsepower
In SI units,
boiler horsepower = heating load (kW)\% boiler efficiency
´ 9.81 kW per horsepower
Efficiency depends partly on the number of passes that the hot gases (from on-site combustion) make through the water—the more passes, the higher the efficiency. It also depends on burner efficiency and on regular maintenance. Finally, efficiency is highest when boiler equipment is operating near its capacity. Figure 12.10 compares typical boiler types, including two- and three-pass boilers.
Fig. 12.10 Comparison of boiler types. (a) Cast-iron sectional type. (b) Two-pass fire tube. (c) Three-pass fire tube. (d) Three-pass wetback Scotch marine. (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 9th ed.; © 1994 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)

Fire Tube Boilers: The hot gases of the fire are taken through tubes that are surrounded by the water to be heated. Fire tube boilers can be either dryback or wetback. Dryback designs have chambers outside the vessel to take combustion gases from the furnace to the tank. Wetback designs have water-cooled chambers that conduct the combustion gases. Firebox boilers place the boiler shell on top of the combustion chamber. Scotch marine boilers feature multiple passes of the combustion gas through tubes.
Water Tube Boilers: The water to be heated is taken through tubes that are surrounded by the boiler’s fire. They hold less water than the fire tube models and so respond faster and can generate steam (where desired) at higher pressures.
Cast-Iron Boilers: Often used in residential and light-commercial applications, these boilers operate at lower-pressure and lower-efficiency. They do have the advantage of being modular.
In addition to basic type ofboiler construction, there are choices of burner types (depending on the fuel used), burner controls, and boiler feedwater systems. Consult the latest ASHRAE Handbook—HVAC Systems and Equipment for details.
Fossil fuel–burning boilers need flues for exhaust gases, fresh air for combustion, and required air pollution control equipment. The exhaust gas is usually first taken horizontally from the boiler; this horizontal enclosure, or flue, is called the breeching. The vertical flue section is called the stack. Guidelines for sizes and arrangements of breeching and stacks are shown in Fig. 10.17. Local codes determine the quantity of air required for combustion; local air pollution authorities set pollution control requirements. As a general rule, combustion air can be supplied in a duct to the boiler at an average velocity of 1000 fpm (5.1 m/s). The duct should be large enough to carry at least 2 cfm (1 L/s) per boiler horsepower. Furthermore, ventilation air to the boiler room should be provided; preferably, the inlet and outlet should be on opposite sides of the room. Minimum sizes: enough for 2 cfm (1 L/s) per boiler horsepower at a velocity of about 500 fpm (2.5 m/s).
Fig. 10.17 Breeching and stack size guidelines for fossil-fuel-fired boilers. (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 11th ed.; © 2007 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
Space requirements for boilers are summarized in Fig. 10.18, which shows multiple boilers. Note that clear space within the room must be provided so that the tubes of the boiler can be pulled when they must be replaced. Access for eventually replacing entire boilers must be considered.
Fig. 10.18 Boiler room space requirements. Dimension A includes an aisle of 3 ft 6 in. (1 m) between the boiler and the wall. Dimension B between the boilers includes an aisle of at least 3 ft 6 in. (1 m), up to 5 ft (1.5 m) for the largest boilers. (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 10th ed.; © 2000 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
Several types of single boilers are discussed here. The final boiler type discussed, the modular boiler, is preferred for energy conservation.
1.   High-output, package-type steel boiler. For large buildings that use steam as a primary heating medium, one or several such boilers may be used. Direct use of steam can be seen in Fig. 10.15b, supplying preheat and reheat coils and also a humidifying unit. The relative lightness of this boiler type, compared to the older styles with ponderous masonry bases (boiler settings), makes it suitable for use on upper floors of tall buildings. Figure 10.6 shows two such boilers on the 13th floor of the Fox Plaza Building.
2.   Converter, steam to hot water. When, in a building that uses primary steam boilers, secondary circuits that use hot water for heating are required, a converter (Fig. 10.19) is used. It is considered a heat exchanger. In Fig. 10.6, there is downfeed steam supply for the two boilers on the 13th floor to two such converters, one for hot water heating in the apartments and one below the garage ceiling for hot water heating in the commercial area. A converter may also be used to transfer heat from steam to domestic (service) water. Converters are frequently used where central steam supply systems are available, as in large-city downtown areas. The easier, quieter distribution of heat by hot water has largely replaced steam heating distribution trees within buildings.
Fig. 10.19 Conversion unit that transfers heat from steam to hot water. (a) Section illustrating the principle of heat transfer from steam to water. (b) A converter connected to the steam supply and equipped with all devices necessary for a complete hot water heating system. (Courtesy of ITT Bell and Gossett.)
3.   Electric boilers. Where electricity costs are competitive with those of fossil fuels, electric boilers are sometimes used. Both hot water and steam electric boilers are available. The advantage of electric boilers is the elimination of combustion air, the flue, and air pollution at the building. The disadvantages are the use of a high-grade energy source for a relatively low-grade task and the pollution impact at the electric generating plant. In order to protect against high electric demand charges, a large number of control steps are desirable.
4.   Compact boilers. Smaller-dimension boilers (Fig. 10.20) with high thermal efficiencies are available. In addition to their space-saving footprint, they feature a variety of venting options that make them easily adaptable to smaller equipment rooms.
Fig. 10.20 Burkay Genesis hot water boiler, fueled by either natural gas or propane, is available in ratings from 200,000 to 750,000 Btu/h (58,620 to 219,825 W). All units are 30 in. high ´ 24 in. deep (762 mm ´ 610 mm); the smallest boiler is 23 in. (584 mm) wide, and the largest is 57 in. (1454 mm) wide. The copper heat exchanger has an 83.7% thermal efficiency rating, and a variety of venting options are available. (Courtesy of A.O. Smith Water Products Company, Irving, TX.)
5.   Modular boilers. The primary advantage of modular boilers (Fig. 10.21) is efficiency. Boilers achieve maximum efficiency when they are operated continuously at their full-rated fuel input. The single boilers discussed previously operate this way only under outside design conditions, which by definition occur, at most, during 5% of a normal winter. In a modular boiler design, each section is run independently. Therefore, only one section need be fired for the mildest heating needs; as the weather gets colder, more sections are gradually added. Because each section operates continuously at full-rated fuel input, efficiency is greatly increased (Fig. 10.22). Each module, being rather small, requires little time to reach a useful temperature and (unlike the larger single boilers) does not waste a lot of heat as it cools down. Thus, modular boilers usually produce a 15% to 20% fuel savings for the heating season relative to single boilers. Their other advantages include ease of maintenance (one module can be cleaned while others carry the heating load) and small size (allowing easy installation and replacement in existing buildings).
Fig. 10.21 Modular boilers. (a) A bank of four modules—with a total input 1.5 million Btu/h (439 kW). (b) Details of one module (20 x 32 x 48 in. H [510 x 812 x 1220 mm]) with a 385,000 Btu/h (113 kW) input. (c) Schematic of flow conditions in mild weather, with only one module in operation.
Fig. 10.22 One large boiler versus many smaller ones. (a) Boilers rarely operate at full capacity; instead, they respond to part loads the majority of the time. (b) Under part load conditions, a boiler will often short-cycle, which on a single large boiler could drop the annual efficiency into the 66% to 75% range.
Modular boilers also eliminate the initial cost of oversizing heating equipment. In cold climates, conventional boiler systems often use two or three large boilers to ensure that heat is available even if one large boiler fails. When two such boilers are used, it is common practice to size each boiler at two-thirds of the total heating load; an oversize of one-third results. When three such boilers are used, it is common practice to size each boiler at 40% of the total heating load; an oversize of 20% results. However, when a minimum of five modular boilers are used, oversizing can be eliminated because the failure of a single module will not have a crippling impact on the overall heat output.
Gas-fired pulse boilers are an even smaller and more energy-efficient choice for modular boilers. Pulse boilers utilize a series of 60 to 70 small explosions per second, making the hot flue gases pulse as they pass through the firetube. This makes for very efficient heat transfer. Pulse boilers are available up to about 300,000 Btu/h (88 kW).
Pulse boilers operate with lower water temperatures so that water vapor in the flue gas can condense and drain. This change of state liberates additional heat, allowing these pulse boilers to achieve efficiencies up to 90%. They exhaust moist air, not hot smoke, so flues can be small-diameter plastic pipe rather than large-diameter, heat-resistant materials.

(b) Source Components: Coolth
An HVAC system that will be asked to provide cooling to offset building heat gains needs a coolth source. There are three conceptually (and physically) different means of introducing coolth into a building. These three basic approaches mushroom into dozens of specific equipment options; and these options expand into more dozens of specific equipment offerings.
Vapor compression refrigeration: the vapor compression cycle is a mechanical-electrical circuit in which a refrigerant is circulated under temperature conditions that allow it to pick up heat from within a building and dump  heat to the outside environment (under weather conditions that won’t permit passive heat flow from in-to-out); vapor compression is by far the most commonly used and encountered means of producing a cooling effect for an HVAC system; all vapor compression systems require an externally placed heat rejection unit (a condenser); the capacity of this type of equipment covers a wide range (from small to huge); the specifics of equipment types is also diverse; the vapor compression cycle can be used directly to cool air or to produce chilled water; capacity is expressed in tons of cooling and “efficiency” via COP (which will be above 2.0);
Absorption refrigeration:  the absorption refrigeration cycle is diagrammatically similar to the vapor compression cycle—but employs a chemical refrigerant flow process driven by heat (versus mechanical compression); absorption refrigeration equipment is less efficient (has a lower COP) than comparable vapor compression equipment, but may be driven by waste heat, solar hot water, or natural gas; absorption equipment today is typically used to produce chilled water in moderately large capacity applications; the equipment can be quieter than comparable vapor compression equipment  
Evaporative cooling: in an HVAC system, evaporative cooling effect will be produced by equipment called an evaporative cooler; this approach to coolth production can be very energy efficient (operating as it does along a line of approximately constant enthalpy); COP values can reach xx; with variations in equipment type, evaporative cooling can be used to directly cool air or to cool water; the performance of an evaporative cooler is more dependent upon climate than either vapor compression or absorption refrigeration (low relative humidity is generally desirable, although indirect evaporative cooling can extend the climate range of applications); where feasible, evaporative cooling has interesting potential as a low-GWP (global warming potential) high-efficiency coolth source
Several examples of coolth source equipment and applications are provided by way of introduction to this part of an HVAC system. 
(b) Chillers
These devices remove the heat gathered by the recirculating chilled water system as it cools the building. The selection of chillers depends largely on the fuel source and the total cooling load. Chillers include both absorption and compressive refrigeration processes in a wide range of sizes.
New developments in chillers continue to result from a combination of concerns about the role of CFCs and HCFCs in global climate change and from changes in utility regulations that are producing unstable energy prices in many areas. Chillers capable of changing quickly between electricity and natural gas are becoming available as a result.
The single-effect, indirect-fired absorption chiller (Fig. 10.23) is attractive where central steam or high-temperature water (from solar collectors, as waste heat from an industrial process, a fuel cell, etc.) is available. This device uses the absorptive refrigeration cycle (explained in Fig. 9.2). Direct-fired absorption chillers use natural gas to power the cycle. In general, absorption equipment is less efficient than compressive refrigeration cycle equipment, although a cheap or even free heat source to power the cycle can rapidly overcome efficiency disadvantages. Absorption machines have fewer moving parts (and therefore require less maintenance) and are generally quieter than compressive cycle equipment. They are environmentally attractive, despite their much higher waste heat output (about 31,000 Btu/ton, compared to at most 15,000 Btu/ton for compressive cycle equipment), because they do not use CFCs or HCFCs and because they require far less electricity to operate. Newer developments include the double-effect absorption chiller (see Fig. 9.3) and the triple-effect chiller, each accompanied by an increase in efficiency.
Fig. 10.23 (a) An absorption chiller driven by heat to produce chilled water. (The Carrier Corporation; courtesy of Ingersoll-Rand.) (b) Two-stage absorption chiller utilizing steam, producing 200 to 800 tons (700–2800 kW) of cooling. (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 11th ed.; © 2007 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
The compressive refrigeration cycle (explained in Fig. 9.1) is used in the other types of chillers. Larger units are centrifugal chillers (Fig. 10.24), whose compressors either can be driven by an electric motor or can utilize a turbine driven by steam or gas. (When a steam-driven turbine is used, the exhaust steam is often used to run an auxiliary absorption cycle machine. These two devices make an efficient combination, and the steam plant that supplies them in summer can supply heating in winter.) Centrifugal chillers usually require about 1 hp/ton (0.57 kW, or 10 ft3 gas, or about 15 lb of steam per ton). These large chillers usually require a cooling tower. Dual-condenser chillers (Fig. 10.25) can choose whether to reject their heat to a cooling tower (via the heat rejection condenser) or to building heating (via the heat recovery condenser).
Fig. 10.24 (a) A centrifugal chiller—a machine of large capacity using the compressive refrigeration cycle. (Courtesy of the Carrier Corporation.) (b) Centrifugal chiller with a flooded cooler and condenser within a single outer shell. This low-pressure unit typically produces 100 to 400 tons (350–1400 kW) of cooling. Typical dimensions are: 14ft  L x 5ft W x 8ft H (4.3 x 1.5 x 2.4 mm), at 16,000 lb (7260 kg). (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 7th ed.; © 1981 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.
Fig. 10.25 Dual-condenser chiller. Heat drawn from the chilled water system is either rejected to the cooling tower or recovered for use in building heating. (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 11th ed.; © 2007 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
Somewhat smaller chillers use either twin screws or a scroll in place of a piston in the compressor. The screw compressor (Fig. 10.26) has a pair of helical screws; as they rotate, they mesh and thus compress the volume of the gas refrigerant. They are small and quiet, with little vibration. The scroll compressor (Fig. 10.27) uses two inter-fitting spiral-shaped scrolls. Again, the refrigerant gas is compressed as one scroll rotates against the other fixed one. Gas is brought in at one end while the compressed gas is released at the other. Quiet and low-maintenance, they are also more efficient than reciprocating compressors.
Fig. 10.26 A screw, or helical, compressor is a quieter, smaller machine with little vibration. (from Bobenhausen, Simplified Design of HVAC Systems; © 1994 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
Fig. 10.27 Scroll compressor rotates one scroll form against another, with a quiet and efficient compression of the refrigerant. (From Bobenhausen, Simplified Design of HVAC Systems; © 1994 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
Even-smaller compressive-cycle machines are called reciprocating chillers (Fig. 10.28). Usually electrically driven, they are often combined with an air-cooled heat rejection process rather than a cooling tower. This makes them a closer relative of the smaller direct refrigerant machines discussed in Section 9.8.
Fig. 10.28 A reciprocating chiller—a small-capacity machine that uses the compressive refrigeration cycle. Typically, this type of chiller produces less than 200 tons (700 kW) of cooling. Such a machine might be around 8 ft L x 3 ft W x 5 ft H (2.4 x 0.9 x 1.5 m) and weigh 3500 lb (1590 kg). (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 11th ed.; © 2007 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
Chilled water is usually supplied at between 40º and 48ºF (4º and 9ºC). When the chilled water is supplied cold and returns much warmer, the large rise in temperature reduces the initial size (cost) of equipment and increases its efficiency (thereby reducing the operating cost as well). Water treatment may be needed for chilled water to control corrosion or scaling.
Typical cooling capacities and space requirements of chillers are shown in Fig. 10.29—with dimensions as tabulated in Fig. 10.29. Each refrigeration machine in this illustration requires two pumps—one for the chilled water (to cool the building) and one for condenser water (to deal with reject heat). Typically, space is provided for future chiller additions, which may be required by building expansion and/or by higher internal gains from as-yet-uninstalled equipment, such as computer terminals within offices. Improved-efficiency chillers may replace older ones when energy costs and environmental regulations become compelling. Adequate clearance access to the equipment room is a major design issue.
Fig. 10.29 Chiller room space requirements. Each refrigeration machine is served by two pumps (chilled water and condenser water). (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 10th ed.; © 2000 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)

Note: Direct-fired absorption machines are roughly the same size as steam absorption machines.
(c) Condensing Water Equipment
With chillers, there must be a way to reject the heat that is removed from the recirculating chilled water system. Reject heat is handled by the condensing water system, which serves the condensing process within refrigeration cycles. For larger buildings, the condensing water requirement is most likely to be met by a cooling tower.
The cooling tower’s place within the overall equipment layout was shown in Fig. 10.15b; a more detailed guide to sizes and types is given in Figs. 10.30 and 10.31. The object is to maximize the surface area contact between outdoor air and the heat condensing water. In crossflow towers, fans move air horizontally through water droplets and wet layers of fill (or packing), whereas in counterflow towers (prevalent in larger buildings), fans move the air up as the water moves down.
Fig. 10.30 Cooling towers that serve the condensing water system for large buildings. (a) Cutaway view of a large-capacity (200 to 700 tons [700-2460 kW]) crossflow induced-draft package cooling tower. (b) Size ranges for crossflow induced-draft package cooling towers. (c) Size ranges for counterflow induced-draft package cooling towers. (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 11th ed.; © 2007 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
Fig. 10.31 For cooling towers, the more wall clearance, the better the operation. A = maximum height of enclosure above the tower outlet; minimize this dimension. B = as large as possible, especially if walls have no air openings. (Consult the manufacturer for minimum dimensions.) (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 11th ed.; © 2007 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
Cooling towers create a special—and usually unpleasant—microclimate. They demand huge quantities of outdoor air (approximately 300 cfm [142 L/s]), which they make considerably more humid. In cold weather, they can produce fog. They are typically very noisy—a natural consequence of forced-air motion. The condensing water flows are about 2.8 gpm (0.18 L/s) per ton of compressive refrigeration and about 3.5 gpm (0.22 L/s) per ton of absorption refrigeration.
The water that escapes as vapor from the tower is between 1.6 and 2 gph (1.7 and 2.1 mL/s). This water must be replaced, which is done automatically. The steady evaporation and exposure to the outdoors under hot and humid conditions spells trouble for the condensing water: Controls for scaling, corrosion, and bacterial and algae growth are especially important. Ozone treatment systems have the advantage of reliable biological control and leave no chemical residue. Since the discovery of the link between Legionnaire’s disease and cooling towers, biological control has assumed greater importance.

The vapor that escapes the cooling tower should be kept from the vicinity of fresh air intakes, and from neighboring buildings or parked cars, where feasible. The floor space requirements can be approximated from Table 10.3, or use the average of 1/500 of the building gross floor area (for towers up to 8 ft [2.4 m] high) or 1/400 of the building gross floor area (for higher towers).
Although it is tempting to try to block the noise of cooling towers with solid barriers, it is critical that noise control not interfere with air circulation. The manufacturer’s recommended clearances to solid objects near cooling towers must be consulted before a tower is enclosed in any way. The roof is thus a favorite location for cooling towers, where wind can disperse the vapor, and the noise and odor are remote from the street. However, the cooling tower can sometimes be featured; near downtown Denver, the cooling tower for the large performing arts complex sits in a forlorn stretch of grass bordered by arterial streets and away from pedestrians (Fig. 10.32). Its plume adds visual interest as it twists ghostlike above the equipment.
Fig. 10.32 A plume of mist hovers ghostlike above a cooling tower in full public view near the Denver performing arts complex.
When fouling of the condensing water system cannot be tolerated, an alternative approach, called the closed-circuit evaporative cooler, is taken. Its schematic operation is described in Fig. 10.33. Usually used to cool the refrigerant directly, it can also be used for the condenser water, as well as on water loop heat pump systems (see Fig. 9.45). Either refrigerant or condenser water is protected within an always-closed loop, while a separate body of water is recirculated through the cooler, with steady evaporation and attendant problems. It requires much less makeup water than the cooling towers.
Fig. 10.33 Closed-circuit evaporative coolers, which cool the condensing water system while protecting it from contact with outside air. A self-contained water system is circulated through the evaporative cooler; steady evaporation losses are replaced by makeup water. (Based upon AIA: Ramsey/Sleeper, Architectural Graphic Standards, 8th ed.; © 1988 by John Wiley & Sons.)

(c) Distribution Components: Air
Distribution components connect source components to delivery components in a central HVAC system. (Local systems are close-coupled and have little need for distribution; district systems function as central systems at the building scale). The building designer faces a fundamental decision regarding the medium that will be used to convey heat and/or coolth effect around a building. Should this medium be air or water?  Both will work, both have been used effectively in the past, and both are competing for attention in high-performance buildings—see for example the controversial ASHRAE Journal article comparing VAV [air] to chilled beam [water] systems (ASHRAE 2013). Air can be directly introduced into a room, can inherently address indoor air quality concerns, but has very low thermal capacity—requiring large conduits (ducts). Water has the opposite characteristics; it is thermally efficient—permitting small conduits (pipes), but cannot be allowed to flow directly into a room, and does not inherently deal with air quality.
Examples of air-based and water-based systems (including hybrids) are discussed in section xx.x. This section looks at the equipment artifacts associated with air as a distribution medium.
Fans
Air-Handling Units
Ductwork
Accessories

(d) Distribution Components: Water
When water is employed in a central or district HVAC system …
Pumps
Piping
Accessories
Steam ???

The internal space can be used differently according to the function. So, a HVAC designer must determine the general criteria and the special requirements of the spaces. The temperature, relative humidity, sound level, air quality, and noise must be considered with designing the HVAC system. 
NOTE: Adapted from Haines, Roger W. and Myers, Micheal E. 2010. HVAC Systems Design Handbook, 5th ed. McGraw Hill

Residences/Apartments
There is a need for the comfort of the occupants in how they adjust the controller set point. In larger residences (over 2400 square feet) the use of multiple or zoned systems should be considered. Zoning is also essential in housing, apartments and hotel guest rooms. The cost of installation and operating cost are always the main concerns.
Offices
The basic needs are comfort and adequate ventilation rate, especially if smoking is allowed. Controls are usually designed to be nonadjustable by occupants. Zoning must be provided for use, occupancy, and exterior exposure. The HVAC system should be flexible enough to allow for adding or rear-ranging zones as use changes. Background noise level should be in the range of 30 to 40 decibels (DB).
Motels and Hotels
A small motel must usually meets residential criteria in terms of zoning. A large resort or conference hotel has many varying needs. The building may include public areas, such as lobbies, restaurants, health and recreation facilities (such as swimming pools), meeting rooms, retail shops and ballrooms, as well as service areas such as kitchens, laundry, repair shops, offices, storage, employee lounge. Each area has different HVAC requirements. Kitchens are often cooled indirectly by transferring air from adjacent areas. Transfer air should be filtered. Laundries and pool areas have high humidity problems that must be addressed, usually by high exhaust air rates. Meeting rooms have high occupancy rates; cross radiation from people usually suggests lower design temperatures for comfort. Zoning and individual guest room controls are essential. Through-the-wall independent HVAC units are popular in motels and hotels. They have a low first cost but are often noisy.
Educational Facilities
Comfort criteria apply in classrooms and offices, including special-purpose rooms for music, laboratories, practice rooms, study halls, and lunchrooms. Auditoriums, with or without stages, have criteria peculiar to theaters—a need for somewhat lower temperatures due to high occupant density, a low background noise level, and avoidance of drafts in what is typically a high airflow rate situation. Many elementary and high schools have smaller, distributed AHUs, single or multizone, with a central plant source of heating and cooling. In single-story schools, rooftop self-contained units are sometimes used. At the college or university level the facility takes on an institutional character, with emphasis on higher-quality equipment and systems, with longer life and lower maintenance costs. There are many special-purpose buildings with widely varying HVAC requirements. Central HVAC plants are often used with elaborate distribution systems.
Theaters and Concert Halls
The HVAC system must not produce a noise that will interfere with the audience's enjoyment of the performance. This is not easy to achieve; it requires careful design and construction of the building and the HVAC and electrical systems.

Laboratories
Laboratory facilities associated with education, public health, or industry can have very complex requirements, including humidity control and high levels of cleanliness. Most laboratories require high rates of exhaust and makeup air. Because the user in a research laboratory seldom knows exactly what is needed, the HVAC design must be flexible enough to satisfy a wide range of contingencies. This tends to be costly but is necessary in order to properly utilize the lab facility. Heat reclaim systems are especially helpful here.
Hospitals
Hospitals are challenging for the HVAC designer because of the wide variety of environmental conditions required in various departments. The operating suite, with heavily clothed staff working under hot lights, requires a design temperature of 65 to 70°F with a relative humidity of 50 to 60 percent and a high percentage of outside air when in use. This requires at least high-efficiency filters and high airflow rates. Nurseries do not require high airflow rates but do require about 55 percent relative humidity. Patient room requirements vary depending on usage i.e., isolation rooms require exhaust with no recirculation to other parts of the hospital. Public areas and offices may be treated as in any other building. Laboratories and treatment rooms require special treatment with no recirculation. There may be requirements for air pressure relationships to keep air moving from higher-quality to lower-quality environments. Exhaust air from contaminated spaces must be scrubbed.
Factories/Manufacturing Facilities
Process environments dominate in manufacturing applications of HVAC. Typical applications include close control of temperature (plus or minus 1°F) and humidity (plus or minus 5 percent RH). These criteria can be met only by the use of carefully designed HVAC systems with high-quality controls. Clean rooms require high flow rates but have normal or low heating and cooling loads. The typical solution is to provide a small HVAC system with a supplemental high flow rate fan system. Electroplating and painting facilities require high rates of exhaust air flow, usually with filtering. Machining operations generate an oil mist, which is carried in the air and deposited everywhere. The HVAC system can be designed to control this problem to some degree. Flammable vapors are often of concern; some processes generate heat and combustion products. Many processes offer opportunities for heat reclaim. Industrial hygiene criteria complement HVAC criteria in these environments.

11.3  EQUIPMENT LOCATION AND SERVICE DISTRIBUTION
NOTE: Chapters 9.3 and 10.1 (from MEEB 11) combined into one section 11.3
’’ivities have been identified, and the basic siting and overall form of the building have been determined from daylighting and thermal considerations, among others. This section begins by considering the internal yet broad issues of zoning and system choice and ends with a discussion of the more detailed consequences of system choice.
(a) Zoning
differences in scheduling within a zone, such as between offices and stores. As is true of the other occupied floors, apartments have a minimum of five zones (based on orientation); however, the emphasis on individual controls—tterns—
Fig. 10.1 The minimum number of thermal zones for a rather large, conventionally designed, multipurpose building.
(b)       System Anatomy
Table 10.1 describes the basic organization of any HVAC system. Three kinds of common tasks (heating, cooling, and ventilating) are done by production components; usually, they require distribution and delivery components. Intake supplies and exhaust by-products accompany each task. Although the eventual choice of HVAC system should follow an analysis of the zone’s needs, some early concepts underlie system choices.
TABLE 10.1 Basic HVAC Systems: Tasks and Components
(c)       Central versus Local Systems
Central systems require one or several large mechanical spaces (often in basements and/or on roofs), sizable distribution trees, and complex control systems. The noise, heat, and other characteristics of such mechanical rooms can be controlled fairly easily, because the machinery is concentrated at a few locations. Similarly, maintenance is easy to perform without interrupting normal activities, although breakdowns in central equipment can paralyze the entire building. Air quality can be controlled by locating the air intakes high above the pollution at street level and by regular maintenance of the centralized air-filtering equipment. Longer equipment life can be expected with regular maintenance. Energy conservation can be served by the recovery of one machine’s heat by-product for a nearby machine’s heat input. Although there are many ways to provide for the differing thermal needs of the many zones served by central systems, one important drawback of central systems is the size and length of the distribution trees necessary to carry centralized services to many local receivers. Another drawback is a difference in zone scheduling: when the entire system must be activated to serve one zone (such as computer operations in an office building on a weekend), energy is wasted.
Local systems therefore become increasingly attractive as scheduling differences multiply. Also, pronounced differences in other factors—function (with resulting comfort expectations) or placement within the building, for example—can lead to the choice of local systems. Large and centralized equipment spaces are not required with local systems; rather, production equipment is distributed throughout the building (or over the roofs of low-rise structures). Dispersal of equipment minimizes the size of distribution trees and greatly simplifies control systems. Moreover, system breakdowns affect only small portions of the building. However, noise and other by-products of multiple machines pose numerous potential threats to occupied spaces, and maintenance is demanding, because access to so many separate locations is often disruptive or constricted. Then, too, air quality depends on the regular cleaning of many filters scattered over the building, often within occupied spaces. The potential for energy conservation seems promising, because heating or cooling is produced only as locally needed, but there is little chance to use one zone’s waste heat as another’s needed source.
Central heat/cool, local air distribution has become a popular way to take advantage of the favorable characteristics of both central and local approaches. This is shown in Fig. 10.2b, with a central boiler/chiller space remotely located and fan rooms on each floor. This minimizes the bulky distribution tree for air; although the distribution tree for heated and chilled water is extensive, it is also of much smaller diameter and therefore is relatively easily accommodated. The central equipment room makes energy recovery systems from boilers and chillers more feasible.
Fig. 10.2 Fan rooms (F) can either be combined with or separated from boiler/chiller “equipment” rooms (*). (a) Common location for a central combined equipment room. (b) Increasingly common arrangement of a small fan room on each floor, with an equipment room in the basement. (c) An intermediate floor may be able to provide space for a central fan room, while the heavier and noisier equipment remains in the basement. (d) With a top-floor central fan room, the equipment may be located either on the roof or in a mechanical penthouse, or may remain in the basement. (Adapted by permission from E. Allen and J. Iano, The Architect’s Studio Companion, 4th edition; © 2007, John Wiley & Sons, Inc.)
(d)       Uniformity versus Diversity
How similar should the interior environments of buildings be? This question encompasses not only thermal experiences, but visual and acoustical ones as well.
The advantages of uniformity are most evident in a rapidity of design and construction that, through mass production and speed, often brings lower first costs. Uniformity of ceiling heights, light fixture placement, grille locations, and so on promotes flexibility in office arrangements that can extend a building’s usable life span. However, there are at least four types of offices, which may need to be interchangeable within such “flexible” space. The typical enclosed office has the privacy of four walls and a door. The bullpen office has repeated, identical workstations, with low dividers at about the height of the desk surface. The uniform open plan office resembles the bullpen, but with higher divider partitions for added privacy. The free-form open plan office has some individually designed workstations with divider partitions of varying heights (sometimes reflecting the varying status of workers). In the bullpen and uniform open plan office, the resulting uniformity is not always attractive to users, and diversity is often encouraged at a more personal level—with office furnishings, for example. A more thorough approach to diversity can provide stimulus to the user who spends many hours away from the variability of the exterior climate.
If offices must be uniform in ceiling lighting, air handling, and size, the corridors that connect them and the lounges or other supporting service spaces can deliberately be made different. Diversity requires a complete and detailed design of places; it gives the builder a more complex and interesting task; and it can provide orientation and interest to the users. The attractiveness of diversity is evident in most collections of retail shops, in which light and sound—and sometimes heat and aroma—are used to distinguish one shop from the next.
Diversity in the thermal conditions to be maintained, such as warmer offices and cooler circulation spaces in the winter, can be used to enhance the comfort of the office users. Designers have long recognized that a space can be made to seem brighter and higher if it is preceded by a dark, low transition space. Thermal comfort impressions can be manipulated similarly. Less than comfortable conditions in circulation spaces or other less-critical zones not only make the critical spaces seem more comfortable by contrast, but also save significant amounts of energy over the life of a building. Furthermore, such conditions can make passive strategies more attractive.
A large-scale demonstration of diversity in thermal zones is shown in Fig. 10.3. Passive solar heating can make a significant contribution, even through a shallow-sloped, single-glazed cover in cloudy Glasgow, Scotland, largely because the mall area and leisure areas are allowed a much wider thermal range than would be permitted in stores and offices. The overcast skies are quite suitable for daylight, and the addition of summer sunshading makes natural ventilation (through the stack effect, assisted by fans) possible during the cool summers. U.S. Pacific Northwest climate conditions are similar.
Fig. 10.3 St. Enoch’s Square, Glasgow, Scotland: a proposal to use passive solar heating, daylighting, and natural ventilation. Reiach & Hall and GMW Partnership, architects (joint venture); Cosentini Associates, energy consultants; Princeton Energy Group, daylighting consultants. (a) Schematic section showing winter operation; the mall temperature varies around 63ºF (17ºC) during operating hours, while offices are kept near 70ºF (21ºC). (b) Schematic section showing summer operation; the mall temperature varies from about 68 to 74ºF (20 to 23ºC) during operating hours. (c) Estimates of annual energy consumption for a conventional-design base case and several alternative configurations. Note the significantly lower heating energy requirements, resulting in part from the lower winter temperatures allowed for the less-critical zones such as the mall and the leisure areas in configurations A to E.
(e)       Comparing Systems and Zones
In the process of selecting systems from among the wide variety available, it is helpful to consider the match between the zones’ characteristics and those of various systems. Among the considerations are zone placement (close to or away from the building skin), the zone’s thermal loads, the comfort determinants based on the zone’s activities, the space available for system components within the zone, and the life-cycle costs of various system alternatives.
Zone placement will sometimes preclude local systems, which depend on easy access to outdoor air both for fresh air and for a heat source or sink. Local systems for interior (away-from-skin) zones are awkward. Relationships between zone placement and building forms are shown in Fig. 10.4.
Fig. 10.4 Zone placement and building form are related to heating, cooling, and ventilating tasks; some applications take on typical building forms. (From class notes developed by G. Z. Brown, University of Oregon.)
The thermal loads on each zone determine the extent to which heating or cooling is the dominant problem—which, in turn, can influence the choice of system. A zone with little cooling load and low moisture production may be well served by a simple system of fresh air plus heating, with no humidity control. Zones that require cooling will usually also require more complete control of air motion and relative humidity. Although it is risky to generalize about which comfort determinants are most important (given the differences between activities and between individuals), it can generally be assumed that comfort and thermal tasks are related.

Thus, the choice of systems can be based partly on whether the system provides good control of the more important comfort determinants.
(f)        Distribution Trees
Central heating/cooling systems produce heating and cooling in one place, then distribute them to other building spaces according to their respective needs. The distribution tree is the means for delivering heating and cooling: the “roots” are the machines that provide heat and cold, the “trunk” is the main duct or pipe from the mechanical equipment to the zone to be served, and the “branches” are the many smaller ducts or pipes that lead to individual spaces.
For now, the questions to be answered about distribution trees for buildings are: How many? What kind? Where? A building can have one giant distribution tree, several medium-sized trees, or an orchard of much smaller trees. At one extreme, a large mechanical room is the scene of all heating and cooling production; leading from this room is a very large trunk duct with perhaps hundreds of branches. At the other extreme, each zone has its own mechanical equipment (such as a rooftop heat pump), with short trunks and relatively few branches on each tree.
What kind of distribution tree? Most simply, air (ducts) or water (pipes). Air distribution trees are bulky and therefore likely to have major visual impacts unless they are concealed above ceilings, below floors, or within vertical chases. Water distribution trees consume much less space (a given volume of water carries vastly more heat than does the same volume of air at the same temperature) and can be easily integrated within structural members such as columns. Both air and water trees can be sources of noise.
Where does the distribution tree fit in? On the exterior, it can lend a three-dimensional organizational structure to a façade. Exterior trees can take up a smaller amount of rentable floor space but require expensive cladding and are subject to considerable heat losses and gains, which could increase energy usage. Interior trees are often combined with other continuous vertical spaces, such as elevator shafts and stairways. If the choice is an exterior distribution tree, its potential contribution to façade performance should be considered. For example, the distribution tree might act as a sunshade or as a light reflector.
HVAC system choice is influenced by the amount of space the system requires. In some cases, it is easy to provide small equipment rooms at regular intervals throughout a building, such that little or nothing in the way of a distribution tree will be required. In other cases, a network of distribution trees and central, large equipment spaces are easier to accommodate. These central systems typically fall into one of three classifications:
All-air (the largest distribution trees)
Air and water
All-water (the smallest distribution trees, with local control of fresh air)

The details of these systems can be found in Sections 11.9 (a), along with typical applications and space requirements. Figure 10.5 shows the matrix of central–local, air–water influences on distribution trees.
To carry the tree analogy to its logical conclusion, consider the “leaves,” the points of interchange between the piped or ducted heating or cooling and the spaces served. One example is a large, bulky device such as a fan-coil unit on the exterior wall below windows. In contrast, a perforated ceiling system with thousands of small holes acting as a widely spread grille is essentially invisible.
Fig. 10.5 Matrix of distribution trees. (© 1998 John S. Reynolds; drawing by Michael Cockram.)
A simplified procedure for matching zones and systems is shown in Table 10.2, in which preliminary system choices are made for a building such as the multipurpose structure shown in Fig. 10.1. In this process, the original 16 zones are translated into three local systems and three central systems: one all-air, one air and water, and one all-water.
TABLE 10.2 Procedure for Matching Zones and Systems

The Fox Plaza Building in San Francisco, which illustrates many of the matches between systems and zones, is shown in Fig. 10.6. This project includes four major building types in one structure:
1.         Underground garage for the storage of cars
2.         Commercial center at ground level, including a bank, a women’s specialty store, and other commercial establishments
3.         Ten floors of offices
4.         Sixteen floors of apartments
Fig. 10.6 The Fox Plaza Building, San Francisco. Victor Gruen Associates, Inc., architects and engineers. (a) Elongated façade facing northeast shows the 16 floors of apartments above, the 10 floors of offices below, and the 13th-floor mechanical space. In that space are chillers, and pumps for cooling tower water and chilled water, as well as boilers and converters (steam to hot water) for the fan-coil units in the residential stories above and hot water coils in the office stories below. Air handling for the offices is also located here, downfed by high-velocity ducts. Residences are heated; offices are heated and cooled. (The roof has the cooling tower and the domestic hot water generator–storage units for the residential stories.) (Courtesy of Progressive Architecture.) (b) Construction photo with air-handling units visible on the 13th floor, and downfeed ducts that supply high-velocity hot and cold air to the office floors. (Photo by Morley Baer.)
The mechanical level is located between the office portion of the building and the apartments above. The distribution trees—heating, air conditioning, electrical, and so on—are thus directed both upward and downward, resulting in two shorter trees rather than one longer tree. The spatial requirements of offices and those of apartments are quite different; thus, the floor-to-floor heights, window treatment, and heating, cooling, electrical, elevators, and other services are different. The placement of the mechanical level between the offices and the apartments also provides for a definite visual separation between the two functions.
Quite unusual is the placement of the steam boilers on the 13th floor instead of in the conventional basement location. Only a small amount of auxiliary equipment is located on the roof and in a small portion of the garage. Residential areas have hot–water heating (residential cooling being rarely needed in San Francisco), offices have dual-duct, high-velocity heating/cooling, and commercial (ground-floor) tenants are supplied with hot and chilled water for individual climate control requirements.
(g)       Central Equipment Location
The Fox Plaza Building has an intermediate location for the heating and cooling production equipment—one that separates floors of apartments from floors of offices. Other typical locations for central equipment are in the basement (where machine noise is most easily isolated, utilities are easily accessed, and machine weight is little problem) and on the roof, where access to air as a sink for reject heat is easiest of all, and headroom is unlimited. Very tall buildings may require several intermediate mechanical floors. Examples of these approaches are found throughout the rest of this chapter.
The equipment’s considerable heat, moisture, air motion, noise, and vibration potentially annoy occupants on nearby floors (or even neighboring buildings). As shown in the Fox Plaza example, the equipment can be expressive of building services and can play a useful demarcation role between vertical layers of high-rise buildings. Moving the equipment off the roof also frees this prestigious view location for high-rent occupancy and allows a roof form much more expressive of great height than a flat roof with a cooling tower.
(h)       Concealment and Exposure
The pipes, ducts, and conduits that take the necessary resources to and from the interior are often carried within a network of spaces unseen by anyone except builders and repair people. The advantages of concealment include less noise from moving water and air, fewer surfaces requiring cleaning, less care necessary in construction (leaks, not looks, are important), and more control over the appearance of the interior ceiling and wall surfaces. Although maintenance access to such hidden supply lines is more difficult, various types of readily removable covers are available, particularly in suspended ceilings.
However, the exposure of these supply networks provides an honest and direct source of visual (and occasionally acoustical) interest. Exposure in corridors and service areas and concealment in offices constitute an approach used in many office buildings. Flexibility is usually encouraged by exposure; changes can be easily made when there is no need for neatly cut holes in concealing surfaces. However, flexibility from full-height movable partitions requires constant ceiling heights—a feature of the suspended-ceiling approach.
One of the more spectacular examples of exposed mechanical (and structural) systems is shown in Fig. 10.7—the result of a design competition for a museum of modern art, reference library, center for industrial design, center for music and acoustic research, and supporting services in downtown Paris.
Fig. 10.7 Centre Georges Pompidou, Paris. A view of the mechanical support systems. Piano Rogers, architects. (Photo by John Tingley.)
When users are invited to play an active role in adjusting conditions inside, exposure of the switches they manipulate is helpful. Visible mechanisms not only remind users of their opportunities but also encourage user interaction. In this way, adjustments are sometimes discovered that the designer had not anticipated.
(i)        Mechanical–Structural Integration or Separation
The similarity of these two technical support systems—structures and an environmental control—has intrigued designers ever since mechanical systems began to require substantial volume for distribution, as in air-duct systems. As the complexity and size of the mechanical distribution systems was increasing with technological development (typically, more air is required to cool a space than to heat it because of a lower ∆t), the increased strength of materials was reducing the size of the structural system. The uncluttered floor areas between the more widely spaced columns became desirable for flexibility in spatial layout. With the mechanical systems at or within these columns, floor areas remained clear, thus giving mechanical–structural integration further impetus. With the new expectations for cooling, the refrigeration cycle’s cooling tower often moved to the roof, often taking the air-handling machinery with it. This further encouraged the merging of systems, for one system was growing wider as the other diminished (Fig. 10.8). Thus, a fixed-column cross section, consisting mostly of the structural column at the base and the air duct at the top, became possible.
Fig. 10.8 Distribution trees: with rooftop centralized air handling, the supply and return air duct sizes decrease as they approach the ground. Conversely, the structural load increases toward the ground.
Yet the functions of these systems differ widely: compared to the dynamic on–off air, water, and electrical distribution systems, the structural system is static—gravity never ceases. The moving parts in mechanical systems need maintenance far more frequently than the connections of structural components. Changes in occupancy can mean enormous changes in mechanical systems, requiring entirely different equipment; structural changes of such magnitude usually occur only at demolition. Mechanical systems can invite user adjustment; structural systems rarely do.
Thus, although it is possible to wrap the mechanical systems in a structural envelope, it is of questionable long-term value, given the differing life spans and characteristics of these systems. The probability of future change suggests that the mechanical system be the exposed one, despite the appeal to many designers of the structural system’s cleaner lines.
(j)        Distribution Tree Placement Options
These options are summarized in Fig. 10.9. Vertical placement options are important because they affect floor space, influencing the flexibility of spatial layout and the availability of usable (or rentable) floor space. Horizontal placement options affect ceiling height—a particular issue in daylighting design and sometimes a critical factor when overall height limits are imposed yet maximum usable floor space is desired. (In Washington, DC, for instance, no building can rise higher than the Capitol.) Both vertical and horizontal distribution at the edges can have a dramatic impact on building appearance.
Fig. 10.9 Distribution tree placement options, both vertical (with impact on the plan) and horizontal (with impact on the section). (From class notes developed by G. Z. Brown, University of Oregon.)
The history of distribution trees and high-rise buildings is one of trends and countertrends. Initially, multistory buildings relied upon daylight and cross-ventilation, so a thin, relatively high-ceiling plan with much perimeter was favored (refer to Fig. 3.33). The heat gain and loss was all at the perimeter, so perimeter distribution trees (carrying only steam or heated water, and of quite small diameter) were generally used. As electric lighting and thus the need for air conditioning increased, so did the thickness of floor plans; large central internal areas needed a lot of forced, cooled air. Central boilers, chillers, and fan rooms were the norm. Thus, bulky air distribution trees appeared. At about the same time, the glass curtain wall and its slick, two-dimensional look of modernity became fashionable. The air distribution trees were so visually intrusive on the façades that they were pushed to the core, where cooling needs were relatively steady. However, the thin glass perimeter experienced extreme needs for both heating and cooling; getting from vertical trees at the core to the perimeter required larger cavities above suspended ceilings. This pushed the ceiling in the offices down to keep floor-to-floor distances economical. Vast office areas resulted that were visually dull, low-ceilinged, and without daylight.
Now, countertrends include decentralized air handling, with small fan rooms on each floor. Vertical air distribution trees are shrinking, horizontal ones becoming more common. At the same time, daylighting is pushing office ceilings higher; so is a preference for indirect lighting and its compatibility with computer screen visual comfort. Night cooling utilizing thermal mass is encouraging the exposure of concrete structure and favoring raised-floor air supply/ventilation systems. A renewed interest in sun control is encouraging three-dimensional façades, replacing two-dimensional reflective glass façades (which merely redirect the sun toward someone else). With increased three-dimensionality at the façade, perimeter distribution trees are once again conceivable.
It is logical to place at the perimeter the parts of the system that deal with the effects of sun, shade, and temperature change in the several perimeter zones, leaving at the core a separate network to handle the more stable interior areas. The disadvantages of perimeter distribution include (usually) higher construction costs and an environment that is more thermally hostile due to the extremes of outdoor temperature.
Vertical distribution within internal circulation cores is very common, as it leaves a maximum of plan flexibility for the rest of each floor and does not disturb the prized floor areas nearest windows. However, one centralized vertical distribution trunk will require large horizontal branches near the core, so with this choice early thought must be given to the horizontal placement options.
An unusual example of vertical air distribution at the core is shown in Fig. 10.10. The Fox Plaza, Los Angeles, office building’s unique features include both fan rooms on each floor and a large central vertical air shaft. This air shaft begins at the bottom as a fresh air intake to each floor and tapers to become, at the top, an exhaust (heated) air outlet from each floor. Thus, the stack effect is utilized to help supply fresh and exhaust stale air from a large building, with help from small supply fans at each floor.
Fig. 10.10 The Fox Plaza, Los Angeles, office building is a 34-story, 800,000-ft2 (74,320-m2) granite and glass tower (a) with an unusual vertical distribution tree. (b) Typical lower-floor plan (floors 6 to 16) shows both a fan room and a large vertical air shaft. At this lower level, most of the shaft area is supplying outdoor air (from an intake in the bluff face below the building); the remainder is exhausting stale air toward the roof. Note the lack of columns between the core and perimeter, contributing to office layout flexibility. (c) Typical upper-floor plan (floors 31 to 33) shows fewer elevators; by this level, most of the shaft area is exhausting stale air toward the roof. (d) Section shows the tapered interior of the constant-cross-section central air shaft, which relies upon the stack effect to bring in (usually cooler) outdoor air at the base and expels hotter exhaust air at the top. (Courtesy of Johnson Fain Pereira Associates, Architects, Los Angeles, and Kim, Casey and Harase, Inc., Engineers, Los Angeles. Photo by Wolfgang Simon.)
Vertical distribution integrated with structure creates some intriguing possibilities where the structure–HVAC integration concept is suitable. Multiple HVAC trees are implied (because there are multiple columns with which they are integrated), so the horizontal branches tend to be small. However, these branches often join the vertical trunk at the same place where critical column-to-girder structural connections need to be made; interference is common and can be costly to correct. Vertical distribution at the edges is potentially dramatic in form but costly to enclose (if outside) or wasteful of prime floor space (if inside).
Horizontal distribution above corridors is very common, since reduced headroom here is more acceptable than in the main activity areas. Furthermore, corridors tend to be away from windows, so their lower ceilings do not interfere with daylight penetration. Because corridors connect nearly all spaces, horizontal service distribution to such spaces is also provided. Furthermore, exposure of these services above corridors can heighten the contrast between such serving spaces and the uncluttered, higher-ceilinged offices that are served. Horizontal distribution at the structure is sometimes chosen, particularly where U-shaped beams or box beams provide ready channels for HVAC distribution. However, the penetration of horizontal structure members by these continuous service runs must be coordinated. Horizontal distribution at the edges can be integrated usefully with sunshading devices and light shelves; it can also act as a spandrel element that contrasts with the window strips. Horizontal distribution within whole layers below floors (or above ceilings) is often utilized, now increasingly common with displacement ventilation systems.
An example of supply at the edge for both vertical and horizontal distribution is found in the International Building in San Francisco (Fig. 10.11). Here the vertical shafts are prominently exposed at the corners; these shafts carry supply and return ducts serving the four perimeter air-conditioning zones. Air-handling equipment and a 750-ton refrigeration plant are located on the floors just below the terrace level (those least desirable for renting). Each corner duct branches to serve two zones, which are separately controlled. Pressure reduction and blending are done by equipment in the hung ceiling, and from these points airflows to strip-grille diffusers directly above the glass on the four sides of the building. Local controls offer comfort to personnel in each area.
Fig. 10.11 The International Building, San Francisco. Anshen and Allen, architects; Eagelson, Engineers (Charles Krieger, E.E.), mechanical designers. (Courtesy of Progressive Architecture.) (a) Photo of one of the four corner main duct enclosures. (b) Tenth-floor plan. Column bay spacing is 24 ft, 6 in. (7.5 m), with a 16-ft (4.9-m) cantilever on all four façades. The major supply ducts (both hot and cold) to all 21 floors are located in two opposite corners. Each of these supply distribution trees serves two adjacent sides of perimeter offices. The conditioned air is supplied from a third-floor mechanical space. In the opposite two corners, return air from the upper 11 floors is collected and taken down to the mechanical space. The remainder of the return air is taken down through the core.
Interior zones on each floor are supplied by a riser duct in the building core, which branches at each floor to a loop just outside the line of elevators. The loop serves ceiling diffusers.
Between the perimeter loop and the interior loop, a return loop collects air for return to the central station (second and third floors). These return loops on the 11th to 21st floors are picked up by external return risers on alternate exterior corners. From the 10th floor down, the loops are picked up (as shown in Fig. 10.11) by an interior return riser that extends down through the core in front of the blank faces of the high-rise elevators. To provide a clear space between the elevator banks on the main floor (fourth or terrace), the two core ducts’ risers are offset at the ceiling of that story.
In summary, perimeter air for all stories is supplied through corner ducts. Central air for all stories is supplied through a core duct. All return air above the 10th floor is carried down through the return ducts at the other two corners. Return air from the 10th floor and below is carried down by a return duct in the core.

11.4  PSYCHROMETRICS

Adopted from various sources of pdfs (present in Dropbox folder)

A psychrometric chart, which allows a graphical analysis of moist air conditions, is the most useful tool for analyzing the HVAC process. Familiarity and facility in the use of these charts are essential for the HVAC designer to determine the volume flow rates of air to be pushed into the ducting system and the sizing of the major system components. In practical applications, the psychrometric analysis made by HVAC contractors involves measuring the dry and wet bulb temperatures of air entering and leaving a cooling coil. If these temperatures are known along with the volumetric air flow rate through the coil, the cooling capacity of a unit can be verified. Using the dry and wet bulb temperature information, two points can be located on a psychrometric chart and the corresponding enthalpy values read for them.

The goal of HVAC process is to provide a sufficient amount of fresh air at desirable temperature and humidity. Outside air is taken in and, depending on its original temperature and humidity, must undergo one or several of the following processes:

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Approaches to Temperature Control
Temperature control in a HVAC system is achieved by passing the air through the cooling or heating coil, which may use any of the following approaches:
1.         Vary the temperature of air supplied to the space while keeping the airflow rate constant. This is the basic constant volume, variable temperature approach.
2.         Vary the airflow rate while keeping the temperature constant for air supplied to the space. This is the variable volume, constant temperature approach.
3.         Vary the airflow rate and change the temperature for air supplied to the space. This is the variable volume and temperature approach.
4.         Vary both the supply air temperature and flow rate where the airflow rate is varied down to a minimum value, then energy input to reheat the coil is controlled to vary the supply air temperature. This is the variable volume reheat approach.

Approaches to Humidity Control

Humidification is not always required in an HVAC system but, when required, it is provided by a humidifier. Commonly used humidification methods include:
1.         Water spray humidifier
2.         Steam pan humidifier
Humidity control in a conditioned space is done by controlling the amount of water vapor present in the air in the space. When relative humidity at the desired temperature set-point is too high, dehumidification is required to reduce the amount of water vapor in the air for humidity control. Similarly, when relative humidity at the desired temperature set point is too low, humidification is required to increase the amount of water vapor in the air for humidity control.
Commonly used dehumidification methods include - Surface dehumidification on cooling coils simultaneous with sensible cooling and direct dehumidification with desiccant-based dehumidifiers.

HVAC SYSTEM DESIGN
In designing air conditioning systems, the first challenge is to understand the components that affect the building heat gain or heat loss - this process is called heating or cooling load estimation. The reactive challenge is to "design" controlled processes to maintain the desired condition or state-point within the occupied space - these are usually called the system processes that use psychrometrics.
Estimating Cooling & Heating Load
Load estimates are the summation of heat transfer elements into (gains) or out of (losses) the spaces of a building. Each heat transfer element is called load components, which can be assembled into one of three basic groups, external space loads, internal space loads and system loads. To properly understand the workings of the various external, internal and system load components, the following items need to be gathered from a set of plans, existing building surveys or occupant interviews:

• Building square-footage and volume
• Orientation of the building (sun effects on surfaces)
• Year round weather data (design conditions, heat transfer)
• Use of the spaces within the building (offices, conference room, lab, data center)
• Hours of operation (occupied and unoccupied)
• Thermostat set points (main comfort parameter)
• Dimensions of walls, roofs, windows and doors
• Construction materials (gather densities, external color and U-factors or describe material type layer by layer (R-values)
• Stairways and elevators (floor-to-floor openings)
• People occupancy and activity, and when they are present
• Lighting intensity and hours used
• Motor and appliance sizes or kW and times they are used
• Ventilation needs (IAQ and exhaust makeup)

The total cooling load is than determined in kW or tons* by the summation of all of the calculated heat gains. Along with psychrometrics, load estimating establishes the foundation upon which HVAC system design and operation occur.
*One ton is equivalent to heat extraction rate of 12000 Btu’s/hr and 1 kW is equivalent to 3414 Btu’s/hr.

Determine Design Supply Airflow Rate
HVAC engineers use psychrometrics to translate the knowledge of heating or cooling loads (which are in kW or tons) into volume flow rates (in m3/s or CFM) for the air to be circulated into the duct system. The volume flow rate is used to determine the size of fans, grills, outlets, air-handling units, and packaged units. This in turn affects the physical size (foot print) of air handling units and package units and is the single most important factor in conceptualizing the space requirements for mechanical rooms and also the air-distribution ducts.
The psychrometric analysis of a HVAC system determines the volume flow rates of air to be pushed into the ducting system and the sizing of the major system components.

Psychrometric Process

Psychrometric processes bring about changes in air-water vapor properties. The movement of the state point on the psychrometric chart represents changes. Common processes include:

• Sensible Heating or Cooling
• Dehumidification by Cooling
• Adiabatic Humidification (Evaporative Cooling)
• Adiabatic Dehumidification (by Absorbents)
• Mixing of Two Air Streams

1) Sensible Heating or Cooling:

This is the addition or removal of heat, without any change in the moisture content (at constant humidity ratio), resulting only in the change in DBT. The status point will move horizontally to the left (cooling) or to the right (heating). While the AH does not change, the change in temperature means the relative humidity (RH) changes. The relative humidity increases if the temperature lowers and vice versa. As the dry-bulb temperature increases, the air will hold more moisture at saturation.

11.5  REFRIGERATION CYCLES

Unlike the slow, diffuse heat transfer processes that characterize passive heating and cooling approaches, mechanical equipment can rapidly concentrate heating or cooling on demand. The refrigeration cycle is a particularly useful mechanical process in heating as well as cooling applications. The two types of heat transfer process commonly used in mechanical equipment for buildings are the compressive and the absorption refrigeration cycles.
(a)       Compressive Refrigeration
As shown in Fig. 9.1, the compressive refrigeration cycle is a scheme for transferring heat from one circulated water system (chilled water) to another (condenser water). This is done by the liquefaction and evaporation of a refrigerant, during which processes it gives off and takes on heat, respectively. The heat it gives off must be disposed of (except in the heat pump), but the heat it acquires is drawn out of the circulated water known as the chilled water, which is the medium for subsequent cooling processes.
Fig. 9.1 Schematic arrangement of the compressive refrigeration cycle, providing chilled water for a building cooling system.
Refrigerants are gases at normal temperatures and pressures, and must be compressed and liquefied to be of service later as heat absorbers. To be liquefied (see Fig. 9.1), the refrigerant must first be compressed to a high-pressure vapor; then, by means of cool water, latent heat is extracted from the refrigerant, which condenses it to a liquid. This high-pressure liquid is a potential heat absorber because, when it is released through an expansion valve, it springs back mechanically to its gaseous form. In this change of state, it must take on latent heat by drawing heat out of the circulated water of the chilled water system.
It may be said that the refrigeration cycle pumps the heat out of the chilled water system into the condenser water system. Indeed, by special (reverse cycle) arrangements of the water systems, a heat pump is the result. (The compressive refrigeration cycle can be used to transfer heat between almost any media; whereas Fig. 9.1 illustrates a water–water cycle, Fig. 9.38 shows the heat pump in both air–air and water–air applications.)
The piston-type compressor in Fig. 9.1 can instead be one of several other types: rotary, scroll, or screw compressors, each with characteristics suitable to particular applications.
(b)       Alternative Refrigerants
Unfortunately, the most commonly used refrigerants of the early 1990s are a threat to our atmosphere, because they can escape from the equipment as chlorofluorocarbon (CFC) gases. The threats are stratospheric ozone depletion and global warming. A related global warming threat is the CO2 released from the production of electricity that powers chillers and other refrigeration machines. There is a potential conflict if lower-threat refrigerants are less efficient and therefore produce higher energy consumption. This conflict assumes continued power generation by fossil fuels; if photovoltaics (PV) are used to power these cooling devices, this aspect of the threat to global warming disappears.
Production of CFC refrigerants was banned in the United States by the mid-1990s. (A black market quickly developed for widely used CFC refrigerants such as Freon.) Fears that replacement refrigerants would be less efficient have largely disappeared; combinations of better chillers and new refrigerants produce energy savings.
Considerable efforts are under way both to reduce the likelihood of escaping refrigerant and to develop more efficient non-CFC refrigerants. The first alternative was hydrochlorofluorocarbon (HCFC) refrigerants—still a threat to our atmosphere, but better than CFC. HCFC still contains chlorine, a major influence on ozone depletion. Thus, HCFCs themselves are due to be phased out in the first decades after 2000. A longer-term replacement is hydrofluorocarbons (HFCs); even this improvement still threatens our atmosphere. Yet another possibility is natural hydrocarbons (HC). In general, comparing these two alternatives, HFCs have higher global warming potential and long atmospheric lifetimes, but they have low toxicity and are nonflammable. HCs have negligible global warming potential and short atmospheric lifetimes, but are flammable and explosive.
One alternative is ammonia, used in the early years of compressive refrigeration but discontinued because of its acute toxicity and (relative to CFCs) its flammability. It is now returning in some absorption cooling cycles. Ammonia does have the advantage of having a strong and unmistakable odor; it warns of its leakage, unlike the other refrigerants. Use of ammonia will require regular maintenance, good ventilation, and good access/escape routes. It will encourage systems that keep the refrigerant loop within controlled spaces such as mechanical rooms.
The search for the ideal refrigerant continues and may never end. This is another area in which rapid developments may be expected in the near future. For a comprehensive review of refrigerants, see Calm (1994).
(c)       Absorption Refrigeration Cycle
This process is illustrated in Fig. 9.2. No CFCs or HCFCs are used here; the process uses distilled water as the refrigerant and lithium bromide (salt solution) as the absorber. In order to remove heat from chilled water, this cycle uses still more heat in regenerating the salt solution. Typically, it is less efficient than the simple compressive cycle and needs about twice the capacity for rejecting heat. However, the heat for salt solution regeneration may be provided by solar energy or by relatively high-temperature waste heat from another source, such as steam or hot water, such as from a fuel cell. Because the high-grade energy (electricity) needed to run a compressor is replaced by the lower-grade heat needed to run the generator, the absorption cycle can enjoy an energy advantage over the compressive cycle, even though it is less efficient.
Fig. 9.2 The steps taken to build a single-effect absorption refrigeration cycle. The refrigeration load from this cycle (step 4) is a building chilled water supply system. The heat source for the generator can be indirect fired (steam, hot water, waste heat, or solar energy) or direct fired (natural gas). (Adapted courtesy of the Carrier Corporation.)
There are several variations on the absorption cycle. When a fuel such as natural gas is used as the heat for the generator, it is called direct fired. When another heat source (such as waste heat) is used, it is called indirect fired. The relatively simple (!) cycle shown in Fig. 9.2 is a single-effect absorption cycle using one heat exchanger between the strong and weak salt solutions. A double-effect absorption cycle (powered by steam in Fig. 9.3) adds a second generator and condenser that operate at a higher temperature. It approximately doubles the efficiency of the single-effect cycle. A triple-effect cycle, in turn, provides a 50% efficiency improvement over the double-effect cycle.
Fig. 9.3 Indirect-fired double-effect absorption cycle. Lithium bromide and water are used in this cycle. (Courtesy of the Carrier Corporation.)

NOTE: Adapted from HVAC Components and Systems, Walter Grondzik and Richard Furst
11.6  SOURCES OF ENERGY
Four distinctly different types of heat sources are employed in buildings. Heat may be generated by the combustion of some flammable material (a fuel) such as coal or natural gas. Electricity may be converted to heat through the process of electric resistance. Solar radiation or other renewable energy resources may be collected on site and converted to heat. Heat may be removed from some material on site and transferred into a building. All four of these fundamental heat sources find common use in all scales of buildings. The choice of a heat source for a given building situation is usually based upon source availability, required system capacity, and equipment and fuel costs.

Cooling is technically just a reverse heat flow, the flow of heat into a sink, rather than from a source. Sources of cooling (heat sinks) are not always readily available. The term “coolth” is sometimes used to identify the product of heat flowing to a sink. Identification of coolth sources is actually the identification of available heat sinks. Heat sinks are either naturally occurring environmental phenomena or artificially induced phenomena. Naturally occurring heat sinks, include outside air, its sensible (temperature) and latent (humidity) conditions, the night sky, on-site water bodies, and on-site soil. Use of such natural sinks is the basis for passive cooling systems. Active system heat sinks are artificially established through the operation of some type of refrigeration device. The choice of a coolth source is usually based upon resource availability, energy and equipment costs, and appropriateness to the building context.

11.7  HVAC COMPONENTS

HVAC system components may be grouped into three functional categories: production/motion components, distribution components, and delivery components. (Table 10.1) Source components provide or remove heat or moisture. Distribution components convey a heating or cooling medium from a source location to portions of a building that require conditioning. Delivery components serve as an interface between the distribution system and occupied spaces.

(a)       PRODUCTION COMPONENTS

Fireplaces

Fireplaces are simple on-site combustion used for producing heat. A typical fireplace consists of a niche or well constructed of non-combustible materials that will withstand the temperatures generated during the combustion process. Although freestanding fireplaces are sometimes used, most fireplaces are installed in exterior walls. By its design and scale a fireplace functions as a passive heating system, directly providing heat for a limited area of a building often totally by natural radiation and convection. Adding fans to circulate heated air can move a fireplace into the realm of hybrid or active systems and increase efficiency.
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Wood Stoves

Wood stoves are sophisticated on-site combustion devices, normally self-contained and freestanding, that provide higher efficiency than fireplaces. Tight control of combustion air permits more substantially complete combustion, resulting in improved resource utilization and less potential for problems from infiltration.
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Furnaces

Furnaces heat air for distribution to various building spaces. A furnace is a packaged assembly of components that includes a heat-source element (burner or coil), a fan (for central units), and an air filter. A burner consists of an arrangement of nozzles that permits the efficient combustion of liquid or gaseous fuels by providing good mixing between the fuel and the oxygen necessary for combustion. A coil consists of a series of heat exchange surfaces that are either the heat source (electric resistance coils) or provide close thermal contact with a heat distribution medium (hot water or steam coils in air handling units or fan-coils).
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Portable Heaters

Portable heaters are normally occupant selected and “installed”, often to supplement conditions provided by another (presumably less than successful) heating system. Portable heaters are designed to operate as local systems serving a fairly small area. Room air heated by the resistance element rises and is replaced by cooler room air, establishing a continuous convective flow of warm air while in operation.
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Boilers

Boilers are heating system components designed to heat water for distribution to various building spaces. As water cannot be used to directly heat a space, boilers are only used in central systems where hot water is circulated to delivery devices (such as baseboard radiators, unit heaters, convectors, or air-handling units). Heat transfer systems (heat pumps) likewise may serve as a substitute for a boiler.
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Heat Pump

Heat pumps are reversible cycle vapor compression refrigeration unit. Through the addition of a special control valve, heat flow in a mechanical refrigeration loop can be reversed so that heat is extracted from the outside air (or ground water or soil) and rejected into a building. The purpose of a conventional refrigeration cycle is to establish heat flow in the opposite direction (from cool to warm).
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Solar Thermal Collector

Solar collectors may be used to heat air or water for building heating purposes. Water-heating collectors may replace or supplement a boiler in a water-based heating system. Air-heating collectors may replace or supplement a furnace.
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Refrigeration Unit

Two refrigeration units are employed for the cooling process. The vapor compression refrigeration cycle induces heat to move in a direction contrary to gross environmental temperature differences through a series of artificially maintained temperature and pressure conditions in a heat transfer fluid (refrigerant). The refrigeration system induces heat to flow from inside a cooler building to a warmer outside environment. The basic concept behind an absorption refrigeration unit is the same as that for a vapor compression unit, but the difference is water acts as the refrigerant, which is circulated between a generator, a condenser, an evaporator, and an absorber. The driving force in the absorption refrigeration cycle is chemical, as opposed to the mechanical driving force in a vapor compression unit.
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Evaporative Coolers

In hot dry climates, cooling effect may be obtained from the evaporative cooling process. Dry air is pulled into the evaporative cooler by a fan. The dry air is passed through some porous media that is wetted with water. As the air contacts the water spread over the media, much of the water evaporates. The energy required to evaporate the water comes from the air. As the air passes through the cooling unit it is humidified – but also cooled.
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DX Systems

In DX (direct expansion) systems the evaporator of the refrigeration cycle (in the form of a cooling coil) is placed in an air handling unit so that the room cooling effect is produced directly by room air flowing across the evaporator. Window air-conditioners, unitary or through-the-wall air conditioners, rooftop package units, and split systems are typically DX systems. A limitation of approximately 100 feet for maximum separation between compressor and evaporator applies to all DX system installations.
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Air-Cooled Condensers

Air-cooled condensers are heat rejection device, installed outside of the building envelope, through which refrigerant is circulated. As the refrigerant comes into indirect contact with outside air, heat is exchanged from the relatively hot refrigerant to the relatively cooler air. Heat exchange is enhanced by fan-forced flow of large volumes of air across the heat exchange coils.
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Chiller

Chillers are a refrigeration unit designed to produce cool (chilled) water for space cooling purposes. The chilled water is then circulated to one or more cooling coils located in air handling units, fan-coils, or induction units. Chilled water cooling systems are typically used in larger buildings. Capacity control in a chilled water system is usually achieved through modulation of water flow through the coils; thus, multiple coils may be served from a single chiller without compromising control of any individual unit. Chillers may operate on either the vapor compression principle or the absorption principle.
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Cooling Towers

Cooling towers are a heat rejection device, installed outside of the building envelope, through which condenser water is circulated. Refrigerant in the refrigeration cycle is condensed in a refrigerant-to-water heat exchanger. Heat rejected from the refrigerant increases the temperature of the condenser water, which must be cooled to permit the cycle to continue. The condenser water is circulated to the cooling tower where evaporative cooling causes heat to be removed from the water and added to the outside air. The cooled condenser water is then piped back to the condenser of the chiller.
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(b)       DISTRIBUTION COMPONENTS

The distribution components convey the heating or cooling effect from the source to the conditioned locations. The transmission of heating and cooling effect are air, water, and steam. Hot air or water and steam can be used as a heating medium, cold air or water as a cooling medium.

In a water-based central system, pipes are used to convey water from the source to the final delivery components. A minimum of two pipes is necessary, one for supply water and one for return water, to establish a distribution loop. When both heating and cooling are required in a building, 3-pipe and 4-pipe distribution systems may be used to increase system flexibility. A 3-pipe system has two supply pipes (hot and cold water) and a single return. A 4-pipe distribution system has two supply pipes and two separate return pipes (hot and cold). The 4-pipe arrangement provides the greatest control flexibility in the most energy-efficient manner. Several piping materials are used in HVAC distribution systems. Steel pipes are the most common, although copper is used for economy or environmental conditions. Hot and cold (chilled) water pipes are normally insulated. Valves are used to control water flow as a means of adjusting system heating or cooling capacity to the demands of the building thermal zones. Valves are also used to shut off water flow so that equipment may be maintained. A range of gauges is used to balance system flows and verify temperature and pressure conditions. A pump is used to provide the energy input required to overcome friction losses and circulate water through a system.

In an air-based central system, ducts are used to convey air from a primary or secondary source to the final delivery components. Typically, two duct paths are necessary, one for supply air and one for return air. Air distribution loops often recirculate as much indoor air as possible, as it is more economical to heat or cool return air than outdoor air. Ducts are classified as either high-pressure or low-pressure systems and as high-velocity or low-velocity systems, depending on their static pressure and air speed design parameters, respectively. Supply ductwork will usually be designed to operate at low-velocity and low-pressure. Increasing air flow velocity allows the use of smaller duct cross sections, which may be necessary in buildings with constricted distribution spaces. Higher pressures are required as the pressure loss in the distribution system increases; a long distribution path or the use of system powered terminal devices may necessitate increased distribution pressures. Increasing distribution system pressure, however, increases HVAC system energy consumption. Return ductwork is usually low-velocity and low-pressure. Several materials are commonly used to construct ducts. Sheet metal (galvanized steel) is probably the most common material. Glass fiber insulation board, which provides containment and insulation in a single material, is also a common duct material for low-pressure systems. Flexible ducts, comprised of plastic wrap over a spiral metal framework, are often used to connect terminal or delivery devices to main distribution ducts. Duct shapes include square or rectangular, circular, and flat oval cross sections. A circular cross section is most economical with respect to material and friction losses. A rectangular cross section, however, is often more likely to fit in the types of spaces available for duct placement. Supply ducts are insulated to reduce heat gain from unconditioned spaces and warm plenums through which they may be routed. Return air ductwork is normally uninsulated. Accessories found in many duct distribution systems include dampers, splitters, and turning vanes.

Dampers are used to control air flow, either to balance flows throughout a system or to adjust air flow in response to changing building loads. Specialized fire dampers and smoke dampers are used to reduce the spread of fire and smoke through the building air distribution system.

Splitters and turning vanes are used to reduce friction losses by reducing turbulence within the ductwork; they also can reduce noise generated within the ducts. As with water, air flowing through a duct system will encounter friction losses through contact with the duct walls and in passing through devices such as dampers, diffusers, filters, and coils.

A fan is used to provide the energy input required to overcome friction losses and circulate air through a system. The typical central HVAC system may require the use of several fans: for supply air, for return air, and for exhaust air. Fans come in a variety of capacities and designs, including centrifugal and axial. Fans are normally driven by electric motors.

Air handling units are equipment packages that house several major components necessary for the operation of air-based central HVAC systems. An air handler consists of a sheet metal enclosure, a fan, a heating coil or heat source and/or a cooling coil (as required), an air filter, occasionally a humidifier, and necessary control devices. The fan provides the motive energy for air circulation. A filter is provided to remove indoor pollutants from the air stream.

The heating or cooling coil, act as secondary sources receiving heating or cooling media from a boiler or a chiller and transferring the conditioning effect to the air stream. Electric resistance coils may also be used as a heat source. On-site combustion at the air handling unit (typically a gas burner) serves as a common heat source for rooftop air handling units. A humidifier may be required to add moisture to the air under certain conditions. Dehumidification (moisture removal) is accomplished through the cooling coil. Control devices such as mixing dampers and valves are often part of an air handling unit.
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(c)        DELIVERY COMPONENTS

The heating or cooling effect produced at a source and distributed by a central system to spaces throughout a building needs to be properly delivered to each space to promote comfort. Some means of transferring the conditioning effect from the media to the space is required.

Diffusers

Diffusers are a device designed specifically to introduce supply air into a space, to provide good mixing of the supply air with the room air, to minimize drafts that would discomfort occupants, and to integrate with the ceiling system being used in the space in question. Diffusers are intended for ceiling installation and are available in many shapes, sizes, styles, finishes, and capacities. In many buildings, the only portions of an HVAC system seen by occupants on a day-by-day basis are the supply diffusers and return air registers or grilles.
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Registers

Registers are similar to diffusers except that they are designed and used for floor or sidewall air supply applications or as return air inlets.
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Grilles

Grilles are simply decorative covers for return air inlets; they are used to block sightlines so that occupants cannot see directly into return air openings.
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Baseboard Radiators

Hydronic baseboard radiators may be used as the delivery device in a hot water or steam heating system. Hydronic baseboard units are similar in general appearance to electric resistance baseboard units. Finned tube heat exchange elements transfer heat from the hot water distribution system to the room air. Baseboard radiators induce natural convection as an important means of heat distribution within a space, with warmer air exiting at the top of the unit and cooler air entering at the bottom.
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Convectors

Convectors are basically high capacity heat exchange elements consisting of one or more finned-tube heat exchange elements, housing, and possibly a fan. Convectors are used in steam or water central heating systems to provide high capacity heat delivery.
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Unit Heaters

Unit heaters are industrial style heat delivery devices, consisting of a fan and coil packaged in housing, used in water or steam central heating systems.
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Radiant Panels

It is possible to embed pipes in wall or floor constructions to develop a radiant heat delivery approach for steam or water central heating systems. Radiant heat delivery is generally considered to provide an exceptionally comfortable environment. Packaged electric resistance radiant panels are also available; such units would normally be used to provide supplemental heating for a localized area of a building. Electric resistance cables can also be used with gypsum board construction to provide large-area radiant heating systems.
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Valance Units

Occasionally, retrofit space cooling installations are required in buildings with no floor space for equipment in the conditioned zones and no room for duct routing. Valance units are finned-tube heat exchangers installed high on a wall near the intersection with the ceiling that are designed for use with an all-water cooling system.
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Heat Recovery Devices

A number of heat recovery approaches may be used to reduce energy consumption in buildings. Common heat recovery devices include heat wheels, run-around coils, and heat pipes. The purpose of heat recovery devices are to capture some of the energy contained in air about to be exhausted from a building, normally so that the heat may be used to pre-heat incoming ventilation air. A similar approach may be used in hot climates to pre-cool ventilation air. Some types of heat recovery equipment can transfer both sensible and latent energy.
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11.8  HVAC SYSTEM TYPES AND SMALL BUILDINGS

Smaller buildings are typically skin load dominated: that is, for them, the climate dictates whether heating or cooling is the major concern. In some climates, only heating systems are needed; the building can “keep itself cool” during hot weather without mechanical assistance. In other climates, only cooling is needed. In still others, both heating and cooling are required.
(a)       HEATING ONLY SYSTEMS AND DISTRIBUTION
There are substantial areas of North America in which the summers are so mild, but winters so cold, that heating systems are installed but not cooling systems. This is most common for residences and small commercial buildings.
Where heating devices should be placed within a space? The relative comfort in heated rooms from entire ceilings or floors as a heat source is discussed in Chapter 4. Figure 4.10 shows dissatisfaction with increased air velocity (the warmer the air, the higher the velocity tolerated), Fig. 4.11 shows dissatisfaction with asymmetric radiation (warmer ceilings were less tolerated), and Fig. 4.12 shows dissatisfaction with vertical air temperature differences (cold floors were less tolerated). Figure 9.12 shows why designers usually locate heat sources below windows, despite the fact that warmer temperatures just inside an exterior wall will drive more heat through the wall in cold weather (heat loss = U ´ A ´ t). As windows and walls become better insulated, their interior surface temperatures rise, and the need for heat at the edge grows less. Indeed, with super insulated components, the need for space heat disappears because internal gains from the sun, lights, appliances, and occupants can heat the space.
Fig. 9.12 Locating a heat source near an interior wall (a) encourages a cold draft along the floor in winter. Below a window (b) it evens the temperature throughout the room but also causes more heat loss through the window. 
(i)        Wood Heating Devices
After the sun, the most ancient method of heating is the radiant effect of fire. With each step from campfire to fireplace to wood stove, more of the fuel’s heat was captured for the room rather than wasted to the outdoors (Fig. 9.13). Although many people enjoy the sight, sound, and smell of the open fireplace, the tightly enclosed wood stove with a catalytic combustor is a substantially more efficient and less polluting approach to heating. Indoor as well as outdoor air pollution is a serious issue with fireplaces and stoves. Combustion generates carbon monoxide, breathable particulates, and, at times, nitrogen dioxide. Wood smoke can cause nose and throat irritation; it can remain in the lungs, and it can trigger asthmatic attacks. Keeping a clean chimney, burning small, hot fires rather than large, smoky ones, using seasoned wood, and ensuring adequate ventilation to the wood-burning device are strategies to minimize pollution and risk.
Fig. 9.13 Wood-burning devices have substantially increased in efficiency since the time of the open fireplace. (Copyright © 1978 by Alternative Sources of Energy, Issue 35.)
Open fireplaces may be lovely to look at, but the amount of air exhausted up the chimney can quickly cause more heat losses than heat gained from the fire. The colder the outside air, the greater the net heat loss. Masonry mass around fireplaces can store and release some heat; for energy conservation, the mass should be surrounded by the building rather than located on an exterior wall. ANSI/ASHRAE Standard 90.2 requires fireplaces to have a tight-fitting damper, firebox doors, and a source of outside combustion air within the firebox.
Wood stoves are available in a wide variety of styles and are made of several materials. The sizes of such stoves are often difficult to determine. Manufacturers rarely specify the Btu/h output, which depends on the density, moisture content, and burn time of the wood fuel. Wood that has been split, loosely stacked, and covered from rain for at least 6 months should achieve a moisture content of about 20% by weight. The following sizing procedure assumes no more than this 20% moisture content. For more details, see Issue 35 of Alternative Sources of Energy Magazine (1978).
The formula for the hourly heat output to a room from a wood stove is
Btu/h = (V)(E)(D)(7000)\T
where
V = useful (loadable) volume of the stove (ft3)
E = percent efficiency, expressed as a decimal (1.0); see Fig. 9.13
D = density of the wood fuel (Table 9.2)
T = burn time (hours) for a complete load of firewood; usually assumed at an 8-hour minimum
7000 = Btu/lb of firewood, 20% moisture content
TABLE 9.2 Approximate Average Wood Density

aThese values are approximate; they vary a great deal. SI units added by the authors of this book.

“Bone-dry” wood can be assumed to have 8600 Btu/lb (20,000 kJ/kg). Note that a drop in burn time increases the heat output; it is evident that when the air supply is increased to the stove, the fire burns hotter, consuming the wood more quickly. To meet the design heat loss (worst condition) for a room, burn times of 8 to 10 hours should be assumed. Stoves rarely need relighting with a 10-hour burn time.
Pellet stoves were introduced in 1984 and have several desirable characteristics. The pellets are made from densified quality sawdust, a manufacturing by-product. The form and content of this fuel produce a highly efficient burn with less pollution emitted. The fuel is cleaner and takes less storage space than cordwood; an electric auger automatically feeds fuel into the burnplace to maintain a fire. From 10,000 to 50,000 Btu/h (2930 to 14,650 W) are produced, depending on the model and operating settings.
Wood stoves are frequently used as the sole mechanical heat source for an entire building, such as a residence or a small commercial building that is passively solar heated. Because radiant heat is the dominant form of heat output, the areas that “see” the stove get most of the benefit. However, circulating stoves convert a larger portion of their heat to convected heat, which produces a layer of hot air at ceiling level. By providing a path between rooms at the ceiling, this hot air will slowly spread throughout a building; it also easily finds its way upstairs, because warm air rises. The more thermally massive the ceiling construction, the longer it will store and reradiate the heat from this warm air mass.
The flue leading from a wood stove carries very hot gases that are a potential source of heat (and pollution). The flue can be exposed to a space, making its radiant heat available, or simple heat exchangers can be constructed (such as the preheating of domestic hot water).
Catalytic combustors, a recent development, reduce the air pollution from wood burning. These devices are honeycomb-shaped, chemically treated disks as much as 6 in. in diameter and 3 in. thick (150 mm in diameter, 75 mm thick). They are either inserted in the flue or built into the stove itself. When wood smoke passes through the combustor, it reacts with the chemical and ignites at a much lower temperature; this causes gases to burn that otherwise would have gone up the flue. The result is more heat produced, less creosote buildup in the flue, and fewer pollutants in the atmosphere. Like the catalytic converters in autos, these devices impose limits on the fuel: plastic, colored newsprint, metallic substances, and sulfur are ruinous to combustors, which mean that the stove must be used as a wood burner, not a trash incinerator.
Wood stoves have a larger impact on building design than do most other heating devices. Either noncombustible materials must be placed below and around them or minimum clearance to ordinary combustible building materials must be provided. Furniture arrangements and circulation paths must be designed with the very hot stove surfaces in mind. Hot spots occur near the stove; cold spots occur whenever visual access to the stove is blocked. Thermally massive materials near the stove are advantageous in leveling the large temperature swings that can accompany the on–off cycle of the stove; this affinity for thermal mass has made the wood stove a popular choice for backup heat in passively solar-heated buildings. Finally, the amount of space required for wood storage should not be overlooked; recall the impact of the wood storage space on the house shown in Fig. 2.2. A covered, well-ventilated, easily accessible, and quite large space is optimum.
Masonry heaters overcome many of the metal wood stoves’ disadvantages. Their footprint is rather small compared to their height; typically, they are used to heat the entire building (such as a residence). An inner vertical firebox supports a hot, clean burn, resulting in efficient combustion; combustion gases then flow downward in outer chambers, transferring heat to exterior masonry surfaces. In Finnish masonry heaters, this is termed contraflow (Fig. 9.14). Cool air at the floor of the room flows upward as it is heated by contact with this masonry; the temperature difference between masonry and air remains fairly constant, with the highest temperatures at the top. Heat is gentle and even; dangerously hot surfaces are avoided. Fires may be built at 6:00 p.m., and combustion is completed by a family’s bedtime; the heat continues to radiate all night, but no fire is burning while people sleep. Research at Finland’s Tampere University of Technology has resulted in optimum masonry heater designs, described in Barden and Hyytiainen (1993).
Fig. 9.14 The Finnish contraflow masonry heater.
(ii)       Electric Resistance Heaters
These common devices carry the disadvantage of using high-grade energy to do a low-grade task, as shown in Fig. 2.6. Their advantages, however, are impressive: low first cost and individual thermostatic control that can easily be used to make each room a separate heating zone. Thus, the energy wasted at the electricity-generating plant (usually 60% to 70%) can be partially “recovered” at the building, where unused rooms can remain unheated. A few of the many types of electric resistance heaters are shown in Fig. 9.15. As in the case of metal wood stoves, surfaces can sometimes reach high temperatures, requiring care in the location of heaters relative to furniture placement, draperies, and traffic flow. Electric heaters are sized by their capacity in kilowatts (1 kW  3413 Btu/h). The maximum watt density allowed is 250 W per linear foot of heater (820 W per linear meter).
Fig. 9.15 Varieties of electric resistance heating units. (Reprinted by permission from AIA: Ramsey/Sleeper, Architectural Graphic Standards, 10th ed.; © 2000 by John Wiley & Sons, Inc.)
(iii)      Gas-Fired Heaters
These are often found in semioutdoor locations such as loading docks and repair shops. They are fired with either natural gas or propane. When vented, they can be used in more traditional environments such as the retail store in a remodeled warehouse (Fig. 9.16). Their advantage is that they heat surfaces first rather than air, so that comfort is obtained without the need for high air temperatures. When high rates of air exchange are expected, high-intensity radiant heaters are often used.
Fig. 9.16 A high-intensity infrared heater adds to the historic atmosphere of this retail store in an old warehouse (a). Exposed mechanical equipment includes the chains that operate the clerestory windows. The vented gas heater has an adjustable reflector that enables its radiant heat to be directed. (Clark-Ditton Architects, Eugene, OR.) (b) Vented gas high-intensity infrared heaters are available in both straight-line and U-shaped units. (Courtesy of Solaronics, Inc., Rochester, MI.)
Radiant heaters should be sized by the surface temperature change they produce. Many manufacturers specify this surface ∆t for specified mounting heights and angles relative to the surface to be heated.
Gas-fired baseboard heaters are also available, using either natural gas or propane. They heat by both convection and radiation, as do their electric counterparts. At a steady-state efficiency of 80%, they use a lower-grade resource (than electricity) to do this lower-grade task. They are direct vented (using a built-in fan) to the outside, and therefore must be installed on or near an exterior wall; vents are 1 1/2 in. in diameter, with a maximum length of 19 in. (38 mm in diameter, 483 mm in length). With a cross section of 9 in. high by 5 in. deep (230 mm X 130 mm), a 48-in. (1220-mm) length will deliver 5800 Btu/h (1700 W); a 72-in. (1830-mm) length will deliver 9400 Btu/h (2755 W).
(iv)      Ceiling Electric Resistance Heat
Ceilings can be constructed to include electric resistance in wiring (Fig. 9.17). Because the ceiling is not touched, it can be safely heated to a rather high temperature. The primary disadvantage of ceiling heat is that hot air stratifies just below the ceiling, so that air motion is discouraged; remember also that Figs. 4.11 and 4.12 predict more discomfort with the warmer ceiling. Finally, the wires are hidden within the ceiling surface, and unwary occupants can puncture wires while installing hooks or additional light fixtures.
Fig. 9.17 Radiant heating electrical cable is installed prior to completion of a plaster ceiling.
(v)       Hot Water Boilers
The remaining choices for whole-building heating systems are discussed in the following order: (1) hot water and (2) forced air. Such systems include a fuel, a heat source, a “mover” (such as a pump or fan), a distribution system, a heat exchanger or terminal within the space, and a control system.
Hot water boilers are rated according to heating capacity by several different categories. Heating capacity is the rate of useful heat output with the boiler operating under steady-state conditions, often expressed in MBh (1000 Btu/h). This “useful heat” assumes that the boiler is within the heated envelope of the building; thus, the heat that escapes from the boiler walls is available to help heat the building. AFUE, the annual fuel utilization efficiency, is defined as 100% minus the losses up the stack during both the on and off cycles, and the losses due to infiltration of outdoor air to replace the air used for combustion and for draft control. Finally, the net I  B  R rating (a designation of the Institute of Boiler and Radiator Manufacturers) is published by the Hydronics Institute Division of the Gas Appliance Manufacturers Association (GAMA). The net I  B  R rating load is lower than the heating capacity rating, because it consists only of the heating to be delivered to the spaces and excludes the heat loss of the boiler itself.
Select a boiler whose rating matches the calculated critical heat loss of the house or building; too small a boiler results in lower indoor temperatures at design conditions; too large a boiler costs more and is a waste of space. When using AFUE to select an efficient boiler, take care to see that the assumptions about “inches of water draft” and percentage CO2 are similar for the boilers being compared. Minimum AFUEs are specified both in ANSI/ASHRAE Standard 90.2, Energy-Efficient Design of Low-Rise Residential Buildings, and in ANSI/ASHRAE/IESNA 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings.
Boilers and their accessories comprise a wide inventory. A few selected types are discussed:
1.         Oil-fired steel boiler. A refractory chamber receives the hot flame of the oil fire. Combustion continues within the chamber and the fire tubes. Smoke leaves through the breeching at the rear. Water, outside the chamber, receives the heat generated in the combustion chamber. If a domestic hot water coil is connected for use, a larger-capacity boiler is selected. An aquastat (water thermostat) turns on the burner whenever the boiler water cools off, thereby maintaining a reservoir of hot water ready for heating the building.
2.         Gas-fired cast-iron hot water boiler (Fig. 9.18). Cast-iron sections contain water that is heated by hot gases rising through these sections. Output is related to the number of sections. Additional heat is gained from a heat extractor in the flue. With induced draft combustion, condensing unit in the flue, and intermittent electronic ignition instead of a pilot light, up to 90% AFUE is attainable. The American Gas Association (AGA) sets standards for gas-fired equipment.
3.         Oil-fired, cast-iron hot water boiler. Primary and secondary air for combustion may be regulated at the burner unit. Flame enters the refractory chamber and continues around the outside of the water-filled cast-iron sections.
Fig. 9.18 Gas-fired cast-iron sectional boiler for hot water heating. Very high operating efficiency is possible; no chimney is required, as lower-temperature exhaust gases can be vented through the wall to the exterior.
(vi)      Hot Water Baseboard and Radiator Systems
Hot water heating circuits that serve baseboards or radiators come in four principal arrangements. Figure 9.19a shows the series loop system, usually run at the building’s perimeter. The water flows to and through each baseboard or fintube in turn. Obviously, the water at the end of the circuit is a little cooler, but because in all hot water systems the water temperature drop seldom exceeds 20Fº (11Cº) in residences, the average temperature can usually be used to select the baseboard or other elements. Valves at each heating element are not possible, because any valve would shut off the entire loop. Adjustment is by a damper at each baseboard, which reduces the natural convection of air over the fins. This is a one-zone system—all elements on, or off, together. There is no general rule about the maximum allowable length of a water circuit, but for long runs, the pipe size can be increased or several loops used in parallel to create more than one thermal zone.
Fig. 9.19 Diagrams of hydronic distribution options (a, b, c, d), seen in the plan. Baseboard convectors are shown here. Controls are not shown.
The one-pipe system shown in Figs. 9.19b and 9.20 is a very popular choice. Special fittings act to divert part of the flow into each baseboard. A valve may be used at each one to allow for reduced heat or for a complete shutoff to conserve energy—an advantage that the loop system does not provide. The one-pipe system uses a little more piping and thus is not as economical to install as the loop system, in which piping is minimal. Again, the supply water temperature will be lower at the end of the run than at the beginning.
Fig. 9.20 One-pipe hydronic system.
The two-pipe reverse–return shown in Fig. 9.19c provides the same supply water temperature to each baseboard or radiator, because it is not cooled either by passing through a previous baseboard or accepting the cooler return water. Equal friction, resulting in equal flow, is achieved through all baseboards (numbers 1 to 5) by reversing the return instead of running it directly back to the boiler. This equality is effected by equal lengths of water flow through any baseboard together with its lengths of supply-and-return main. More pipe is required for this system than for the systems shown in Fig. 9.19a or 9.19b.
Figure 9.19d shows an arrangement that is not usually favored because the path of water through baseboard number 1 is much shorter than that through the others, especially number 5. Baseboard number 5 could easily be undesirably cool, because it is short-circuited by the others.
Pipe expansion requires expansion joints in long runs of pipe and clearance around all pipes passing through walls and floors. Each time a hydronic system changes from room temperature to a heated condition, the piping will undergo the following expansion, assuming a 70ºF (21ºC) initial temperature:

An air cushion tank, compression tank, or expansion tank is a closed tank containing air, usually located above the boiler. When the water in the system is heated, it expands, compressing the air trapped in the tank. This tank allows for the usual range of temperatures within the system, including temperatures above the usual boiling point of water, without the frequent opening of the pressure relief valve. One type of air cushion tank, called a diaphragm tank, separates the air and water with an inert, flexible material; this prevents reabsorption of the air by the water.
For a conventional (unpressurized) air cushion tank, allow 1 gallon of tank capacity for every 5000 Btu/h (1 L for every 385 W) of the total system heat loss. For a pressurized tank of at least 8 lb/in2 (55 kPa), allow 1 gallon of tank capacity for each 7000 Btu/h (1 L for each 540 W), or see the manufacturer’s recommendations.
Air vents and water drains are part of the distribution system. Except for the necessary air cushion in the upper part of the compression tank above the boiler, air must not be allowed to accumulate at high points in the piping or at the convector branches. Air vents at all high points relieve these possible air pockets that would otherwise make the system air-bound and inoperative.
If a system is drained and left idle in a cold house, water trapped in low points can freeze and burst the tubing or fittings. Operable drain valves must be provided at such locations and, of course, at the bottom of the boiler, as shown in Fig. 9.21.
Fig. 9.21 An oil-fired boiler and its hydronic and electrical controls.
Hydronic and electrical controls allow automatic operation, described in Fig. 9.21. There are two options for system control:
1.         As in Fig. 9.21, the thermostat controls the circulating pump and the boiler. In colder weather, the system operates almost continuously, and the average temperature in the system gradually rises.
2.         The thermostat controls only the boiler, and the circulating pump operates continuously. This uses more energy for the pump but minimizes system temperature variation and thus the possibility of expansion noises.

Makeup water is added as required, the air level in the tank is regulated by the air control fittings, and the circulator and burner operate as controlled by the aquastat and thermostat. If air vents in the piping are not automatic, they will require periodic manual “bleeding” of unwanted air.
Circulating pumps are used to overcome the friction of flow in the piping and fittings and to deliver water at a rate sufficient to offset the hourly heat loss of the house or building.
Pipe insulation is required whenever the pipes are outside the heated envelope of the building; ANSI/ASHRAE Standard 90.2 specifies insulation appropriate to the temperature of the heated water.
(vii)     Radiant Panels
Radiant floors have several comfort advantages over radiant ceilings (see again the introduction to Section 9.7). The components are largely the same as in the baseboard/radiator systems, except that now whole coils of pipes replace the individual radiators and baseboards. A balancing valve should be installed on each coil. Where uninsulated spaces underlie floors (or are above ceilings), special attention should be paid to adequate insulation, because the radiant panel will generate especially high temperatures, and thus a very high ∆t through the floor (or ceiling).
Thanks to higher insulation, today’s buildings often require panels smaller in area than the floor (or ceiling) area available. In a conventional radiant panel system, the panel is placed nearest the exterior walls, where the heat loss is greatest. In a solar-heated building, this is not so clear. If the panel heats the floor surface just inside south-facing glass, how much warming will be left to the sun? A preheated slab will absorb much less solar radiation. However, in cloudy cold weather, the area closest to the south glass could become uncomfortably cool.
In the past, copper tubing was widely used for the heating coils. This typically involved a number of connections within a coil; each represented a potential point of failure over the life of the panel. Today, coils are typically one-piece and are made of synthetic materials such as cross-linked polyethylene tubing. When the floor is concrete or other cast-in-place material, the coil is either directly embedded in the slab (tied down to resist floating during the pour) or stapled to an underfloor, over which the slab is poured. Radiant floors with coils underneath wood floors are increasingly popular.
Rugs or carpets over radiant floors are a mixed blessing; they are soft on the feet but interfere with the exchange of heat. Special undercarpet pads can facilitate heat transfer; higher water temperatures can be used in the coils, because skin contact with the floor is prevented by the carpet.
(viii)    Hydronic Heating Sizing
The calculations for the sizing of a water distribution system are based upon the required flow and the friction in the piping. (For a domestic water supply, another factor is the vertical distance the water must be raised. In these closed loop heating systems, however, the weight of the cooler water falling back to the boiler essentially counterbalances the weight of the hot water being raised. Furthermore, gravity helps the hot—lighter—water rise and the cooler water fall.)
The key to pipe sizing is the overall required flow rate. Ordinarily, the temperature drop that occurs as the hot supply water gives up heat to the space (through the convector) is about 20Fº [11Cº] in residences; in commercial applications, 30, 40, or 50Fº (17, 22, or 28Cº) temperature drops are also common, as recommended by the manufacturers of unit heaters and convectors. Because the entire building’s design heat loss is overcome by this system, in I-P units, total flow rate, gpm
= design heat loss, Btu/h\20Fº ´ 60 min/h ´ 8 lb/gal ´ 1 Btu/lb F
= design heat loss\9600
In SI units, total flow rate, L/s
= design heat loss, W\11Cº ´ 1 kg/L ´ 4180 W sec/kg Cº
= design heat loss\45,980

Then, using Section 21.11, we can account for friction through piping and fittings and can size the main supply and return pipes. The same procedure can be applied to branches, proportioned to the heat they must deliver. The total friction to be overcome in the most distant run is then converted from total psi to feet of head, with each foot of head  0.433 psi (the pressure exerted by a foot-high column of water). In SI, 1 ft of water  2.99 kPa.
With both the friction of the system expressed in feet of water, or head, and the flow rate established, a pump can be selected. Figure 9.22 shows typical performance curves for four pumps. The designer enters the curve with a desired flow rate, then selects the pump with a head capacity greater than or equal to the head required. Pump performance curves are provided by the manufacturer.
Fig. 9.22 Typical pump capacity (or performance) curves for four pumps used in hydronic heating systems. One foot of water  2.99 kPa; 1 gpm  0.0631 L/s. (Courtesy of the Hydronics Institute Division of GAMA, Berkeley Heights, NJ.)
The critical choice, however, is not pipe size. It is relatively easy to distribute such small-diameter pipes within wall and floor/ceiling construction. Rather, the critical choice is the hot-water supply temperature: the higher this temperature, the smaller the convector units that discharge the heat to each space. However, higher temperatures endanger occupants, who may suffer skin burns if they touch exposed parts of the convectors or the distribution tree. Higher temperatures also can lead to steam within the boiler/distribution tree, although the system is under pressure and therefore the boiling point is greater than 212ºF (100ºC). These systems are not designed to accommodate steam, and serious injury can sometimes result. A safer choice of average water distribution temperature is 180ºF, even though this temperature results in larger convectors. A slightly lower annual average energy consumption should accompany the lower distribution temperature.
Baseboard convector selection is then made from the manufacturer’s data tables, such as the one shown in Table 9.3. The two common baseboard types are RC, usually cast iron with a water-backed front surface and an extended rear heating surface, and finned tube, a metal tube with an extended surface in the form of fins, usually placed behind a metal enclosure.
TABLE 9.3 Hydronic Baseboard Convectors

aUse flow rate of 0.6 L/s (1 gpm) unless flow rate is known to be 0.25 L/s (4 gpm).
Pressure drop is as follows: flow 0.6 L/s (1 gpm) = 3.76 mm/m (47 mil.in./ft).
flow 0.25 L/s (4 gpm) = 42 mm/m (525 mil.in./ft).
bBaseboard finished length will be 76 mm (3 in.) longer than the length required to meet the heating need.

Panel (radiant ceiling or floor) heating design usually depends on a water temperature of 120ºF (49ºC) for heated floors and 140ºF (60ºC) for heated ceilings. An uncarpeted concrete floor slab using 3/4-in.-diameter pipe or tube on 12-in. centers (20-mm diameter on 300-mm centers), with an average water temperature of 120ºF, will deliver 50 Btu/h ft2 (158 W/m2) of floor panel.
A ceiling panel with nominal 3/8-in. tube on 6-in. centers (10-mm diameter on 150-mm centers), with an average water temperature of 140ºF, will deliver 60 Btu/h ft2 (189 W/m2) of ceiling panel.
To determine the area of heated panel, divide the room’s design heat loss by the rate of panel heat delivery. Although radiant ceilings deliver more heat per unit area, they also discourage air motion because the warmest air rises to lie against the warmest surface. In contrast, at a radiant floor the coolest air drops to contact the warmest surface, is then warmed, and rises to be cooled at the ceiling, drops to the floor, and repeats this cycle continually.
Each panel contains one or more coils. In floor panels, each coil delivers 10,000 Btu/h (2930 W) and should be no longer than about 200 linear feet (60 m). In ceiling panels (with smaller-diameter tubes), each coil delivers 3000 Btu/h (880 W) and should be no longer than about 100 linear feet (30 m).
Sizing of the main pipes and pump is based on the longest circuit, measured along the length of the supply pipe from the boiler to the coil and back along the return line to the boiler. Two guidelines apply:
1.         Do not include the length of the coil itself in this circuit length.
2.         No section of a floor panel main should be less than 3/4 in. (20 mm) in diameter.

(ix)      Hydronic Zoning
Figure 9.23 shows that zoning is relatively easy to accomplish with hydronic systems. The installation shown in the figure is made up of three separately heated areas—the first, second, and third floors. Each can be heated to different temperatures as called for by thermostats in each separate apartment. For example, if only the thermostat serving the second floor (zone B) calls for heat, it turns on pump B. Flow-control valves B open, admitting hot water from the boiler header to main B. Flow-control valves A and C remain closed, preventing flow in mains A and C. Any or all of the zones may operate at one time. The boiler keeps a supply of hot water continually ready to supply any zone on demand. This is achieved by an aquastat immersed in the boiler water. When the boiler water drops below the prescribed temperature, it turns on the firing device, such as an oil burner or a gas burner, which brings the water up to the temperature setpoint. If an overhead main supplies downfeed, as in the first floor of this installation, special downfeed supply and return fittings are necessary. For the second- and third-floor zones, one special return tee is sufficient. If the designer also elects to use a special upfeed supply tee of the venturi type, higher outputs of the convectors will result.
Fig. 9.23 Three-zone, multicircuit, one-pipe system. (a) Each convector has connections to the one pipe. (b) Boiler, piping, and water controls suitable for this three-zone, one-pipe system. Each one-pipe circuit should be provided with two flow-control valves and a circulator (also called a booster or pump) on the supply (or return) pipe. (Courtesy of ITT Bell and Gossett.)
Two of the more famous residences in the Midwest utilize hydronic systems. Frank Lloyd Wright’s Robie House (Chicago) has wall radiators integrated below the north windows in the living room. Underfloor radiators with grilles in the floor were provided for below the full-height south windows but were apparently never installed. The boiler sits in a basement room. Mies van der Rohe’s Farnsworth House (Fox River, Illinois) preserves its four walls of ceiling-to-floor glass by concealing radiant heating pipes in the floor slab. The boiler sits within the central utility “closet.”
Today’s radiators are designed to reflect the sleek and simple lines of contemporary architecture. The radiator is getting new exposure with some colorful and pleasing products (Fig. 9.24). The Mayer Art Center at Phillips Exeter Academy in New Hampshire (Fig. 9.25) features new exposed radiators in some older buildings. These radiators are based on simple components (typically, 2 3/4 in. [70 mm] wide) that can be combined in many heights and widths, inviting the designer to feature them rather than to hide them in metal cabinets.
Fig. 9.24 Hot water radiators are available in bright colors and are based on simple components. (Reprinted by permission from AIA: Ramsey/Sleeper, Architectural Graphic Standards, 10th ed.; © 2000 by John Wiley & Sons, Inc.)
Fig. 9.25 Hot water radiators fit below a window (a) at the Mayer Art Center, Phillips Exeter Academy, Exeter, New Hampshire. Amsler Hagenah MacLean, Architects. (Photo by Alex Beatty.) (b) Radiator formed by flat tubes. (Courtesy of Runtal/North American Energy Systems.)
(x)       Heating Equipment Efficiency, Combustion, and Fuel Storage
As fuels burn to produce heat, they require oxygen to support the combustion. Because oxygen constitutes only about one-fifth of the volume of air, reasonably large rates of airflow are required. The air should be drawn in from outdoors at a position close to the fuel burner or (preferably) led to this location by a duct. ANSI/ASHRAE Standard 90.2 calls for 0.5 cfm/1000 Btu/h (1 L/s per 1242 W). This supply duct should be arranged to remain open at all times. Although permitted by Standard 90.2, this combustion air should not be drawn from the general building space. It is a waste of energy, and contemporary “tight” construction inhibits such airflow. A dangerous condition is created whenever stack flow is restricted.
High-efficiency boilers and furnaces manage to remove so much heat from the exhaust gases that smaller flues at much lower temperatures result. These relatively small pipes can be vented through a wall to the exterior. Eliminating a chimney has lessened the impact on the building design of such boilers and furnaces.
For older or less-efficient fuel-burning equipment, it is important that when chimneys carry high-temperature flue gases, they be safely isolated from combustible construction to prevent the possibility of fire. The size of the flue will depend upon the boiler or furnace selected. Flue height (Fig. 9.26a) had traditionally been 35 to 40 ft (11 to 12 m). The function of providing a draft, for which chimney height was an important consideration, is now provided by fans. For example, oil is injected under pressure, accompanied by air, and forced in by a fan. Often a draft adjuster in the breeching (smoke pipe) that carries the flue gases to the chimney is arranged to open slightly to reduce the normal stack draft. If increased draft is ever required, an induced draft fan that puts suction on the flue side of the fire is usually chosen instead of greater stack height. Draft hoods above gas burners prevent downdraft from blowing out the flames.
Fig. 9.26 The need for 40-ft (12-m) chimneys (a) has been eliminated by controlled draft in burners (b) and by the use of high-efficiency heating equipment that can be directly vented to the exterior. Note the buried fuel oil tank, a potential environmental threat.
Prefabricated chimneys (Fig. 9.26b) are replacing with increasing frequency the bulkier and heavier field-built masonry. They offer a number of advantages and can be easily supported on a normal structure.
The storage space to be allowed for fuel oil depends on the proximity of the supplier and the space available at the building. For oil, when more than 275 gal (1040 L) was stored, it was common practice to use an outside tank buried in the ground. This practice eventually led to leaking tanks and contaminated soil and groundwater. Thus, several factors converged to discourage the use of oil: a cleaner and more efficient alternative (natural gas), few basements in new construction where oil tanks might be stored, and unsightly outdoor above-ground oil tanks.
Both ANSI/ASHRAE Standard 90.2 and ANSI/ASHRAE/IESNA 90.1 specify minimum efficiency ratings for heating and cooling equipment. Depending upon the size and the type, one of these terms will apply:
Annual fuel utilization efficiency (AFUE) is the ratio of annual fuel output energy to annual input energy, which includes any nonseason pilot input loss.
Coefficient of performance (COP) is defined slightly differently, depending upon the task. For cooling, it is the ratio of the rate of heat removal to the rate of energy input in consistent units, for a complete cooling system (or factory-assembled equipment), as tested under a nationally recognized standard or designated operating conditions. For heating (heat pump), it is the ratio of the rate of heat delivered to the rate of energy input in consistent units, for a complete heat pump system as tested under designated operating conditions. Supplemental heat is not included in this definition.
Energy efficiency ratio (EER) is the ratio of net equipment cooling capacity in Btu/h to the total rate of electric input in watts under designated operating conditions. (When consistent units are used, this ratio is the same as COP.)
Integrated part load value (IPLV) is a single-number figure of merit based on part-load EER or COP expressing part-load efficiency for air-conditioning and heat pump equipment on the basis of weighted operation at various load capacities for the equipment.
Seasonal energy efficiency ratio (SEER) is the total cooling output of an air conditioner during its normal annual usage period for cooling, in Btu/h, divided by the total electric energy input during the same period, in watt-hours.
(xi) Warm Air Heating Systems
These systems began to supersede the open fireplace in about 1900. Originally, an iron furnace that stood in the middle of the basement was hand-fired by coal. Surrounding it was a sheet metal enclosure. An opening in its side near the bottom admitted cool combustion air that gravitated to the basement. A short duct from the top of the enclosure delivered the warm air by gravity to a large grille in the middle of the floor of the parlor. Other rooms, including those in upper stories, shared a little of this warmth when doors were left open.
Very gradual changes had culminated by the middle of the twentieth century in systems essentially like the ones described in Fig. 9.27. The improvements included:
Automatic firing of oil or gas
Operational and safety controls
Ducted air to and from each room
Blowers to replace gravity
Filters
Adjustable registers
Fig. 9.27 Typical furnace types and duct distribution arrangements. (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 10th ed.; © 2000 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
By the 1960s, the basement was beginning to disappear as subslab perimeter systems became popular for basementless houses (Fig. 9.28). The heat source was located centrally and fully within the insulated volume of the house; heat escaping from the unit itself merely helped heat the building. In general, air was delivered from below, upward across windows, to be taken back at a central high-return grille.
Fig. 9.28 Forced-warm-air, perimeter loop system, adaptable for a cooling coils at the furnace. No returns are taken from the kitchen, baths, or garage.
When electricity was used instead of oil and gas for space heating, provisions for combustion, chimneys, and fuel storage were unnecessary. Horizontal electric furnaces began to appear in shallow attics or above furred ceilings. Air was delivered down from ceilings across windows and taken back through door grilles and open plenum space. Electric resistance furnaces use more electricity than heat pumps to do the same heating task (see again Fig. 2.6), so heat pumps have largely supplanted such furnaces.
As energy-saving design gained strength, insulated windows and well-insulated roofs, walls, and floors lessened the need for space heating. From a central furnace or heat pump, short ducts could deliver warm air to the inner side of each room, because warming at the insulated windows was less essential. Air returned to the unit through open grilles in doors and at the furnace or heat pump enclosure.
Comfort is one of warm air heating’s advantages. The motion of air in the space helps to ensure uniform conditions and reasonably equal temperatures in all parts of a building. The building can quickly be warmed with a forced-air system. It is possible to clean both the recirculated air and the outdoor air by means of filters and other special air-cleaning equipment. Air may be circulated in nonheating seasons. Fresh air may be introduced to reduce odors and to make up the air exhausted by fans in kitchens, laundries, and bathrooms. Central cooling can be incorporated or introduced if ducts are designed originally to do so; cooling often calls for greater rates of air circulation. Humidification can be achieved by a humidifier in the air stream, and if cooling is included in the design, dehumidification can be accomplished in summer. For both heating and cooling, a common arrangement is to place the supply registers in the floor, below areas of glass. This is important for winter operation. With adequate attention to supply register placement, return grilles can be located so as to minimize return air ductwork. High return grilles pick up the warmer air for reheating at the equipment. In many systems, air circulates at all times and is warmed or cooled as required.
Planning for warm air systems begins with the attempt to locate the furnace reasonably close to the center of the building. After the system is designed, a furnace must be selected. It should be capable of burning fuel at a rate suitable to make up the building’s hourly heat loss. The rate of air delivery depends on the air temperature rise that is planned. Finally, the motor and blower must be powerful enough to overcome the friction of air against metal in both the supply and return duct systems, as well as the friction of air flowing through the furnace, filters, registers, and grilles (see later Fig. 9.31). Minor adjustments can be made at the furnace to adapt to the demands of the system and the building.
Some of the system components are discussed in the material that follows.
Furnaces have become much more efficient in recent years, thanks to forced-draft chimneys and heat exchangers, as shown in Fig. 9.29. Seasonal efficiencies of up to 95% are possible, in contrast to about 62% for older furnaces. The AFUE ratings for furnaces are based on an isolated combustion system that requires that all combustion and dilution air be drawn from outside.
Fig. 9.29 Furnaces with greatly increased operating efficiencies are now available. (a) An Amana gas furnace. (b) The small, high-efficiency heat exchanger utilized by this furnace, which recovers heat from exhaust gases. (Courtesy of Amana Refrigeration, Inc.)
Figure 9.30 shows the relationship between a furnace, the duct distribution tree, and some elements of the spaces they serve.
Fig. 9.30 Conventional warm-air furnace and ducts. Low supply registers under windows and high return grilles at interior walls help ensure distribution within the spaces.
Ducts are constructed of sheet metal or glass fiber and are either round or rectangular. Ductwork will conduct noise unless these suggestions are followed:
Do not place the blower too close to a return grille.
Select quiet motors and cushioned mountings.
Do not permit connection or contact of conduits or water piping with the blower housing.
Use a flexible connection between bonnet and ductwork.
Ducts also can be lined with sound-absorbing material to further discourage noise transfer, but beware of materials that encourage mold and mildew growth.
Duct sizes may be selected on the basis of permissible air velocity in the duct (Table 9.4).
TABLE 9.4 Air Velocities for Ducts and Grilles

aBranch duct velocities should be about 80% of those listed, whereas velocities in final runouts to outlets should be 50% or less. Elbows and other fittings can increase noise substantially, so airflows should be reduced accordingly.
bThese are “free” opening velocities. Diffusers and grilles can increase sound levels, in which case these velocities should be reduced accordingly.
cSee also Table 19.8 for recommended NC levels.

Figure 9.31 illustrates a device that simplifies duct sizing. At a glance, it shows many duct cross-sectional configurations that will satisfy the combined requirements of friction, airflow, and air velocity.
Fig. 9.31 The Ductulator is a duct-sizing device available from the Trane Company. The designer selects any two factors (e.g., friction and airflow volume), and the device yields all the other factors (e.g., air velocity, diameter of the round duct required, or combinations of rectangular-duct cross sections required).
Dampers are necessary to balance the system and adjust it to the desires of the occupants (Fig. 9.32). Splitter dampers are used where branch ducts leave the larger trunk ducts. The flow of each riser can be controlled by an adjustable damper in the basement at the foot of the riser. Labels should indicate the rooms served. Some codes require dampers of fire-resistant material actuated by fusible links to prevent the possible spread of fire through a duct system (see Fig. 24.6). Figure 9.32d shows how turning vanes can be used to assist airflow at sharp turns in ductwork. Such assistance reduces friction within the ductwork, thus reducing the total static head (Fig. 9.33) against which the supply fan must work.
Fig. 9.32 Air controls in ducts. (a) Air adjustment by opposed-blade dampers. (b) Air adjustment by a splitter damper. (c) Conventional turns in ducts. (d) Right-angle turns with turning vanes—a more compact method.
Fig. 9.33 The static head is the pressure, measured in inches of water (or pascals), available to overcome friction in the entire system.
Supply registers (Fig. 9.34) should be equipped with dampers, and their vanes should be arranged to disperse the air and to reduce its velocity as soon as possible after it enters the room. This is commonly done by providing vanes that divert the air, half to the right and half to the left. When a supply register is in the corner of a room, it is best if the vanes deflect all the air away from the corner. Return grilles are of the slotted type in walls and of the grid type in floors. All registers and grills should be made tight at the duct connection. See Tables 9.4 and 9.5 for selection of registers based on output and recommended face velocity.
Fig. 9.34 The typical floor register (diffuser) in its 21⁄4-in.  12-in. (60-mm X300-mm) size (a). It has diverting vanes for spread and an adjustable damper. See Table 9.5 for characteristics. (b) Spread and throw; cooler room air is induced by aspiration to join the stream of warm air, resulting in a bland, pleasant air stream that crosses the room.
TABLE 9.5 Typical Residential Forced-Air Register, 2 1/4  12 in. (60  300 mm)

SI conversions by the authors.
Controls: The burner is started and stopped by a thermostat, which is usually placed in or near the living room at a thermally stable location that is protected from cold drafts, direct sunlight, and the warming effects of nearby warm air registers. A cut-in temperature of between 80º and 95ºF (27º and 35ºC) is selected for the fan switch in the furnace bonnet. After the burner starts, the fan switch turns on the blower when the furnace air reaches the selected cut-in temperature. Burner and blower then continue to run while heat is needed. When the burner turns off, the blower continues to run until the temperature in the furnace drops to a level a little below the cut-in temperature of the fan switch. If, during operation, the temperature unexpectedly exceeds 200ºF (93ºC), a high-limit switch turns off the burner in the interest of comfort and safety. As in all automatically fired heating units, a stack temperature control in the breeching cuts off the fuel if ignition fails.

(b)       COOLING ONLY SYSTEMS AND DISTRIBUTION
(i)        Fans
Before the advent of mechanical air conditioning, cooling was commonly achieved with simple air motion provided by fans. The summer comfort chart shown in Fig. 4.8 encourages increased air motion as a way to extend comfort into air temperatures in the mid-80s ºF. As a general rule, people will perceive a 1Fº decrease in air temperature for every 15 fpm increase in that air’s speed past the body (about a 1Cº decrease for every 1 m/s increase). Ceiling fans are often installed and run at slow speed to destratify warm air at the ceiling in winter; they can be run at higher speed in summer to provide added comfort through increased air motion. The air motion produced by ceiling fans will vary with the fan height above the floor, the number of fans in a space, and the fan’s power, speed, and blade size. Figure 9.4 shows expected air speeds with one 48-in. (1220-mm) ceiling fan in a typical residential living room.
Fig. 9.4 Ceiling fans are useful in heating and cooling. In winter, at slow speeds, they destratify warm air at the ceiling. In summer (a) they extend the comfort zone by providing increased air motion. Room size is typical of residential living rooms. (Reprinted with permission of the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., from 1997 ASHRAE Handbook—Fundamentals.) (b) A ceiling fan is a visual feature in this Oregon residence. (Photo by Nathan Majeski.)
In the hot summer climate of Davis, California, an experimental house (sponsored by Pacific Gas & Electric, designed by the Rocky Mountain Institute) has eliminated a conventional cooling system through a series of alternative approaches. Fans play a major role: in addition to ceiling fans, a whole-house fan removes the hottest air from the central hallway, exhausting to the ventilated attic. More thermal mass (tile floors, double drywall), an attic radiant barrier, and low-, gas-filled windows are also used.
(ii)       Unit Air Conditioners
The device shown in Fig. 9.5 is perhaps the most commonly seen piece of mechanical equipment in the United States. Perched in windows in full view of passersby, these window-box air conditioners noisily remind us that many of our buildings still are not centrally mechanically cooled. Mechanical cooling was considered a luxury until long after World War II.
Fig. 9.5 Diagram of a typical through-the-wall air-conditioning (cooling) unit. Direct heat exchange occurs between air and the process of evaporation (on one side) and condensation (on the other) of the refrigerant. The unit is self-contained, requiring access to outside air and electricity to power the compressor and the two fans. Typical capacities are 1 to 2 tons (12,000 to 24,000 Btu/h [3.5 to 7 kW]).
Built-in, through-wall air conditioners offer a low-first-cost way to provide separate zones for individual apartments, motel rooms, and so on (see Section 9.8). In noisy cities, the drone of these units masks street noise for the interior, thus potentially helping to promote relaxation. Unfortunately, such scattered units rarely afford the chance to conserve energy through the exchange of waste heat or the higher efficiencies that can accompany larger equipment. However, if turned on only when cooling is needed (i.e., when people are present), they can provide substantial savings over the larger always-on systems.
(iii)      Evaporative Cooling: Misting
As explained in Section 8.12, the net effect of evaporative cooling is no total change in the heat content (enthalpy) of the treated air; its DB temperature is lowered, but there is an increase in RH. People feel cooler, although no change in total heat has occurred. One of the most direct approaches is a misting or fogging system whereby a fine spray of water droplets is blown into the air. A common application is for small outdoor areas—the team benches of football stadiums or refreshment pavilions. However, very large spaces in hot, dry climates can also benefit; Fig. 9.6 shows mist descending from a skylight diffuser in the Atocha railroad station in Madrid, Spain. A large space using fogging (in a Michigan conservatory) is shown later in Fig. 10.52.
Fig. 9.6 Mist is sprayed from a diffusing system located high in the Atocha railroad station in Madrid, Spain. It provides psychological reinforcement of evaporative cooling in a hot, dry summer.
(iv)      Evaporative Cooling: Roof Spray
In the past, when roofs were poorly insulated, roof sprays were rather common ways to reduce heat gains. New variations promise new energy savings.
Roof color is the first consideration; white or near-white roofs are the first step toward energy savings through control of sol-air temperature. However, emissivity is also involved; the higher the emissivity, the faster a roof surface reradiates its heat to the sky. Table 9.1 compares the solar absorptance, albedo (overall solar reflectance), and emittance of some common roofing materials. The combination of high albedo and high emittance resists solar heat gain most effectively. (It also promises to reduce the heat island effect in urban areas; see Chapter 3.) A solar reflectance index (SRI) was developed to allow quick comparisons between roofing products. The SRI scale ranges from 0 (approximately the combination of 5% albedo and 90% emittance; roughly equal to black asphalt shingles) to 100 at 80% albedo and 90% emittance (roughly equal to T-EPDM).
TABLE 9.1 Solar Performance of Roofing Materials

The night roof spray thermal storage system (NRSTS) cools water on a roof by night, using both night sky radiation and evaporation. The water is then stored for use the next day in building cooling. The water can be stored either on the roof (below floating insulation, above a structural ceiling) or in a tank below the roof. When stored on the roof, it becomes a variant of the roof pond cooling system. When stored in a tank, the water can be circulated through a cooling coil to precool air before it enters the chiller-fed cooling coils. It is thus used to assist mechanical cooling. Three variations are shown in Fig. 9.7.
Fig. 9.7 Night roof spray thermal storage systems (NRSTS). (a) This version approximates the performance of the roof pond. (b) Remote water storage allows use of the cooled water at any time. (c) The floor slab is used as thermal storage in this version. (From Technical Installation Review, December 1997: WhiteCap Roof Spray Cooling System. Federal Energy Management Program, U.S. Department of Energy.)
At a Nogales, Arizona, border patrol station, a 6500-ft2 (604-m2) flat white roof was retrofitted with an NRSTS, utilizing a 10,000-gal (37,850-L) above-ground tank. Over the summer of 1997, it averaged 250 Btu/ft2 day of cooling. Water use at various sites has averaged 4 to 5 gal/h (15 to 19 L/h) per 1000 ft2 (per 93 m2) of roof area. Details are presented in Bourne and Hoeschele (1998).
(v)       Evaporative Coolers
These are also affectionately termed swamp coolers and desert coolers and are familiar devices in hot, arid climates (Fig. 9.8). (They are used in other climates for special high-heat applications such as restaurant kitchens.) They require a small amount of electricity to run a fan and some water to increase the RH of the air they supply to the building. This process of cooling is explained in Section 8.14.
Fig. 9.8 The UltraCool evaporative cooler has a low profile and uses internal components of plastic or stainless steel to reduce corrosion. Typical dimensions for a 4000-cfm (1890-L/s) unit are 35 in. H x 42 in. W x 48 in. D (890 x 1070 x 12220 mm). (Courtesy of Champion Cooler Corporation, El Paso, TX.)
The typical evaporative cooler (shown in Fig. 9.8) needs full access to outdoor air and is thus often set on the roof; through-the-wall units are also available. Great quantities of dry, hot outdoor air are blown through pads kept moist by recirculated and makeup water. The “cooled” air is then delivered to the indoor space. The effect of the gently moving cool air is to cool the body and to produce further cooling by evaporation of body moisture.
Air introduced into the indoor space must then be exhausted for the system to operate properly. By selecting the room through which the air is exhausted, one can route cool air as desired in any chosen path from unit to relief opening. The closer to the relief opening, the warmer the indoor air.
(vi)      Indirect Evaporative Cooling
The preceding discussion concerns the simplest application of evaporative cooling, known as the direct process (once-treated outdoor air directly introduced to the space). Unfortunately, some areas have such hot summer daytime conditions that these simple evaporative approaches cannot produce comfort indoors. Direct and indirect processes have been combined to achieve more “real” cooling and better indoor comfort conditions; a psychrometric process diagram is shown in Fig. 5.16.
One of several such approaches is shown in Figs. 9.9 and 9.10. Warm, rather dry night outside air is evaporatively cooled and fed into a rock bed. The air’s temperature is low enough to cool the rock bed, and its RH is moderate. (At the same time, the house is directly evaporatively cooled by a second cooling unit.) Figure 9.10 traces the process by day. Extremely hot, dry outdoor air (A) is drawn into the rock bed, where it is cooled by contact (D). It can then be passed through an evaporative cooler to achieve a better combination of RH and DB temperature (E). After picking up both sensible and latent heat, the air is exhausted (at approximately temperature F). Note that at condition F, it is still much cooler than outdoor air.
Fig. 9.9 (a, b) Direct–indirect evaporative cooling utilizing a rock bed for storage and heat exchange. (Reprinted by permission of the Environmental Research Laboratory, University of Arizona.)
Fig. 9.10 Comparing direct (A to B to C) and indirect cooling (A to D to E to F) on the psychrometric chart.
By comparison, simple direct evaporative cooling by day would have produced indoor supply air too hot and humid for comfort (B). Again, upon exhaust (C), it is still cooler than outdoor air.
Indirect evaporative cooling is combined with a direct refrigerant system in an innovative tent structure over a San Francisco department store (Fig. 9.11). Two layers of fiberglass, Teflon-coated fabric, separated by an average of 12 in. (300 mm), are supported by a network of cables hung from eight masts. The tent roof covers about 70,000 ft2 (6500 m2) of sales floor; its 7% translucency to sunlight provides 450 to 550 footcandles of daylight. This greatly reduces the need for electric lighting, although some electric lamps, clipped onto exposed fire sprinkler pipes, are still used as accent lighting. About 3.5 W/ft2 of solar gain, mostly in the form of this diffused daylight, penetrates the tent cover. When this solar gain is combined with heat gains from people and electric accent lights, a cooling load is always generated in San Francisco’s mild climate, which rarely falls below 45ºF (7ºC). The roof’s relatively high U-factor is thus advantageous in helping to lose heat. Because San Francisco overheats even more rarely, such low resistance to heat flow is not seriously disadvantageous in summer.
Fig. 9.11 Bullock’s Department Store, San Mateo, California. (a) The eight-masted white fabric roof, highly reflective to ward off solar heat, transmits about 7% of daylight to provide ample diffuse daylight for sales areas. (Photo by Steve Proehl. Environmental Planning and Research, Inc., architects; Giampolo and Associates, Inc., mechanical and electrical engineers.) (b) Cooling is provided by a two-stage, indirect evaporative cooling system, which uses much less energy than does conventional compressive refrigeration.
To remove this heat, four sets of direct refrigerant equipment are provided at the tent perimeter on grade. These feed into a perimeter plenum, from which the entire store is supplied with cooled air. The exhaust air is collected at the center and returned to help with the task of cooling. Indirect evaporative cooling (also called sensible evaporative refrigeration) units are used to cool air (see Fig. 9.11b). In this application, two units work in tandem. Air to be cooled (supply air) enters the heat exchanger of the first unit, which is cooled by evaporative cooling of outside air. During peak temperature periods, the supply air is only somewhat cooled by this process; sufficiently low temperatures for use on the interior are obtained by passing it through the second unit’s heat exchanger. This second unit is cooled by evaporative cooling of the exhaust air from the store, which is cooler than outside air under summer conditions. Thus, the exhaust air does some work beyond the direct cooling of the tent’s interior. This two-stage, indirect evaporative cooling process allows the supply air to be cooled without the RH being raised, as would be the case if direct evaporative cooling were used.
(c)        HEATING/COOLING SYSTEMS AND DISTRIBUTION
Mechanical cooling is a much more recent development than heating; in its early days, it was widely adopted as a retrofit to existing heating systems. The first two approaches to mechanical cooling reflect this early attitude.
(i)        Cooling Coils Added to Warm Air Furnaces
This common system utilizes a rather simple arrangement of the refrigeration cycle. Figure 9.1 illustrated the circuit of a refrigerant in compression, condensing, and evaporation, in which the condenser heat is carried away by water and the evaporation process draws heat out of water in another circuit to produce chilled water. Thus, the heat is moved to a heat-rejection location outdoors. Figure 9.35 is a schematic diagram of an air-to-air (in contrast to a water-to-water) refrigeration device. Air instead of water can be used to cool the condenser, and indoor air can be cooled directly by being passed over the evaporator coil in which the refrigerant is expanding from a liquid to a gas. Thus, heat is moved from the indoor air to the outdoor air by the step-up action or heat-pumping nature of the refrigeration cycle. When indoor air is cooled directly in this manner, by the expanding refrigerant, the process is usually known as direct expansion. The cooling coils therefore are often referred to as DX coils. Figure 9.36 shows another popular arrangement in which the airflow through the furnace and coils is horizontal.
Fig. 9.35 The compressive refrigeration cycle (a) used in an exterior air-cooled heat-rejection unit and an interior cooling coil suitable for placement in a central air stream. (b) Cooling/heating air-handling unit. Cutaway view shows an upflow air pattern. At the lower left is the return air intake and a filter; adjacent are the fan and motor. Above these components are the gas-burning elements. At the top is the direct-expansion cooling coil. (Courtesy of American Furnace Division, Singer Company.)
Fig. 9.36 Compact horizontal flow combinations for forced-air heating and cooling. The small pipe between the two refrigerant pipes of the cooling unit is a water drain that carries away the condensed moisture from the recirculated and outdoor air. The heating unit requires gas and flue or exhaust gas connections. Refrigerant pipes connect to the outdoor compressor–condenser unit.
Meanwhile, the compressor–condenser unit is placed outdoors on a concrete slab or on the roof. The unit creates a noisy, hot microclimate in summer—an influence on both site and building planning.
(ii)       Hydronic and Coils
This system combines a perimeter hot water heating pipe with an overhead air-handling system. A boiler with a tankless coil supplies domestic hot water. The boiler’s heat output supplies both the perimeter loop and a coil in the air-handling unit of the duct system. The total heating load is met by the combination of radiant heat generated by the perimeter loop and heated air from the overhead air-handling system.
As installed in the Levittown standardized houses, the perimeter loop consists only of 1/2- or 3/4-in. (13- or 19-mm) tubing embedded 4 in. (100 mm) below the top of the floor slab to overcome the cold slab effect. It has the capacity to maintain a 35Fº (19Cº) differential between the inside and outside temperatures at the perimeter.
The air-handling unit and overhead duct system, incorporating supply outlets in each room and central return, is used throughout the year. Its cooling coil is connected to an adjacent outdoor condensing unit (Fig. 9.37).
Fig. 9.37 Combination hydronic and forced-air system (a) using a perimeter hot water loop in the floor slab and an overhead air supply. (Courtesy of Levitt and Sons. Design by mechanical engineer John Liebl.) (b) Close-up of the heating and cooling components. The boiler heats both the slab perimeter and the coils in the air stream.
Because the heating load is shared by the slab loop and by warm air, the winter indoor temperature remains more constant. The air can be distributed at no more than about 120ºF (49ºC), or 20Fº (11Cº) less than with a conventionally ducted system.
(iii)      Air–Air Heat Pumps
These use the refrigeration cycle to both heat and cool, thus eliminating the distinction between furnace (or boiler) and DX cooling coils. As shown in Fig. 9.38, heat is “pumped” from indoors to outdoors in summer (Fig. 9.38a) and from outdoors to indoors in winter (Fig. 9.38b). Heat pumps can transfer heat air–air, air–water, and water–water. The most common application for smaller buildings is the air–air heat pump, shown in Figs. 9.38 and 9.39. In a single-package (also called unitary) system (Fig. 9.39a), only one piece of equipment is involved. A single-package air–air heat pump moves heat between an outdoor air stream and an indoor air stream; although kept separate, both streams pass through a single outdoor unit. A system with both outside and inside components is called a split system (Fig. 9.39b). A split-system air–air heat pump moves heat via a refrigerant loop between the outdoor unit (which also contains the compressor), through which outdoor air passes, and the indoor unit (which usually contains backup heating coils) for the treatment and circulation of indoor air.
Fig. 9.38 Heat pump applications of the compressive refrigeration cycle. The simple air–air heat pump provides cooled air (a) or heated air (b). Teamed with a solar collector and water storage tank (c), the heat pump can yield usefully warm temperatures in the air stream while increasing the collector efficiency by lowering the storage tank temperature. (Reprinted with permission from Popular Science; © 1978 by Times Mirror Magazines, Inc.)
Fig. 9.39 The package unit (a) and the split system (b) are popular applications of the air–air heat pump. The package unit and the outdoor unit of the split system are also typically placed outside walls, as well as on roofs, as shown here.
Single-package heat pumps are commonly located on roofs, where they have unlimited access to outdoor air, and where their noise is less likely to annoy—provided they are sufficiently isolated from the building’s structure. This approach is shown in the daylighted, passively solar-heated Mount Airy, North Carolina, library (Fig. 9.40). This 14,000-ft2 (1300-m2) building also has a solar preheating system for its hot water. The five air–air heat pumps utilize economizer cycles (up to 100% outside air when temperatures are favorable). The average annual building energy consumption has been monitored at about 17,000 Btu/ft2 (53,635 W/m2)—approximately one-third of nearby similar-function buildings.
Fig. 9.40 Mount Airy Library, Mount Airy, North Carolina. Mazria/Schiff & Associates, Architects. (a) View from the southwest. (Photo by Gordon H. Schenck, Jr.) (b) Plan. (c, d) North–south sections relate sunlight and natural heat flow. Individual package heat pumps are set on the flat-roof sections.
Individual air–air heat pump units are especially common in building types with all-perimeter spaces with varying orientations and numerous thermal zones. Motels are a prime example. In Fig. 9.41, separate air–air heat pumps serve each motel room; at best, their constant noise helps mask the intermittent sounds from the adjacent parking lot/circulation space. Opportunities for heat exchange between these heat pumps are scarce; if a central water loop (see Section 9.8e) were substituted for outdoor air as the heat source/sink, energy costs would go down, although the first cost would rise.
Fig. 9.41 Air–air heat pumps serve a motel. (a) A panel set just forward of the exterior wall allows the heat pump to inhale and exhale outdoor air around the panel edges. In summer it discharges warm air; in winter, cool air. (b) Interior cabinet contains the heat pump. Room air is taken in as shown, then discharged (after cooling or heating) upward across the glass surface—the likely point of maximum heat loss or gain.
Split systems are popular because the noise of the compressor and the outdoor air fan are removed from the interior, and the size of the indoor unit can be quite small. This indoor element is often mounted either high on the wall or on the ceiling. Such an indoor unit is available with automatically changing louvers; when it is in the cooling mode, it delivers cool air along the ceiling, from where it sinks to the level of occupancy; cold air blowing directly on people is avoided. In the heating mode, the louvers shift to direct hot air steeply downward. The greater the distance between the indoor and outdoor units, the greater the strain on the refrigerant loop.
Heat pumps have a high initial cost, and they have shown a relatively high frequency of compressor failure. Noise from the compressor and the outdoor air fan may affect site planning, especially for residences.
One of the primary attractions of the heat pump is that in its heating mode it can give more energy than it receives (electrically). Although energy (usually electricity) is required to run the cycle, the pump draws “free” heat from a source such as outdoor air. The total heat delivered to the building is more than the heat (electricity) required to run the cycle. The measure of this heat advantage is called the coefficient of performance (COP), defined as
COP = heat delivered to space\necessary work input
(See the earlier discussion in Section 9.7j.)
In typical space-heating applications, a seasonal COP of 2 or more is common in mild-winter areas. Because the COP changes with outdoor conditions and indoor load, a seasonal energy efficiency ratio (SEER) rating system has been established. SEER measures the number of Btu/h removed for each watt of energy input, averaged over the cooling season. The higher the SEER, the more efficient the heat pump’s seasonal performance. SEER ranges are roughly as low as 5 and as high as 15. The heating cycle of the heat pump has a similar rating system, called the heating seasonal performance factor (HSPF).
As might be expected from a device that draws heat from winter outdoor air, there are limitations to its heating performance. As outdoor temperatures approach 32ºF (0ºC), the COP drops and the outdoor coil tends to ice over. Built-in electric resistance coils must then be used; this, of course, ends the efficiency advantage that made the heat pump attractive. See Fig. 9.42 for a demonstration of falling performance with falling temperatures. Because of this characteristic, air–air heat pumps are less frequently used in cold-winter climates. They also generally make questionable backup choices for passively solar-heated buildings in colder climates, because backup sources are typically needed only in the coldest weather.
Fig. 9.42 Typical air–air heat pump operating characteristics; heat delivery falls with falling outdoor temperature. (Redrawn by Jonathan Meendering.)
Heat pumps that pump heat from water sources, such as wells or solar-heated storage tanks, or from the ground, are much more dependable cold weather performers. Water–air heat pumps (shown in Fig. 9.38c) and solar collectors make an effective team. As these rather high-COP heat pumps remove heat from the solar storage tank (to deliver it to the indoor air), the resulting lower temperature of the solar-heated water increases the solar collector’s performance. (See Fig. 21.36 for solar collector efficiencies.) Assume that on a cold, partly sunny day, the collector being fed water from the tank at 90ºF is able to raise its temperature by 4Fº to 94ºF (from 32º to 35ºC). This improvement is slight because of the rather high heat loss that a 94ºF collector experiences when surrounded by cold air. If, however, the collector were to be fed 59ºF (15ºC) water, its heat loss would be greatly reduced. The heat that the collector does not lose to the cold air can be invested in the 59ºF water, which will thus leave at a considerably higher temperature than 59ºF 4  63ºF. Thus, more solar energy is collected, and more is available for transfer to the building via the heat pump. The heat pump, in turn, can heat the 59ºF tank water to much higher temperatures to serve the coils in the air-handling unit.
Although most of today’s heat pumps utilize electricity to drive the compressive refrigeration cycle, there are also absorption cycle heat pumps that utilize natural gas. The GAX heat pump (Fig. 9.43) not only uses this lower-grade resource for heating and cooling, it avoids CFCs and HCFCs and functions at outdoor temperatures much lower than those of the electric air–air heat pump. Eventually, solar energy can be used to drive the absorption cycle. Solar-driven refrigeration is a particularly elegant blend of energy source and task—the hotter the sun, the higher the cooling capacity.

Fig. 9.43 An air-cooled advanced GAX (generator-absorber heat exchange) heat pump uses aqua ammonia as the absorbent and is operational down to 10ºF (23ºC). The target cooling COP is 0.95; heating COP is 1.55. Designed for light commercial applications, it ranges from 5 to 25 tons capacity. (RHX  refrigerant heat exchanger; IP ABS  intermediate pressure absorber.) (Courtesy of Energy Concepts Company, Annapolis, MD.)

(iv)      Ground Source Heat Pumps
Ground–air heat pumps, also called geothermal heat pumps or Geo Exchange systems, are found in several configurations throughout North America. They often provide domestic hot water in addition to heating and cooling. An environmentally safe refrigerant is circulated through a loop installed underground (or in a pond or lake), taking heat from the soil in winter and discharging heat to the soil in summer. The loop is often high-density polyethylene (HDPE). Below the surface, soil temperatures are more stable year-round than outdoor air temperatures, thus raising the COP relative to that of air–air heat pumps. Annual well-water (deep soil) temperatures were shown in Fig. 8.56. This system is almost completely out of sight, with no maintenance or weathering of exterior equipment. Noise is confined to the compressor in a small indoor mechanical room.
Some common configurations are shown in Fig. 9.44. In the closed systems (a–c) the flow rate is typically 2 to 3 gpm/ton of refrigeration (0.3 to 0.5 mL/J), with lower flows in the open loop systems.
Fig. 9.44 Configurations of ground source heat pumps. (a) Horizontal ground source closed loop heat pump laid in trenches. (b) Vertical ground source closed loop heat pump placed in boreholes. (c) Pond or lake closed loop heat pump. (d) Groundwater source heat pump taking water from one well and discharging it to another. (e) Standing column groundwater source heat pump taking water from, then discharging to, the same well.
The horizontal ground source closed loop heat pump (Fig. 9.44a) requires trenches 3 to 6 ft (1 to 2 m) deep; typically, 400 to 650 ft (120 to 200 m) of pipe are installed per ton (12,000 Btu/h or 3.5 kW) of heating and cooling capacity. To squeeze more pipe length into a trench, a “slinky” coiled pipe is sometimes used. The trenches can be placed below parking lots or lawns and gardens.
The vertical ground source closed loop heat pump (Fig. 9.44b) is particularly applicable where the site area is limited. Vertical holes are bored from 150 to 450 ft (46 to 137 m) deep. Each hole contains a single full-depth loop and is backfilled (or grouted) after the loop is installed. Because the temperature is much lower at greater depths, less pipe length is required than for horizontal loops. The distance between boreholes varies from a minimum of 15 ft (4.6 m) with high water table and low building cooling loads to as much as 25 ft (7.6 m) for buildings with high cooling loads. A minimum distance of 20 ft (6 m) is usually recommended.
The pond or lake closed loop heat pump (Fig. 9.44c) is sometimes used when a building is close to an adequately large body of water. The loop is submerged, and the surrounding water conducts heat far more rapidly than does soil. The resulting shorter length required, and the low cost of placing the coils in water, can make this attractive. However, the water level in the pond should never drop below a minimum of 8 ft (2.5 m) and must have sufficient surface area for heat exchange.
The groundwater–source open loop heat pump (“pump and dump”) is suitable only where groundwater is plentiful, and may be prohibited by local codes and environmental regulators. One variant of this system (Fig. 9.44d) takes water from one well, through a heat exchanger within the building, then discharges it to a second well. Another variant (Fig. 9.44e) takes water from the bottom and discharges back into the top of the same (standing) well, typically 6 in. (150 mm) in diameter and as deep as 1500 ft (460 m).
The Wildlife Center of Virginia at Waynesboro is a wildlife teaching–research hospital. Its 5700-ft2 (530-m2) floor area is served by four geothermal heat pumps (two at 4 tons, two at 5 tons) connected to 11,350 ft (3460 m) of underground horizontal pipe, laid in a “slinky” configuration and thus fitting within about 2500 ft (760 m) of trench. The trench was dug around existing trees and placed under future roadways in its forest setting. Energy simulations estimate that annual heating, cooling, and hot water will use about 35,000 kWh. Had air–air heat pumps and an electric water heater been used instead, the estimate is 66,000 kWh, a 47% yearly savings advantage for the geothermal system.
Ground source heat pumps are often used in retrofits, especially in schools where site areas are plentiful, or historic structures where small-size interior mechanical equipment is highly desirable. The Daniel Boone High School near Johnson City, Tennessee, was built in 1971 with a two-pipe chilled water system and electric resistance heat. A 1998 ASHRAE Technology Award was won when this 160,000-ft2 (14,864-m2) school was retrofitted with a ground source vertical closed loop heat exchanger that is fed by 320 boreholes, each 150 ft (46 m) deep. Each borehole loop is 300 ft (91 m) of 3/4-in. (19-mm) polyethylene pipe. The boreholes are arranged in sections of 20 holes each. The holes are 15 ft (5 m) on center. The 20-hole sections are separated by 20 ft (6 m). The loops all connect to an 8-in. (203-mm) supply and same size return line to the new heat exchanger within the existing mechanical room. Within the building, a new water loop heat pump system is installed, one heat pump in each zone.
(v)       Water Source Heat Pumps
The school described previously uses an interior closed water loop that connects all the heat pumps of all thermal zones. This system facilitates heating in one zone while another zone is being cooled, because simultaneous heat “deposits” and heat “withdrawals” actually help the system to function most efficiently. Figure 9.45 diagrams the typical water loop system, where a supplementary heat source (such as a boiler) in cold weather, and a supplementary heat rejector (such as a cooling tower) in hot weather, are installed to maintain usable water temperatures within the loop. A large office building in Pittsburgh that uses such a system is shown in Fig. 10.63.
Fig. 9.45 Water source heat pumps. (a) Each water–air heat pump either deposits heat into the loop (while cooling) or withdraws heat (while heating). This system is particularly well suited to buildings where simultaneous heating and cooling needs occur. (b) Supplementary heat sources (boilers) and heat rejectors (cooling towers) are usually provided. (Reprinted by permission from AIA: Ramsey/Sleeper, Architectural Graphic Standards, 9th ed.; © 1994 by John Wiley & Sons, Inc.)
Motels are good candidates for water loop heat pumps, because some rooms face sunny conditions, others are shaded; some are occupied, some unoccupied; and substantial domestic hot water needs all combine to make a heat-sharing water loop attractive.

(d)       CONTROLS
In the past, when buildings had mechanical equipment for heating only, thermostats were simple on–off devices; when they dropped below a setpoint, the heat was turned on. With the advent of mechanical cooling and then concerns about energy conservation, thermostats added a deadband separating heating need temperature from cooling need temperature, and automatic setback provisions that allow buildings to change internal temperature between occupied and unoccupied hours. ANSI/ASHRAE Standard 90.2, Energy-Efficient Design of Low-Rise Residential Buildings, calls for thermostats capable of being set from 55ºF to 85ºF (13ºC to 29ºC) as well as an adjustable deadband, the range of which includes a setting of 10Fº (5.6Cº).
Building management systems (BMS) are now capable of regulating far more than temperature. They are capable of remote control, allowing systems to be activated in advance of the occupants’ arrival; this is particularly useful for weekend and vacation homes, where heating/cooling and domestic hot water systems, if left on, could waste much energy during unoccupied periods. Door and window locks, security cameras, lighting, and appliances are potential partners in a comprehensive control system.
Future developments include neural networks, where automation systems are capable of learning while being used. They thus predict usage patterns, adjusting in advance without needing specific commands from occupants. When the building use pattern is highly predictable, as with many retail and commercial occupancies, these self-programming systems should learn very quickly how to anticipate needs while conserving energy.
In residences, usage patterns are likely more varied and less predictable. Experiments at the University of Colorado have revealed that even here, patterns may be more predictable than was first thought; for details, see Mozer (1998). The Adaptive Control of Home Environments (ACHE) system has two objectives: to anticipate inhabitants’ needs and to save energy. Lighting, air temperature, ventilating, and water heating are controlled so that just enough energy, just when needed, is provided. Lights will be set to the minimum required; hot water maintained at the minimum temperature to meet demands, only occupied rooms will be kept at an optimum comfortable temperature, and so on.
The control framework compares energy costs to “misery” costs. Minimum settings and their time patterns are developed as the building is occupied over time. Whenever a preset minimum is overruled by the occupant, the system learns and readjusts accordingly; however, it occasionally tests lower minimum settings to be sure that energy conservation is not being unduly sacrificed to past desires for more.

(e)        GUIDELINE FOR SIZING MECHANICAL AREAS
The content of 9.3 (b) Central Heating or Cooling Equipment) moved here
In the early design stages, determining an approximate size for the largest equipment is sometimes useful. Once the heating or cooling capacities are known, manufacturers’ catalogs can be consulted for the dimensions of the heating and cooling equipment.
The critical decision in sizing the heating equipment is the design temperature: what is the lowest reasonable outdoor temperature for which a heating device can be sized, if the desired interior temperature is to be maintained? These winter design temperatures are listed in Appendix B. When this is known, the next step is
design ∆t = inside temperature  outside design temperature
In Section 8.4, the criteria for Btu/DD ft2 were listed in Table 8.3. To convert to the required capacity of a building’s heating equipment, calculate
Btu/DD ft2\24 h ´ ∆t ´ ft2 floor area = Btu/h heating capacity
(Note: For passively solar-heated buildings, the backup heating unit is sometimes sized for the heat loss of the entire envelope and sometimes for the envelope minus the solar wall, just as the criteria were defined in Table 8.3. Similarly, buildings with reliable internal gains (lights, equipment, people, etc.) are sometimes designed with smaller heating units because internal gains supply a constant portion of the space-heating needs. For a more complete discussion, see Section 8.8.)
The sizing of cooling (mechanical refrigeration) units is not so straightforward, as was evident when detailed hourly heat gain procedures were presented (Sections 8.11 and 8.13). However, a very approximate early estimate can be obtained from the estimated hourly gains in Table F.3. (Warning: This estimate is likely to be lower than that obtained using the peak heat gain hour, for which cooling equipment is often sized.)
sensible Btu/h cooling capacity = [approx. heat gain (Btu/h ft2)][floor area (ft2)]
Another common unit used for sizing mechanical refrigeration is tons of cooling capacity, 1 ton being equivalent to the useful cooling effect of a ton of ice, or 12,000 Btu/h (3516 W). Therefore, the required capacity in tons is
heat gain, Btu/h\12,000 = tons of cooling
With this information about needed Btu/h (or watts), equipment is then selected; physical size and service access areas are specified in catalogs. For a more general floor area estimate, see the sizing nomographs in Tables 10.3 and 10.4. Even more generally, equipment size may be estimated using these guidelines:
For ordinary equipment: 500 ft2/ton (46.5 m2/ton)
High-efficiency chillers: 1000 ft2/ton (93 m2/ton)

The procedure for sizing cooling equipment is more complicated than for heating because latent as well as sensible heat must be considered. The example following refers to the psychrometric chart; a review of Figs. 8.35 to 8.42 may be helpful.

Fig. 9.46 Sizing cooling equipment using the psychrometric chart (see detailed charts in Appendix F). (a) Finding the conditions for the supply air. SI values are: 12.8, 23.9, and 26.7oC DB. (b) Finding the conditions for the return air–outdoor air mixture. SI values are: 23.9 and 32.2oC DB; 23.9oC WB. (c) Points A, B, C, and D, shown within the building and its cooling equipment. SI values are: Point A (23.9/16.9oC); Point B (12.8/10.7oC); Point C (29.4/21.8oC); Point D (32.2/23.9oC); outdoor air (1321 L/s); exhaust air (1321 L/s); supply air (2010 L/s); return air (689 L/s); room (23.9oC).

11.9  SYSTEM TYPES AND LARGE BUILDINGS
(a)       HVAC SYSTEM TYPES
Large buildings have so many thermal zones, and there are so many ways to move heat from one place to another, that hundreds of HVAC system variations have been devised. A few of the most typical are introduced in this section; the following section treats in detail the major components of HVAC production and delivery. Finally, some common variations on each of the four main system classifications are presented.
One way to classify HVAC systems is by the media used to transfer heat. Although thousands of liquids and gases can be used as carriers of heat, the three most common in building applications are air, water, and refrigerant. Traditionally, there are four main system classifications:
Direct refrigerant systems
All-air systems
Air and water systems
All-water systems
In the last three cases, the heating/cooling production equipment typically is located centrally in a large building, often rather far from the thermal zones it serves. The air-handling components may be either centrally served or served floor by floor. Distribution tree size and placement thus become important issues when those systems are selected. In direct refrigerant systems, the heating/cooling machine usually is located adjacent to the zone(s) it serves; thus, the machine’s environment—the microclimate it creates and its needs—relative to the zone’s environment becomes an important consideration.
(i)        Direct Refrigerant Systems
These systems nearly eliminate the distribution trees of air or water, relying instead on a heating/cooling device adjacent to or within the space to be served. Thus, they are prevalent in skin-load-dominated buildings with extensive perimeter zones; these tend to be smaller buildings.
(ii)       All-Air Systems
The more common variations on all-air systems are shown in Fig. 10.12. Because air is the only heat transfer medium used between the mechanical room (central station) and the zones it serves, and because air holds much less heat per unit volume than water, the distribution trees for this class are quite thick—therefore more building volume—but can promise comfortable results because they effectively regulate IAQ, temperature, and humidity. This approach concentrates the mechanical equipment and its maintenance within a central mechanical room, freeing the rest of the building from repair interruptions and worries about water leaks from heating/cooling pipes. However, this family does less well with perimeter zones in cold climates.
Sometimes, to reduce duct sizes, higher velocities are used for supply air. This generates more noise and higher friction, resulting in more energy used by fans; higher velocity should be used only sparingly, where space limitations are extreme.
Fig. 10.12 (a–f) Schematic diagrams of all-air HVAC systems. An underfloor air supply is shown here to simplify the diagram, but a ceiling supply is much more common.
For comfort, however, these systems are, over all, the best. The quantities of air moved through the central station(s) are heated or cooled, humidity-controlled, filtered, and freshened with outdoor air—all under controlled conditions. Within the zones, supply registers and return grilles allow a well-planned stream of conditioned air to thoroughly permeate all work areas.
With all-air systems, air quality can be manipulated through pressure control: negative pressures in odorous or excessively humid locations (kitchens, toilet rooms, pet shops in shopping centers, etc.), positive pressures in shopping malls, corridors of apartment houses, stair towers, and so on. The difference in pressure sets up an overall direction of airflow that helps prevent the spread of odorous or otherwise contaminated air and can help manage smoke in a fire (see Chapter 24). Positive pressures in connecting spaces help each adjacent space keep its air to itself.
Single-Zone Systems
This (Fig. 10.12a) is the common small-building forced air system controlled by a single thermostat. This system offers a very low first cost.
Single-Duct, Variable-Air-Volume (VAV) Systems
This (Fig. 10.12c) is the most popular large-building system of recent years. Its single duct requires less building volume for distribution than do multiduct systems, and the variation of air volume flow rate (rather than of air temperature) saves energy relative to the single duct with reheat (Fig. 10.12e). Depending on outdoor conditions and prevailing indoor needs, the central station supplies at normal velocity either a heated or a cooled stream of air. Automatic volume controls (linked to each zone’s thermostat) adjust the volume admitted to that zone within an air terminal diffuser (often located above a suspended ceiling). When the central station is supplying cold air, a zone that needs more cooling will get more air; an unoccupied room with no internal gains, or a space with heat loss through an exterior wall, will get less air. Clearly, such a system is well suited to serve the interior, always-hot zones of internal-load-dominated buildings. Less clear is its suitability for the perimeter zones of buildings in cold, cloudy conditions.
This system is an energy-saving option ideally suited to the internal zones of large buildings, where cooling is always needed. However, it can also be adapted to serve the entire building; with important savings over the constant-volume (CV) systems. The air-handling units (fans, etc.) for each thermal zone are sized to meet the peak demand on that zone. In most CV systems, the fans always run at this peak-condition speed, even though peaks are often of rather short duration. In VAV systems, fans run at peak speed only during peak hours. This obviously saves considerable energy needed to run fans. When the building uses only one or a few central fans, a VAV system will require smaller fans because the fans need to meet only one peak condition at a time and do not have to be sized for all zones’ peak flows. The variation in demands on the fan can be met either by selecting variable-pitch blades or (less expensively) by varying the speed of the fan.
Where a VAV system is used, provision must be made for at least code-minimum fresh air levels, sometimes resulting in overcooling or wasteful reheating. Although VAV systems are typically less noisy than CV systems (because less air is moving), the air motion noise varies with the volume, and variable noise sources are inherently more noticeable than steady ones.
Within (or near) the spaces it serves, the VAV system typically needs a mixing box or terminal; this is often placed above a suspended ceiling or below a raised floor. Of the several terminal types available, the standard and most simple one is shown in Fig. 10.53. This terminal serves not only to vary the quantity of air, but also to both attenuate the noise and reduce the velocity of air from the main trunk of the distribution tree. High velocity is commonly used in main ducts because it reduces the size (thickness) of these critical large portions of the tree.
Fig. 10.53 The simplest type of VAV terminal, which pinches back the volume of incoming air as cooling loads decrease. Boxes can range in size: 8–18 in. H by 24–67 in. L by 14–54 in W (200–460 x 610–1700 x 360–1370 mm). (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 9th ed.; © 1994 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
The Utah Department of Natural Resources in Salt Lake City (Fig. 10.54) utilizes VAV throughout its three-story, 105,000-ft2 (9755-m2) office building. Air is supplied from linear ceiling diffusers located between the rows of indirect luminaires parallel to the windows. Return air is taken into the suspended ceiling plenum. Highly insulated walls, windows, and roof lessen the need for perimeter heating, although a supplementary warm water heating system is built in below the windows to offset perimeter heat losses. The lower 1 ft (300 mm) is a slot where room air is drawn in by convection, and a perforated metal grille at the top forms the windowsill.
Fig. 10.54 The Utah Department of Natural Resources building features lightshelves, direct and indirect evaporative cooling, and an economizer cycle. Supply air is distributed by a VAV system with ceiling diffusers located between luminaires. (Based upon the design by Gillies Stransky Brems Smith Architects, Salt Lake City, UT.)
The building is elongated east–west for best daylight control and winter solar gain; lightshelves even the daylight penetration. With almost no east- or west-facing glass, the building relies on 4200 ft2 (390 m2) of south glass and 3800 ft2 (353 m2) of north glass to serve its width of 125 ft (38 m). Supplementary indirect lighting is controlled by photocells and dimming ballasts.
Utah’s hot, dry summers permit a four-stage approach to cooling. First is the economizer cycle at higher temperatures, direct evaporative cooling is used; then indirect evaporative cooling (see Figs. 9.10 and 9.11) utilizing an oversized cooling tower; and finally (estimated at about 10 days per year), a conventional chiller.
There are several variations on the basic VAV system, some of which respond to the problem of minimum airflows for rooms with little thermal load or to the desire to serve both interior and perimeter areas with the same HVAC system.

Fan-Powered VAV Systems
This variation (Fig. 10.12f) allows individual units to heat when the main supply system is cooling; it might therefore serve perimeter zones. In this case, the cool air is reduced to the minimum required for acceptable indoor air quality (IAQ), and the unit’s fan draws additional air from a ceiling (or floor) plenum, heating it as required.
To maintain minimum fresh air, VAV terminals are usually set so that they cannot be entirely closed off, ensuring some outdoor air at all times. If this fresh air minimum, entering at low velocity, does not provide the desired air motion and mixing within the room, VAV terminals can be equipped with fans (Fig. 10.12f), which are activated as needed with decreasing incoming air volume. These self-contained fans mix room air with incoming air to provide an air stream of the right temperature and velocity to maintain comfort.
This approach to VAV is sometimes taken when simultaneous heating and cooling are needed, such as at perimeter zones, which can generate sizable heating needs while the rest of the building needs cooling. Another approach is to utilize an induction-type VAV terminal, with which air heated by electric lights is induced to join the incoming cool air stream. Greater heating needs often require the use of reheat terminals supplied by a circulating hot water system or by electric resistance heating. This reheat application is more energy efficient than the standard CV reheat system described in Section 10.5e, because a much smaller volume of air is first cooled, then reheated. The water or electric coils can be incorporated either in the VAV terminal or in the ductwork between the terminal and the space it serves.
An example of a floor-by-floor VAV system (Fig. 10.55) is a 1-million-ft2 (92,900-m2) 28-floor Chicago office building designed by Skidmore Owings & Merrill. This mid-rise approach to office towers utilizes three “stacked” atriums to relieve the monotony of the wide interior floors. Another result is lower structural and energy costs per square foot relative to conventional high-rise structures. The cube like shape of the building exposes less skin area (38% of which is in insulating glass) to Chicago’s cold winters; electric lighting at about 1.8 W/ft2 holds down internal gains. To accommodate the differing schedules and comfort needs of a variety of tenants, each floor is provided with two VAV supply fans that can be operated at night and on weekends, independently of the rest of the building. Each floor’s mechanical core has one exterior wall (on an alley) to facilitate fresh air intake/stale air exhaust. The perimeter heating system is electric resistance fin radiation; an economizer cycle provides cooling with outdoor air below 55ºF (13ºC).
Fig. 10.55 Floor-by-floor fan rooms supply VAV for this Chicago office building. (a) Exterior view of 33 West Monroe. (Photo by Merrick, Hedrick-Blessing.) (b) Plan of a typical lower floor; two fan rooms occupy the space along the exterior wall. (c) Section perspective showing stacked atriums. (Courtesy of Skidmore, Owings & Merrill, Architects-Engineers, Chicago.)
Multizone Systems
A multizone system (Fig. 10.12b) is a collection of single-zone systems served by a single supply fan; such systems rarely exceed eight zones per air-handling unit. Because each zone has an individual centrally conditioned air stream, the total distribution tree volume grows to astonishing size with only a few zones. The central station produces both warm and cool air streams, which are mixed at the central location to suit each zone. The single-return air stream collects air from all zones (as is the case for the other systems in this class). Energy savings result when a “bypass” deck is added, allowing each zone to choose some unthermally treated return air as part of the supply air. Leakage between zones at the decks of hot and cold coils is common. Return air from all zones is mixed within one return duct; a bypass at the heating and cooling decks requires more space but allows such air to be mixed with supply without undergoing unnecessary heating or cooling. These systems are more likely to be found on medium-sized buildings or on larger buildings in which smaller central stations are located on each floor.

Single-Duct with Reheat
This (Fig. 10.12e) system (along with VAV) has the smallest distribution tree of this class, because at each zone the only object added to the duct is a small reheat coil (heat provided by steam, hot water, or electric resistance). (Technically, this could also be called an air and water system.) The central station provides a single stream of cold air that must be cold enough to meet the maximum cooling demand of any one zone. All other zones reheat this air as needed. In cold weather, outdoor air at temperatures as low as 38ºF (3ºC) can be used; the colder this single central air stream, the less air need be circulated (and the smaller the ducts). For buildings with large interior zones in most U.S. climates, however, the central air stream must be cooled most of the time; then more energy must be spent to reheat the air stream at most zones. Once a widely-used system and usually supplied at constant volume (CV), single-duct reheat systems (Fig. 10.12e) now are severely restricted by many building codes and energy standards because of its energy wastage. For high-pressure and high-velocity main ducts, CV reheat boxes (Fig. 10.56a) are used to control the temperature, pressure, and velocity of the supply air, and to mitigate noise. For simpler all-low-velocity systems, simple duct insert heaters (Fig. 10.56b) can be used. These are sized to fit the duct, which needs to be enlarged only slightly to accommodate them.
Fig. 10.56 Single-duct system with reheat coils (a) requires a simple terminal where velocity and pressure are reduced. Box capacity ranges from 200 cfm (95 l/s) at 12 in. by 50 in. by 22 in. (305 x 1270 x 560 mm) to 5000 cfm (2360 L/s) at 20 in. by 60 in. by 80 in. (510 x 1520 x 2030 mm) (b) Electric coil within a duct. Size varies to suit duct dimensions (and capacity, which can range from 0.3 to 2000 kW). (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 9th ed.; © 1994 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
Dual-Duct, Constant-Volume Systems
This (Fig. 10.12d) is still considered the “Cadillac” of HVAC systems, not only because of its superior comfort control and flexibility for simultaneous heating/cooling zones, but also because of its high first cost, large size, and high-energy usage. The dual-duct system is rarely installed now, except in hospitals. Building volumes needed for a dual-duct system’s three (two supply, one return) full-sized air distribution trees are harder to justify, given that VAV systems can provide acceptable comfort for most common spaces. An example of a dual-duct office building was seen in Fig. 10.11, San Francisco’s International Building.
The mixing boxes (terminals) of dual-duct systems are similar to those of other all-air systems (Fig. 10.57) but are generally larger for the same airflow capacity. This adds still more to their impact on building volume. They are expensive and may require maintenance. Although most dual-duct systems are constant volume, they can be VAV when the reduction in airflow is no more than 50% below the maximum. For details on other double-duct variations, see Grondzik (2007).
Fig. 10.57 High-velocity, dual-duct terminal providing mixing and attenuation (pressure and sound reduction). (a) Controlled from a thermostat, this unit blends the supply air streams and delivers air to the zone at the selected temperature. (Courtesy of Anemostat.) (b) Typical mixing box dimensions are: 10 by 50 by 30 in. (250 x 1270 x 760 mm) for a 400 cfm (190 L/s) unit to 18 by 60 by 66 in. (460 x 1520 x 1680 mm) for a 5000 cfm (2360 L/s) unit (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 9th ed.; © 1994 by John Wiley & Sons, Inc. Reprinted with permission from John Wiley & Sons, Inc.)
(iii)      Air and Water Systems
Several variations on air and water systems are shown in Fig. 10.13. Most of the heating and cooling of each zone is accomplished via the water distribution tree, which is much thinner than the tree needed by air. For air quality—filtering, humidity, freshness—a small, centrally conditioned air stream, equal to the total fresh air required, is provided. Thus, several distribution trees are involved, yet the total space they require is almost always less than that required by all-air systems.
Fig. 10.13 (a–c) Schematic diagrams of air and water HVAC systems. An underfloor air supply is shown here to simplify the diagram, but a ceiling supply is more common (NOTE: why not show all ceiling?). In (b) the supplementary air is often delivered directly to the fan-coil unit.
Exhaust air may be gathered in a return air duct system, making heat recovery possible. Or (as a cheaper alternative) air can be exhausted locally to avoid the construction of yet another distribution tree. If the water distribution provides either heating or cooling only, it is called a two-pipe system (shown throughout Fig. 10.13). If it provides simultaneous heating and cooling, it is a four-pipe system. (Three-pipe systems are a lower-first-cost alternative allowing simultaneous heating and cooling [from two supply pipes with a single return pipe], but they waste energy by mixing hot return and cold return water flows in one return pipe. They are no longer permitted in most locales.)
These systems have the design complexity—and first cost—of supply-and-return distribution trees for both water and air. This disadvantage is offset by the space-saving advantages of thinner water trees, the possibility of only one air tree (supply), and the superior comfort characteristics offered by air. When only fresh air is centrally treated and distributed, only an equal quantity of air need be exhausted, which rarely needs an extensive distribution tree. Because this exhaust air is not recirculated, these systems are attractive for hospitals and other buildings in which the mixing of air between zones is undesirable. When return air is used rather than exhausted, it forms a small percentage of the supply air.
These systems are most often used in perimeter zones, where extensive extra heating (winter) and cooling (summer) is readily provided by water. If only one water supply tree is used (two-pipe system), there is a problem of deciding when to change over between heating and cooling. In-space maintenance is required (filters in particular), and humidity is less tightly controlled than in all-air systems. In most air and water systems, the air is circulated in a cooled and dehumidified condition and heated by the water for most of the year.
Induction Systems
The induction terminals typically are located either below perimeter windows (Fig. 10.58) or above a suspended ceiling. Condensation from the cooling coil in summer and the need to clean the filters of the induced room air make under-window locations much easier to maintain, even if they intrude on the floor area available.
A high-velocity (and high-pressure), constant-volume fresh air supply is brought to each terminal, where it is forced through an opening in such a way that air already within the room (bypass, or secondary, air) is induced to join the incoming jet of air. Each terminal then mixes 20% to 40% incoming fresh air with the 80% to 60% room air that is induced to flow along with the fresh air, all passing over finned tubes for heating or cooling and circulating this mixture of air to the space. Thermostats control the unit’s output by controlling either the flow of the water or the flow of secondary air.
In two-pipe systems, either hot or cold water—not both—is available to temper this air mixture ordered by the thermostat linked to each terminal. Because cool air is distributed for most of the year, the two-pipe system is largely in heating mode (in colder climates).
In four-pipe systems, the availability of both hot and chilled water makes it possible to switch instantly from heating to cooling for excellent thermal control.
Fig. 10.58 Installation of a high-velocity induction unit. Conditioned 100% outdoor air is brought in through a high-velocity duct to provide ventilation and to induce the circulation of room air. It is attenuated and silenced in the chamber (2), and then, through jets in the front of the plenum, it induces the flow of room air, which is heated or cooled at the fin coil (3). The lint screen (11) requires periodic maintenance for proper airflow. (Courtesy of Carrier Corporation. Redrawn by Erik Winter.)
The CBS Tower in New York City (Fig. 10.59) uses a high-velocity induction system for its perimeter zones. The triangular black granite-faced exterior columns give the façade a three-dimensionality quite unlike the slick glass boxes of its contemporaries (it was designed by Eero Saarinen and built in 1962). These thick columns enclose the perimeter’s distribution trees; every other column contains the high-velocity air supply; in between, every fourth column contains the supply water and every alternate fourth column the return water trees. These constant column sizes belie the thicker air, thinner water distribution trees. The interior zone of this tower is served by a VAV system; return air from both perimeter and interior zones is collected at the core.
Fig. 10.59 The identical triangular exterior columns of the CBS Tower in New York City (a) enclose a variety of distribution trees. (b) Plan of a typical floor; perimeter zones are served by a high-induction air and water system, interior zones by VAV. All return air is collected at the core. (c) At the perimeter, column type A contains an air supply tree feeding the high-velocity induction unit on either side of the column. Column type S contains the supply water tree, hot in winter and cold in summer. Column type R contains the return water tree. (From David Guise, Design and Technology in Architecture; © 1985, John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
Fan-Coil with Supplementary Air
This system (Fig. 10.13b) is closely related to the preceding induction system; however, this system uses a fan at each unit, rather than high-velocity primary air, to move the mixture of supply and room air through the unit. A typical arrangement is a supply air plenum over a corridor, feeding horizontal fan-coils above the suspended ceilings of spaces on either side. These fan-coils draw some supply air and mix it with room return air to maintain desired temperatures. If condensation and resulting pans of standing water are to be avoided, the supply air must be sufficiently dehumidified.
Fan-coils are very widely used, both in conjunction with conditioned supply air (air and water) or as stand-alone units that take in their own fresh air at the perimeter (all-water), or even as room-air-only units. A building that uses both fan-coil units (at entries) and radiant panels (in offices) is shown in Fig. 10.60.
Fig. 10.60 An innovative Canadian office building, Green on the Grand at Kitchener, Ontario. The cooling system combines a radiant panel system (a) with a dehumidified summer ventilation system (b). The building also features daylighting, a gas absorption chiller/heater, rainwater pond heat rejection, and two-stage heat recovery. (Courtesy of Enermodal Engineering, Ltd., Kitchener, ON.)
NOTE: Plan and Section of the Building???
Radiant Panels with Supplementary Air.
Radiant heating systems (Fig. 10.13c) have a large following of comfortable users; radiant cooling systems are fairly commonly used in Europe and are getting a closer look in the United States. Large areas of radiant surface can be used to offset large losses of bodily radiant heat, as in the case of large areas of cold glass on a winter day, or when users are both scantily clothed and sedentary. In summer, such panels can help offset radiant gain from electric lights or large glass areas. The ceiling is often favored for the panel location, because it is uncluttered by the furniture, tackboards, and other items that cover floors and walls. At the 56-story Commerzbank in Frankfurt, Germany, chilled ceilings serve all office floors in conjunction with the ventilation system.
Green on the Grand (Fig. 10.60) is a 23,465-ft2 (2180-m2) two-story office complex in Kitchener, Ontario. Its design combines a very highly insulated envelope, daylighting, water conservation, a gas absorption (non-CFC, non-HCFC) chiller/heater with a heat rejection pond, a separate IAQ system, and heat recovery of exhaust air. It also features a dedicated air system for ventilation and a hydronic system for heating and cooling. The hydronic system uses fan-coils for the entryways and radiant panels on ceilings of office spaces. (The conditioned air in this example is provided by the ventilating system.) Fan-coils are better able to handle the extremes of entryway heat losses and gains. The radiant panels in the offices are used for both heating and cooling, but are sized to meet cooling loads and cover 30% of the ceiling area. Tenants were given their choice of two designs: steel panels suspended below a drywall ceiling (painted to match the ceiling color) or extruded aluminum panels fit into a suspended ceiling system. To prevent condensation on these radiant panels during cooling, the ventilation air is dehumidified.
The ventilation system supplies low on the walls and exhausts high on the walls, a form of displacement ventilation. There are two rates of fresh air: 20 cfm (10 L/s) per person, and (when economizer cycles operate or when more fresh air or more cooling is required) 40 cfm (20 L/s) per person. The exhaust air then flows through two heat exchangers: the first reheats fresh air that has been greatly cooled for dehumidification; the second preconditions the incoming fresh air at the point of entry. The latter heat exchanger is a rotary-wheel ERV capable of transferring both heat and moisture.
Several approaches to the radiant cooling (and heating) panel, independent of a dehumidified air supply, are shown in Fig. 10.61. These approaches assume a minimal floor/ceiling thickness, which could contribute either to reducing the floor-to-floor height or to greater ceiling height (for daylight penetration or displacement ventilation). If a raised floor is used above these concrete slabs, even higher percentage heat gains come from the ceiling.
Fig. 10.61 Approaches to radiant cooling systems. (a) Concrete core system: water is circulated through plastic tubes embedded in the concrete floor/ceiling slab. (b) With carpet, pad, and insulation on the floor, most of the heat radiates to the ceiling. (c) Panel system, usually using aluminum facing, connected to metal tubes. (d) Panel system has the highest percentage of heat to the ceiling. (e) Cooling grid made of capillary tubes embedded in ceiling plaster (or in gypsum board or mounted on ceiling panels). This is the most even surface temperature distribution. (f) Again, a high percentage of the heat is radiated to the cooled ceiling, depending on the use of insulation below the carpet and pad. (From Center for Building Science News, Fall 1994. Lawrence Berkeley Laboratory, CA.)
Radiant cooled floors are sometimes used, as in the Bleshman Regional Day School (Fig. 10.62). The designers of this school for the handicapped recognized that the majority of its users would spend much of their time quite close to the floor and that the colder air near the floor could be uncomfortable, especially in the New Jersey winter. The entire floor is warmed by the supply (ventilation) air in winter, which enters just below windows to counteract the downdraft off cold glass. In summer, the cool supply air first cools the floor and then cools the air in front of the warm glass. The concrete cellular “air floor” provides a thermal mass that helps maintain steady temperatures. Heated or cooled air is provided by rooftop air–air heat pumps, one of which is provided for each cluster of three to six classrooms. This HVAC example is therefore related to the direct refrigerant systems family.
Fig. 10.62 Underfloor air distribution achieved with a cellular concrete air floor serves the Bleshman Regional Day School in Paramus, New Jersey. (Courtesy of Rothe-Johnson, architects.)
Water Loop Heat Pumps
Because individual heat pumps are used, this system is closely related to direct refrigerant systems. It is often considered an all-water system, but may be configured with a central outdoor air supply (as in an air-water system). Heat pumps (water-to-air) either draw heat from the water circulation loop (in heating mode) or discharge heat to it (in cooling mode). For a large building in cold weather, excess heat from the interior zones is used to warm the perimeter zones. The loop’s temperature ranges between 65º and 90ºF (18º and 32ºC); in hot weather, a central cooling tower disposes of the loop’s excess heat, whereas in cold weather a central boiler adds needed heat to the water loop. The loop is sized to carry 2 to 3 gpm (0.13 to 0.19 L/s) per ton, where the total tonnage equals the sum of the capacity of all the individual units (often greater than the actual load).
This approach was diagrammed in Fig. 9.45 and is used in the school shown in Fig. 10.51. In the 175,000-ft2 (16,260-m2) Comstock Center in Pittsburgh (Fig. 10.63), there are six to eight small heat pumps on each of 10 floors, located above the suspended ceiling. Their connecting loop doubles as the building’s wet-pipe sprinkler system supply; this is possible because the heat pumps keep the loop between 65º and 85ºF (18º and 29ºC). Because neither hot nor chilled water supply and water return distribution trees are needed in addition to the sprinklers, there is a substantial first-cost saving, and relatively little building volume is consumed. A 23,000-gal (87,060-L) water storage tank allows daytime heat, rejected to the loop by the heat pumps, to be stored and recalled for nighttime heating. When necessary, penthouse equipment is used: two small boilers to maintain the 65ºF minimum temperature in the water loop or an evaporative condenser to hold temperatures at 85ºF.
Fig. 10.63 The Comstock Center, Pittsburgh, Burt Hill Kosar Rittelmann Associates, architects. (a) The north and west faces. (b) Ground-floor plan. (c) Water loop heat pumps are linked via the sprinkler system on each floor. This is also called a tri-water system. (d) Extract-air windows (see again Fig. 10.50) control infiltration and moderate the perimeter zone temperatures. Fresh air is ducted to the plenum, where it mixes with return air before being treated and recirculated through the heat pumps. (e) The central daylighting atrium is tempered by exhaust air from the offices; exhaust air then leaves via the stack effect. (f) The stack effect also controls summer overheating.
Another feature of the Comstock Center is its use of air extract windows (see Fig. 10.50) to control infiltration and to moderate the perimeter zone’s temperatures. After the return air passes up inside this window and arrives in the ceiling plenum, it is mixed with fresh air, then tempered and recirculated by the heat pumps.
The Comstock Center also utilizes a large daylighting atrium, whose temperature control is provided largely by exhaust air from the offices; the stack effect is utilized to provide natural ventilation on its west face in summer conditions.

(iv)      All-Water Systems
The more simple-appearing all-water systems are shown in Fig. 10.14. These systems only heat and cool; the distribution trees are indeed slim. Air quality is dealt with elsewhere—either locally, by means of infiltration or windows; or by a separate fresh air supply system; or simply by fresh air from an adjacent system, such as a ventilated interior zone. This ambiguity about fresh air leads to similar ambiguities about whether a system is air-and-water or all-water. A fan-coil terminal is often employed so that air motion occurs along with heating or cooling. (Sometimes the fan-coil unit is located against the exterior wall so that fresh air may be brought in and mixed with the room air through the fan.) Both baseboard and valence (above-window) units are also commonly available. Because water is often condensed from the room air when cooling is in progress, a drain line is required; water standing in drain pans is unhealthy. Exterior air intake grilles can easily be added when fan-coil units are located below windows to allow the local provision and tempering of ventilation air; this is especially common in motels, hotels, and apartments.
Fig. 10.14 Schematic diagram of all-water HVAC systems. Four-pipe distribution trees require smaller volumes than do those for air systems; however, less thorough provision of outdoor air is a potential concern.
Because air is handled so locally, there is very little mixing of air from one zone to another, making this attractive where potential air contamination (or smoke from a fire) is a special concern. It is also an easy system to retrofit. However, maintenance is high; filters in each fan-coil must be cleaned, and drain pans are potentially problematic.
Two-pipe water distribution systems were shown throughout Fig. 10.13. They provide either heating or cooling. One pipe is for supply, the other for return. In a typical large-building application, they are used for heating in winter, cooling in summer. This raises the question of what changeover period will be required, a problem made much easier with the following alternative system.
Four-pipe systems are shown in Fig. 10.14. They allow quick changeover heating and cooling, utilizing two supply and two return pipes. A four-pipe system also allows for simultaneous cooling and heating in different zones within a single distribution system.
Fig. 10.64 (a) The innards of a simple fan-coil unit without outdoor air connection; showing a standard below-window unit. Widths can range from 2.5 to 7 ft (0.8 to 2.1 m), height is around 26 in. (660 mm), and depth between 9 and 12 in. (230-305 mm). (b) High-rise unit for corners or cabinet locations. These can be stacked to reduce connecting piping runs and simplify condensate collection. (c) Vertical units for below large windows. Low-profile unit height is around 14 in. (360 mm). (d) Horizontal units for above-ceiling locations. (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 10th ed.; © 2000 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
Fan-coil units are inherently noisy due to the fan, so careful attention to sound ratings is required when background sound levels must be low. In offices, a relatively high background sound level can help to maintain acoustic privacy at the workstation; the sound of the fan-coil provides reassurance that some kind of air treatment system is at work.
All-water perimeter systems with local fresh air can also take the simple form of operable windows with hot water finned-tube radiation. An expressive variation on this approach appears in the reading rooms and staff workrooms for the Seeley G. Mudd Library at Yale University (Fig. 10.65). Limestone spandrels are curved in to allow fresh air to enter these smaller-perimeter rooms just below the windows, where hot water finned-tube radiators are available when needed. The incoming fresh air replaces exhaust air, which flows out of the upper operable windows. The remainder of the building has conventional forced-air heating and cooling.
Fig. 10.65 Local fresh air and finned-tube radiation at the perimeter. The Seeley G. Mudd Library at Yale University; Roth and Moore, architects. (a) Exterior, showing curved limestone spandrels, along which flows incoming fresh air. (Photo © Steve Rosenthal.) (b) Section showing the fresh air intake, finned-tube radiation, and upper operable sash for exhaust air. This system is used for the smaller reading room and staff workrooms at the perimeter.
(b)       AIR DISTRIBUTION WITHIN SPACES
When large office buildings had far greater interior (core) areas than perimeter areas and were filled with less-efficient lighting at high luminance levels, they were considered to always need cooling. Because cool air supplied at the ceiling would naturally fall toward the level of occupants, and because suspended ceilings were ubiquitous, supply air from the ceiling was almost universal. The suspended ceiling was also tempting for the return air provisions, whether as a plenum or for another ducted system. With both supply and return air at the ceiling, the danger of short-circuiting arose: supply air heading quickly for the return opening, with resulting shortages of both cooling and IAQ. In this section, we first look at approximate duct sizing and then consider three air distribution systems for multistory office buildings.
(i)        Air Ducts
Duct sizes (in cross section) are frequently of interest early in the design process. Duct depths can help determine floor heights; duct cross sections influence the sizes and shapes of the vertical cores that serve multistory buildings. An approximation of duct size can be obtained as follows:
1.         Determine the quantity of air to be distributed through the largest duct, using Table 10.5, or the ACH, from a calculation of night ventilation of thermal mass. This will usually be expressed in cubic feet per hour (cfh).
2.         If necessary, convert cfh to cfm:
Match to format (including italics) and symbols in 10e
cfh ´ 1 h\60 min  cfm
3.         Find the maximum velocity of this air within the duct from Table 9.4, expressed in feet per minute (fpm).
4.         The approximate minimum required cross-sectional area of duct A is then
Ain.2  volume of air (cfm)\velocity (fpm)
´ 144 in.2/ft2 ´ friction allowance
where the friction allowances are

• round ducts  1.0 (may be neglected)
• nearly square ducts (ratio of width to depth, 1:1)

small (1000 cfm)  1.10
large (1000 cfm)  1.05

• thin rectangular ducts (ratio of width to depth, 1:5)  1.25

Then check this against the recommended duct cross-sectional area from Table 10.4. Remember that the minimum duct area will carry a penalty of increased noise and friction. Techniques for noise suppression in ductwork are discussed in Chapter 19.
(ii)       Ceiling Air Supply
This approach is so widespread that many lighting fixtures are made to either serve as diffusers for supply air or as intakes for return air. As return air fixtures, they are especially effective because they remove much of the heat from electric lighting before it can contribute to overheating the office space below. This makes less work for the cooling equipment.
When individual offices are enclosed, a supply diffuser and return grille ensure air circulation. However, as open office space proliferated, concerns grew about whether the workspace cubicles were being adequately served with conditioned air. Air diffusers and grilles (Fig. 10.46) on the ceilings are uncluttered by furniture and independent of rearrangements of workstation cubicles. Canadian research in the early 1990s indicated that the most important variable was the quantity of the airflow, not the location of the diffusers relative to workstations or whether the cubicle partitions have a small gap where they meet the floor.
Fig. 10.46 Common air distribution outlets. (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 10th ed.; © 2000 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
Are higher ceilings better for air distribution? There seems to be a trade-off between more vertical clear space (between ceiling and cubicle partitions) that would allow a wider range of air distribution and the increased distance from the actual occupant who should be receiving the conditioned air. The increased distance would allow colder air to be distributed, resulting in smaller ducts, and could also allow higher velocity of supply air, further reducing duct size. Higher ceilings allow deeper daylight penetration and represent a larger “pool” of air that is slower to become polluted by occupants or office equipment. Higher ceilings also encourage underfloor distribution, as seen in the next subsection.
When the noise from forced-air diffusers is critical, as in telecasting and recording studios, pinhole-perforated diffusers (Fig. 10.47) can provide large quantities of low-velocity air. This is an especially challenging combination of high heat gains (from lights) and low background noise requirements. Again, ceiling distribution leaves the floor unencumbered.
Fig. 10.47 Pinhole-perforated diffusers (above a lighting grid) provide large quantities of low-velocity air to the television studio at the Community Media Center of Santa Rosa, California. The background noise from more typical HVAC systems would be amplified by sound recording equipment and hence would be unacceptable. (© TLCD Architecture, Santa Rosa, CA.)
(iii)      Underfloor Air Supply
As open office spaces have grown larger in area and their ceilings higher (for daylight penetration and indirect lighting), and as concerns about IAQ have deepened, the floor has gained in popularity as the air source location. Underfloor air supply introduces supply air through floor outlets, cooled to just below the design room temperature, at a low velocity. (Displacement ventilation supplies air at very low velocity, such that there is little to no turbulence in the space). Furthermore, fresh air supply rates are typically near 45 cfm (22 L/s), compared to about 30 cfm (15 L/s) often used in ceiling supply systems. This high-volume, low-∆t, low-velocity combination requires greatly increased duct sizes, essentially provided by using the plenum formed by a raised floor system as a duct. The supply air rises quickly to the occupant level, drawn upward by the stack effect of heat gain from lights, people, and office equipment (and by supply air pressure). It continues to the ceiling, where it stratifies and where various approaches to return air collection are located. Freshest and coolest at the floor, hottest and most stale at the ceiling, this system promises better IAQ and thermal comfort than would be available from ceiling supply–return systems. Furthermore, the outlets at the floor are adjustable and easily reached by the office workers.
The raised floor is typically supported on a 2 ´ 2-ft (600 ´ 600-mm) module. The module that contains the diffusing outlet often also contains an outlet for power and cable. This diffuser is placed off-center within the modular unit, further expanding the variability of the floor opening relative to furniture placement, because each plan rotation of the square module places the outlet slightly differently. Some codes restrict the height of the raised floor plenum; many codes require specially coated wiring within the plenum because moving air will wash over the wires for the life of the building.
The overall floor-to-floor height needs to be great enough to accommodate both the raised floor and the higher office ceiling (at least 9 ft [2.7 m] to accommodate warm air stratification). However, elimination of the suspended ceiling and the supply air ductwork also eliminates the suspended ceiling cavity. This tends to expose the lighting, fire sprinkler, and public address systems below the structural ceiling, although the latter can be distributed in the raised floor cavity above and puncture the structural ceiling as required.
The structural slab below the raised floor can be of concrete, and this thermal mass can be night-ventilated to store coolth to assist the ventilation air for the next day. The Inland Revenue Building (discussed in Chapter 8) incorporates a night-ventilated raised-floor section within its precast waveform floor sections.
Library Square in Vancouver, British Columbia (Fig. 10.48), utilizes a raised floor to serve its interior zones of book stacks. The library has seven floors, totals 390,000 ft2 (36,320 m2), and exposes its concrete structure as a finished ceiling. The floor-to-floor height is 16.4 ft (5 m). The underfloor supply is fed by VAV boxes and floor fan units (FFU) that blend low-temperature supply air (about one-third) with local filtered return air (about two-thirds) to provide 63ºF (17ºC) air to the pressurized plenum. Flush to the floor, high-induction swirl floor diffusers (8 in. [200 mm] in diameter) produce an upward air motion, taking the fresh-return air blend directly into the occupancy zone (the first 6.5 ft [2 m] of height on each floor). The stack effect of heat from people, computers, and lights takes hot air on to the stratification zone above. Thus, people-generated pollutants tend to rise to this hot, stale air zone, then into return air slots in the precast concrete ceiling. Return air is taken away in insulated ducts within the supply air plenum of the floor above. The plenum height is an unusually generous 2 ft (600 mm) high; it also contains the sprinkler distribution and all wiring systems. The warm, dry summer climate supports a night ventilation system, flushing the plenum and storing coolth in the exposed concrete structure. This has reduced the use of an ice-storage system.
Fig. 10.48 Library Square, Vancouver, British Columbia, supplies conditioned air to the central book stacks via an underfloor plenum. This displacement ventilation results in return air taken through openings in the exposed structural ceiling. (Section adapted from information supplied by Blair McCarry, Keen Engineering, North Vancouver, BC.)
(iv)      Workstation Delivery Systems
The air delivery system shown in Fig. 10.49 allows a variety of individual controls at each workstation. As developed by Johnson Controls, Inc., these systems are called Personal Environments® systems. A mixture of outdoor and recirculated indoor air, called primary air, is brought from the main duct (or floor plenum) to each Personal Environments system’s mixing box in a duct carrying at most 120 cfm (56 L/s) but typically less. Each worker can adjust the supply air temperature, the mixture of primary and locally recirculated air, air velocity (and therefore volume) and direction, and radiant supplementary heat (below desk level). Task lights can be dimmed, and the background (masking) sound level is adjustable. The worker has almost as much environmental control at each office workstation as a driver has in an automobile’s front seat. When the workstation is unoccupied, an occupancy sensor shuts the system down, maintaining a minimum airflow of 12 cfm (5.7 L/s).
Fig. 10.49 The Personal Environments® system provides each workstation with a fan, air filters, an air-mixing box, and a background sound (white noise) generator. The control panel allows adjustment of task lighting, background sound, fan speed, primary/recirculated air mixture, and radiant heating. Two diffusers distribute both air and background sound. Diffusers are adjustable about both horizontal and vertical axes. The below-desk radiant heating panel warms the lower body. (Courtesy of Johnson Controls, Inc., Milwaukee, WI.)
(v)       Alternative Supply/Return Systems
So far, the supply/return systems have been either both from the ceiling or supply from the floor and return at the ceiling. Supply from the ceiling, return through the floor is another possibility. The fresh air gets to the occupancy level later, and individual control is much more difficult, but debris from the floor falls into a return, not a supply, plenum. There seems less danger of debris falling into a floor supply diffuser while the system operates; it is during fan-off periods, which may include custodial maintenance that this possibility increases.
An alternative approach with return at the perimeter is shown in Fig. 10.50. It is variously called an air-extract window, an air curtain window, or a climate window. Developed in Scandinavia in the 1950s, this is a triple-glazed window that passes room air between a typical outer double-glazed window and an inner single pane. The inner pane thus is kept at very nearly the same temperature as the room air, which greatly increases comfort near windows on very cold (or very hot) days. Venetian blinds are often inserted in this cavity, where they can intercept direct sun and redirect its light toward the ceiling. The solar heat intercepted by the blind is carried off by the room air to a plenum above the ceiling, where such air can be either exhausted or recirculated and its heat content either reclaimed or rejected. The U-factor of these windows is dependent upon the rate of airflow within the glazing panes (Fig. 10.50c); typical flow rates are 4 to 6 cfm per foot (6 to 9 L/s per meter) of window width.
Fig. 10.50 Air extract window (also called an air curtain or climate window). (a) Cross section showing the window as a solar collector. (b) Distribution of solar heat in the collector mode. (c) The window as a “solar chimney” exhausting hot air. (d) The U-factor of these windows varies with the rate of airflow within the glazing: the typical flow rate is 4 to 6 cfm per foot (6 to 9 L/s per meter) of window width. (From articles by D. Aitken and O. Seppanen in Proceedings of the Sixth National Passive Solar Conference; © 1981 by the American Solar Energy Society, Boulder, CO.)
The Ocosta Junior/Senior High School at Westport, Washington (Fig. 10.51), uses these windows on the south façade. (For an example of such windows in a larger U.S. building, see Fig. 10.63, which shows the Comstock Center in Pittsburgh.) The classrooms have photocells that switch off some lights nearest the windows when daylight is adequate, and water loop heat pumps help to transfer passive solar heat from the south side to the sunless north side rooms. Excess heat in the Ocosta school is stored in an underground water tank. The compact plan, highly insulated envelope, and mass capacity within the school reduce the need for a boiler to supplement the winter temperature of the water loop. During consistently warm weather, the BMS exhausts the air from the window to the outdoors.
Fig. 10.51 The Ocosta Junior/Senior High School at Westport, Washington, uses air extract (air curtain) windows to help distribute solar energy from the south side (to the right in section a) to the north classrooms. (b) Detail through the south-facing windows. Photocells control the electric lights during daylight hours; venetian blinds direct sunlight toward the ceiling and deeper into the classroom. (Courtesy of Burr Lawrence Rising Bates Architects, Tacoma, WA. Redrawn by Erik Winter.)
Supply openings in vertical risers are relatively rare but at times function effectively. The Frederick Meijer Gardens (Fig. 10.52) near Grand Rapids, Michigan, is a huge indoor tropical rain forest. This glass structure is more than 65 ft (20 m) high; it has more glass area than floor area—great for a high daylight factor, but imagine the heating loads! High humidity must be maintained, yet water condensation on leaves is undesirable. A strong horizontal airflow could be helpful to the plants, acting as wind would to develop a stronger root structure.
Fig. 10.52 With more glass area than floor area, the Frederick Meijer Conservatory near Grand Rapids maintains a tropical environment in Michigan’s winter. (a) A perimeter tunnel delivers air evenly around the walls, heated by finned tubes just above the planting level. (b) A separate “wind” system provides a continuous breeze from diffusers in the vertical ducts to strengthen plants’ root systems. (c) A fogging system maintains tropical humidity while adding evaporative cooling under summer conditions. (Photos by Roger Van Vleck; courtesy of Fishbeck, Thompson, Carr & Huber, Ada, MI.)
The solutions here begin with a perimeter tunnel serving as an air supply duct, delivering air evenly around the glass walls, heated by finned tubes at planting level that are largely hidden by foliage. The tunnel insulates the planting beds from the extremes of the exterior, maintaining constant ground temperatures year-round. A separate “wind” system continuously feeds air to vertical ducts with several diffusers (Fig. 10.52b), making leaves and branches sway. Humidity is provided by a high-pressure water system that creates fog (very small water particles). This is located high in the space (but also near waterfalls and a stream for special effects). This is also an evaporative cooling system to counteract summer overheating. Peak airflow is needed only for cooling; return air is taken through a large opening hidden behind a waterfall. About one-third of the peak supply air is returned; the remainder is exhausted at the top of the structure through operable vents high in the glass roof. A BMS controls the roof vents, the fogging system, the finned-tube heating system, rolling shading screens and the heating/cooling equipment in a basement, all in response to changing conditions outside this enormous glass house.
(c)        Central Equipment
The many HVAC systems that are included in the categories of all-air, air-and-water, and all-water have in common a dependence on central equipment for the generation of heating and cooling, and/or air quality control. Figure 10.15 shows the basic relationships between some of the major pieces of central equipment and the spaces they serve. This section offers a general guide to some central equipment options and sizes. The consulting engineer chooses such equipment based upon a much more detailed analysis.
Fig. 10.15 Some basic components of HVAC central equipment. (a) A simplified diagram of a cooling cycle, in which chilled water is circulated to air-handling coils and heat is disposed of through a cooling tower. (b) Schematic diagram of major components of central equipment for both heating and cooling.
(i)        Boilers
These devices heat the recirculating hot water system used for building heating. The type of boiler selected depends on the size of the heating load, the heating fuels available, the desired efficiency of operation, and whether single or modular boilers are to be installed. Boiler sizes are commonly stated either in Btu/h of net output or in (gross) boiler horsepower, where, in I-P units,
boiler horsepower  heating load (Btu/h)\% boiler efficiency
´ 33,470 Btu/h per horsepower
In SI units,
boiler horsepower  heating load (kW)\% boiler efficiency
´ 9.81 kW per horsepower
Efficiency depends partly on the number of passes that the hot gases make through the water—the more passes, the higher the efficiency. It also depends on burner efficiency and on regular maintenance. Finally, efficiency is best when the equipment is operating near its capacity. Figure 10.16 compares typical boiler types, including two- and three-pass boilers.
Fig. 10.16 Comparisons of boiler types. (a) Cast-iron sectional type. (b) Two-pass fire tube. (c) Three-pass fire tube. (d) Three-pass wetback Scotch marine. (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 9th ed.; © 1994 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
Fire Tube Boilers
The hot gases of the fire are taken through tubes that are surrounded by the water to be heated. Firebox boilers place the boiler shell on top of the combustion chamber. Scotch marine boilers feature multiple passes of the combustion gas through tubes. Fire tube boilers can be either dryback or wetback. Dryback designs have chambers outside the vessel to take combustion gases from the furnace to the tank. Wetback designs have water-cooled chambers that conduct the combustion gases.
Water Tube Boilers
The water to be heated is taken through tubes that are surrounded by the boiler’s fire. They hold less water than the fire tube models, and so respond faster and can generate steam (where desired) at higher pressures.
Cast-Iron Boilers
Often used in residential and light-commercial applications, these are lower-pressure and lower-efficiency boilers. They do have the advantage of being modular.
In addition to the boilers themselves, there are choices of burner types (depending on the fuel[s] used), burner controls, and boiler feedwater systems. Consult the latest ASHRAE Handbook—HVAC Systems and Equipment for details.
Fossil fuel–burning boilers need flues for exhaust gases, fresh air for combustion, and required air pollution control equipment. The exhaust gas is usually first taken horizontally from the boiler; this horizontal enclosure, or flue, is called the breeching. The vertical flue section is called the stack. Guidelines for sizes and arrangements of breeching and stacks are shown in Fig. 10.17. Local codes determine the quantity of air required for combustion; local air pollution authorities set pollution control requirements. As a general rule, combustion air can be supplied in a duct to the boiler at an average velocity of 1000 fpm (5.1 m/s). The duct should be large enough to carry at least 2 cfm (1 L/s) per boiler horsepower. Furthermore, ventilation air to the boiler room should be provided; preferably, the inlet and outlet should be on opposite sides of the room. Minimum sizes: enough for 2 cfm (1 L/s) per boiler horsepower at a velocity of about 500 fpm (2.5 m/s).
Fig. 10.17 Breeching and stack size guidelines for fossil-fuel-fired boilers. (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 11th ed.; © 2007 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
Space requirements for boilers are summarized in Fig. 10.18, which shows multiple boilers. Note that clear space within the room must be provided so that the tubes of the boiler can be pulled when they must be replaced. Access for eventually replacing entire boilers must be considered.
Fig. 10.18 Boiler room space requirements. Dimension A includes an aisle of 3 ft 6 in. (1 m) between the boiler and the wall. Dimension B between the boilers includes an aisle of at least 3 ft 6 in. (1 m), up to 5 ft (1.5 m) for the largest boilers. (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 10th ed.; © 2000 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
Several types of single boilers are discussed here. The final boiler type discussed, the modular boiler, is preferred for energy conservation.
1.         High-output, package-type steel boiler. For large buildings that use steam as a primary heating medium, one or several such boilers may be used. Direct use of steam can be seen in Fig. 10.15b, supplying preheat and reheat coils and also a humidifying unit. The relative lightness of this boiler type, compared to the older styles with ponderous masonry bases (boiler settings), makes it suitable for use on upper floors of tall buildings. Figure 10.6 shows two such boilers on the 13th floor of the Fox Plaza Building.
2.         Converter, steam to hot water. When, in a building that uses primary steam boilers, secondary circuits that use hot water for heating are required, a converter (Fig. 10.19) is used. It is considered a heat exchanger. In Fig. 10.6, there is downfeed steam supply for the two boilers on the 13th floor to two such converters, one for hot water heating in the apartments and one below the garage ceiling for hot water heating in the commercial area. A converter may also be used to transfer heat from steam to domestic (service) water. Converters are frequently used where central steam supply systems are available, as in large-city downtown areas. The easier, quieter distribution of heat by hot water has largely replaced steam heating distribution trees within buildings.
Fig. 10.19 Conversion unit that transfers heat from steam to hot water. (a) Section illustrating the principle of heat transfer from steam to water. (b) A converter connected to the steam supply and equipped with all devices necessary for a complete hot water heating system. (Courtesy of ITT Bell and Gossett.)
3.         Electric boilers. Where electricity costs are competitive with those of fossil fuels, electric boilers are sometimes used. Both hot water and steam electric boilers are available. The advantage of electric boilers is the elimination of combustion air, the flue, and air pollution at the building. The disadvantages are the use of a high-grade energy source for a relatively low-grade task and the pollution impact at the electric generating plant. In order to protect against high electric demand charges, a large number of control steps are desirable.
4.         Compact boilers. Smaller-dimension boilers (Fig. 10.20) with high thermal efficiencies are available. In addition to their space-saving footprint, they feature a variety of venting options that make them easily adaptable to smaller equipment rooms.
Fig. 10.20 Burkay Genesis hot water boiler, fueled by either natural gas or propane, is available in ratings from 200,000 to 750,000 Btu/h (58,620 to 219,825 W). All units are 30 in. high  24 in. deep (762 mm  610 mm); the smallest boiler is 23 in. (584 mm) wide, and the largest is 57 in. (1454 mm) wide. The copper heat exchanger has an 83.7% thermal efficiency rating, and a variety of venting options are available. (Courtesy of A.O. Smith Water Products Company, Irving, TX.)
5.         Modular boilers. The primary advantage of modular boilers (Fig. 10.21) is efficiency. Boilers achieve maximum efficiency when they are operated continuously at their full-rated fuel input. The single boilers discussed previously operate this way only under outside design conditions, which by definition occur, at most, during 5% of a normal winter. In a modular boiler design, each section is run independently. Therefore, only one section need be fired for the mildest heating needs; as the weather gets colder, more sections are gradually added. Because each section operates continuously at full-rated fuel input, efficiency is greatly increased (Fig. 10.22). Each module, being rather small, requires little time to reach a useful temperature and (unlike the larger single boilers) does not waste a lot of heat as it cools down. Thus, modular boilers usually produce a 15% to 20% fuel savings for the heating season relative to single boilers. Their other advantages include ease of maintenance (one module can be cleaned while others carry the heating load) and small size (allowing easy installation and replacement in existing buildings).
Fig. 10.21 Modular boilers. (a) A bank of four modules—with a total input 1.5 million Btu/h (439 kW). (b) Details of one module (20 x 32 x 48 in. H [510 x 812 x 1220 mm]) with a 385,000 Btu/h (113 kW) input. (c) Schematic of flow conditions in mild weather, with only one module in operation.
Fig. 10.22 One large boiler versus many smaller ones. (a) Boilers rarely operate at full capacity; instead, they respond to part loads the majority of the time. (b) Under part load conditions, a boiler will often short-cycle, which on a single large boiler could drop the annual efficiency into the 66% to 75% range.
Modular boilers also eliminate the initial cost of oversizing heating equipment. In cold climates, conventional boiler systems often use two or three large boilers to ensure that heat is available even if one large boiler fails. When two such boilers are used, it is common practice to size each boiler at two-thirds of the total heating load; an oversize of one-third results. When three such boilers are used, it is common practice to size each boiler at 40% of the total heating load; an oversize of 20% results. However, when a minimum of five modular boilers are used, oversizing can be eliminated because the failure of a single module will not have a crippling impact on the overall heat output.
Gas-fired pulse boilers are an even smaller and more energy-efficient choice for modular boilers. Pulse boilers utilize a series of 60 to 70 small explosions per second, making the hot flue gases pulse as they pass through the firetube. This makes for very efficient heat transfer. Pulse boilers are available up to about 300,000 Btu/h (88 kW).
Pulse boilers operate with lower water temperatures so that water vapor in the flue gas can condense and drain. This change of state liberates additional heat, allowing these pulse boilers to achieve efficiencies up to 90%. They exhaust moist air, not hot smoke, so flues can be small-diameter plastic pipe rather than large-diameter, heat-resistant materials.
(ii)       Chillers
These devices remove the heat gathered by the recirculating chilled water system as it cools the building. The selection of chillers depends largely on the fuel source and the total cooling load. Chillers include both absorption and compressive refrigeration processes in a wide range of sizes.
New developments in chillers continue to result from a combination of concerns about the role of CFCs and HCFCs in global climate change and from changes in utility regulations that are producing unstable energy prices in many areas. Chillers capable of changing quickly between electricity and natural gas are becoming available as a result.
The single-effect, indirect-fired absorption chiller (Fig. 10.23) is attractive where central steam or high-temperature water (from solar collectors, as waste heat from an industrial process, a fuel cell, etc.) is available. This device uses the absorptive refrigeration cycle (explained in Fig. 9.2). Direct-fired absorption chillers use natural gas to power the cycle. In general, absorption equipment is less efficient than compressive refrigeration cycle equipment, although a cheap or even free heat source to power the cycle can rapidly overcome efficiency disadvantages. Absorption machines have fewer moving parts (and therefore require less maintenance) and are generally quieter than compressive cycle equipment. They are environmentally attractive, despite their much higher waste heat output (about 31,000 Btu/ton, compared to at most 15,000 Btu/ton for compressive cycle equipment), because they do not use CFCs or HCFCs and because they require far less electricity to operate. Newer developments include the double-effect absorption chiller (see Fig. 9.3) and the triple-effect chiller, each accompanied by an increase in efficiency.
Fig. 10.23 (a) An absorption chiller driven by heat to produce chilled water. (The Carrier Corporation; courtesy of Ingersoll-Rand.) (b) Two-stage absorption chiller utilizing steam, producing 200 to 800 tons (700–2800 kW) of cooling. (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 11th ed.; © 2007 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
The compressive refrigeration cycle (explained in Fig. 9.1) is used in the other types of chillers. Larger units are centrifugal chillers (Fig. 10.24), whose compressors either can be driven by an electric motor or can utilize a turbine driven by steam or gas. (When a steam-driven turbine is used, the exhaust steam is often used to run an auxiliary absorption cycle machine. These two devices make an efficient combination, and the steam plant that supplies them in summer can supply heating in winter.) Centrifugal chillers usually require about 1 hp/ton (0.57 kW, or 10 ft3 gas, or about 15 lb of steam per ton). These large chillers usually require a cooling tower. Dual-condenser chillers (Fig. 10.25) can choose whether to reject their heat to a cooling tower (via the heat rejection condenser) or to building heating (via the heat recovery condenser).
Fig. 10.24 (a) A centrifugal chiller—a machine of large capacity using the compressive refrigeration cycle. (Courtesy of the Carrier Corporation.) (b) Centrifugal chiller with a flooded cooler and condenser within a single outer shell. This low-pressure unit typically produces 100 to 400 tons (350–1400 kW) of cooling. Typical dimensions are: 14ft  L x 5ft W x 8ft H (4.3 x 1.5 x 2.4 mm), at 16,000 lb (7260 kg). (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 7th ed.; © 1981 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.
Fig. 10.25 Dual-condenser chiller. Heat drawn from the chilled water system is either rejected to the cooling tower or recovered for use in building heating. (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 11th ed.; © 2007 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
Somewhat smaller chillers use either twin screws or a scroll in place of a piston in the compressor. The screw compressor (Fig. 10.26) has a pair of helical screws; as they rotate, they mesh and thus compress the volume of the gas refrigerant. They are small and quiet, with little vibration. The scroll compressor (Fig. 10.27) uses two inter-fitting spiral-shaped scrolls. Again, the refrigerant gas is compressed as one scroll rotates against the other fixed one. Gas is brought in at one end while the compressed gas is released at the other. Quiet and low-maintenance, they are also more efficient than reciprocating compressors.
Fig. 10.26 A screw, or helical, compressor is a quieter, smaller machine with little vibration. (from Bobenhausen, Simplified Design of HVAC Systems; © 1994 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
Fig. 10.27 Scroll compressor rotates one scroll form against another, with a quiet and efficient compression of the refrigerant. (From Bobenhausen, Simplified Design of HVAC Systems; © 1994 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
Even-smaller compressive-cycle machines are called reciprocating chillers (Fig. 10.28). Usually electrically driven, they are often combined with an air-cooled heat rejection process rather than a cooling tower. This makes them a closer relative of the smaller direct refrigerant machines discussed in Section 9.8.
Fig. 10.28 A reciprocating chiller—a small-capacity machine that uses the compressive refrigeration cycle. Typically, this type of chiller produces less than 200 tons (700 kW) of cooling. Such a machine might be around 8 ft L x 3 ft W x 5 ft H (2.4 x 0.9 x 1.5 m) and weigh 3500 lb (1590 kg). (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 11th ed.; © 2007 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
Chilled water is usually supplied at between 40º and 48ºF (4º and 9ºC). When the chilled water is supplied cold and returns much warmer, the large rise in temperature reduces the initial size (cost) of equipment and increases its efficiency (thereby reducing the operating cost as well). Water treatment may be needed for chilled water to control corrosion or scaling.
Typical cooling capacities and space requirements of chillers are shown in Fig. 10.29—with dimensions as tabulated in Fig. 10.29. Each refrigeration machine in this illustration requires two pumps—one for the chilled water (to cool the building) and one for condenser water (to deal with reject heat). Typically, space is provided for future chiller additions, which may be required by building expansion and/or by higher internal gains from as-yet-uninstalled equipment, such as computer terminals within offices. Improved-efficiency chillers may replace older ones when energy costs and environmental regulations become compelling. Adequate clearance access to the equipment room is a major design issue.
Fig. 10.29 Chiller room space requirements. Each refrigeration machine is served by two pumps (chilled water and condenser water). (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 10th ed.; © 2000 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)

Note: Direct-fired absorption machines are roughly the same size as steam absorption machines.
(iii)      Condensing Water Equipment
With chillers, there must be a way to reject the heat that is removed from the recirculating chilled water system. Reject heat is handled by the condensing water system, which serves the condensing process within refrigeration cycles. For larger buildings, the condensing water requirement is most likely to be met by a cooling tower.
The cooling tower’s place within the overall equipment layout was shown in Fig. 10.15b; a more detailed guide to sizes and types is given in Figs. 10.30 and 10.31. The object is to maximize the surface area contact between outdoor air and the heat condensing water. In crossflow towers, fans move air horizontally through water droplets and wet layers of fill (or packing), whereas in counterflow towers (prevalent in larger buildings), fans move the air up as the water moves down.
Fig. 10.30 Cooling towers that serve the condensing water system for large buildings. (a) Cutaway view of a large-capacity (200 to 700 tons [700-2460 kW]) crossflow induced-draft package cooling tower. (b) Size ranges for crossflow induced-draft package cooling towers. (c) Size ranges for counterflow induced-draft package cooling towers. (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 11th ed.; © 2007 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
Fig. 10.31 For cooling towers, the more wall clearance, the better the operation. A  maximum height of enclosure above the tower outlet; minimize this dimension. B  as large as possible, especially if walls have no air openings. (Consult the manufacturer for minimum dimensions.) (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 11th ed.; © 2007 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
Cooling towers create a special—and usually unpleasant—microclimate. They demand huge quantities of outdoor air (approximately 300 cfm [142 L/s]), which they make considerably more humid. In cold weather, they can produce fog. They are typically very noisy—a natural consequence of forced-air motion. The condensing water flows are about 2.8 gpm (0.18 L/s) per ton of compressive refrigeration and about 3.5 gpm (0.22 L/s) per ton of absorption refrigeration.
The water that escapes as vapor from the tower is between 1.6 and 2 gph (1.7 and 2.1 mL/s). This water must be replaced, which is done automatically. The steady evaporation and exposure to the outdoors under hot and humid conditions spells trouble for the condensing water: Controls for scaling, corrosion, and bacterial and algae growth are especially important. Ozone treatment systems have the advantage of reliable biological control and leave no chemical residue. Since the discovery of the link between Legionnaire’s disease and cooling towers, biological control has assumed greater importance.

The vapor that escapes the cooling tower should be kept from the vicinity of fresh air intakes, and from neighboring buildings or parked cars, where feasible. The floor space requirements can be approximated from Table 10.3, or use the average of 1/500 of the building gross floor area (for towers up to 8 ft [2.4 m] high) or 1/400 of the building gross floor area (for higher towers).
Although it is tempting to try to block the noise of cooling towers with solid barriers, it is critical that noise control not interfere with air circulation. The manufacturer’s recommended clearances to solid objects near cooling towers must be consulted before a tower is enclosed in any way. The roof is thus a favorite location for cooling towers, where wind can disperse the vapor, and the noise and odor are remote from the street. However, the cooling tower can sometimes be featured; near downtown Denver, the cooling tower for the large performing arts complex sits in a forlorn stretch of grass bordered by arterial streets and away from pedestrians (Fig. 10.32). Its plume adds visual interest as it twists ghostlike above the equipment.
Fig. 10.32 A plume of mist hovers ghostlike above a cooling tower in full public view near the Denver performing arts complex.
When fouling of the condensing water system cannot be tolerated, an alternative approach, called the closed-circuit evaporative cooler, is taken. Its schematic operation is described in Fig. 10.33. Usually used to cool the refrigerant directly, it can also be used for the condenser water, as well as on water loop heat pump systems (see Fig. 9.45). Either refrigerant or condenser water is protected within an always-closed loop, while a separate body of water is recirculated through the cooler, with steady evaporation and attendant problems. It requires much less makeup water than the cooling towers.
Fig. 10.33 Closed-circuit evaporative coolers, which cool the condensing water system while protecting it from contact with outside air. A self-contained water system is circulated through the evaporative cooler; steady evaporation losses are replaced by makeup water. (Based upon AIA: Ramsey/Sleeper, Architectural Graphic Standards, 8th ed.; © 1988 by John Wiley & Sons.)
(iv)      Energy Conservation Equipment
One big advantage of central equipment rooms is the opportunity they present for energy conservation. Regular maintenance is simplified when all the equipment lives in a generous space kept at optimum conditions; with regular maintenance comes increased efficiency of operation. Another conservation opportunity is that of heat transfer between various machines, or between distribution trees, where one’s waste meets another’s need.
Boiler flue economizers achieve heat transfer by passing the hot gases in a boiler’s stack through a heat exchanger, thus preheating the incoming boiler water (Fig. 10.34).
Fig. 10.34 Heat recovery for boilers. Flue gas entering at 500ºF (260ºC) leaves the “economizer” at 325ºF (163ºC), a temperature still high enough to prevent condensation in the stack. The heat recovered here is added to incoming boiler water, raising its temperature from 200 to 248ºF (93 to 120ºC). (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 11th ed.; © 2007 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.
Runaround coils (Fig. 10.35) can be used for heat transfer between intake and exhaust air ducts when these two air streams are rather far apart. This circulating heat-transfer fluid usually contains antifreeze; it provides simple sensible heat transfer, with no restrictions on exhaust and intake location. No contamination of intake air by exhaust air is caused by this arrangement. The efficiency of such coils runs between 50% and 70%, and they are available in modular sizes up to 20,000 cfm (9440 L/s).
Fig. 10.35 Runaround coils for heat transfer between fresh intake air and stale exhaust air, used where the air streams are in separate locations. Efficiencies can range from 50–70% and coil capacities up to 20,000 cfm (9400 L/s). (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 11th ed.; © 2007 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
Heat exchange between incoming and exhaust air streams, which was discussed in Sections 5.6c and 5.6d, allows heat pipes and thermal transfer wheels to play major roles in energy conservation. The thermal transfer wheels can add to the central equipment space requirements. Other desiccant systems were discussed in Section 5.6d.
Economizer cycles (Fig. 10.36) use cool outdoor air, as available, to ease the burden on a refrigeration cycle as it cools the recirculated indoor air. The economizer cycle can thus be thought of as a central mechanical substitute for the open window; when it is cool enough (below the supply air temperature), 100% outside air can be provided and no chilled water is needed. When the outdoor air temperature is higher than the supply air temperature but lower than the return air temperature, 100% outdoor air is still brought in, but chilled water is used to lower its temperature. Above the return air temperature, outdoor air is reduced to that volume required for IAQ.
Fig. 10.36 The economizer cycle controls the relationships among fresh, exhaust, and recirculated air. (a) When outside air is hot (or very cold), the economizer cycle is inactive, and minimum fresh air is introduced. (b) As very cold outside air gets warmer, it can be blended with recirculated air, and neither heating nor cooling coils are needed. (c) When outside air is cool, it can completely replace circulated air, making mechanical cooling inactive.
Relative to open windows, this cycle has several advantages: energy-optimizing automatic thermal control, filtering of the fresh air, tempering of the cool outdoor air to avoid unpleasant drafts, and an orderly diffusion of fresh air throughout the building. Its disadvantages are the loss of personal control that windows offer and thus loss of awareness of exterior–interior interaction. In hot, humid climates, the moisture brought by 100% outdoor air may be unwelcome.
Economizer cycles are available as options on most direct refrigerant machines (such as single-package rooftop units) and are typically installed for large-building central air supply systems. Buildings with high internal gains (internal load dominated) are particularly good targets for economizer cycles because they need cooling even when the outside temperature is chilly. Economizer cycles lend themselves readily to a cooling strategy of night ventilation of thermally massive structures because they have a built-in option for 100% outdoor air.
(v)       Geoexchange Systems
Using the earth as a heat source and sink for small buildings was explored in Section 9.8d. Four typical applications were shown in Fig. 9.44. Larger buildings can also utilize such systems. A late 1990s building just east of Central Park, in New York City, utilizes two wells 1500 ft (457 m) deep; all but the top 50 ft (15 m) are lined by bedrock. Heat is taken from (or discharged to) water, which is pumped from one well and discharged to the other—a “groundwater source” system. The average year-round temperature in these deep wells (in an intensely urban area) is estimated at 56ºF (13ºC), about the temperature at which chilled air is delivered to a space in summer. Going to such depths is perhaps the only geothermal option in densely built-up areas.
The Cambridge (Massachusetts) Cohousing Project (Fig. 10.37) is also located in a densely settled urban neighborhood. The 41 living units feature passive solar heating, underground parking (this reserves some 20,000 ft2 [1860 m2] of the surface for open green space), and centralized heating/cooling utilizing ground source heat pumps with locally controlled thermal zones. In this urban setting, the relative quiet of the indoor heat pump’s compressors and the lack of hot air (summer) or cold air (winter) noisy discharges are welcome amenities.
Fig. 10.37 Cambridge (Massachusetts) Cohousing development takes advantage of three boreholes on its urban site (a) to provide central heating and cooling, serving individual residential fan-coil units. It also preheats water (b) for the DHW (domestic hot water) system. The residences are sited to provide winter solar access, and a positive fresh air intake is located on the side sheltered from adjacent railroad tracks. Underground parking preserves open space for gardens and recreation. (Courtesy of Building Science Engineering, Harvard, MA.)
In England, the Hyndburn Borough Council decided that their new headquarters building should set an example as a “zero energy” building: that is, over a typical year, it should generate as much energy as it imports. The 38,750-ft2 (3600-m2) building (Fig. 10.38) is elongated east–west and boasts an ambitious section that combines daylighting, photovoltaics (PV), a well-insulated skin, and even rainwater collection to use for flushing toilets. Windpower adds to electricity generation. Summer cooling is by ventilation (assisted by night ventilation of mass) through a raised “Termodeck” system. This is a prefabricated hollow-core slab through which cool night air circulates, storing coolth to assist the next day’s hottest hours. (This is a concept explored by the Oregon office building in Fig. 8.6.) The raised floor then provides a distribution plenum, with a design rate of 4 air changes per hour (ACH).
Fig. 10.38 The Hyndburn Borough Council (England) headquarters faces south toward a reservoir (a) that provides an evaporatively cooled microclimate and also acts as a heat source/sink for a water–water heat pump. (b) As suggested in the diagrammatic section, daylighting, passive solar heating, PVs, and a well-insulated shell are featured. Termodeck is a precast hollow-core slab that stores coolth on summer nights and precools ventilation air. The supply air then rises from the plenum created by the raised floor. (Courtesy of Jestico Whiles Associates, architects, London.)
For more extreme future summer cooling, but now mostly for winter heating (beyond that provided by passive solar heating), an adjacent reservoir acts as a thermal sink. A “lake closed source heat pump” will serve the mechanical ventilation system. The lake is south of the building, providing some local evaporative cooling in summer and inviting a very climate-oriented south façade, behind which are open-office areas. The north façade faces the city and is more traditionally institutional in character, as are the individual offices behind it.
(vi)      Energy Storage
We commonly experience daily changes from warmer to colder conditions, both in winter and in a hot, dry summer. Central storage equipment for large buildings can take advantage of this cycle to increase operating efficiency, save energy, and significantly reduce electricity demand charges. Some electric utilities offer incentives to install thermal storage in order to reduce the peak strain on their generating facilities.
Water storage tanks are one common approach to storage, such as those shown in Fig. 10.39. On typical winter days, the total internal heat generated by a large building can be somewhat greater than its total need for heating at the perimeter zones. Instead of being thrown away as exhaust air, this surplus heat is captured and stored in large water tanks, from which it can be withdrawn and used on cold winter nights and weekends. In the summer, chillers can work at night, when efficiency is high because cool outdoor air helps the refrigeration cycle reject its heat. By storing the coolth produced, less work need be done by chillers during the next day’s peak, when electric rates are highest and operating efficiency is lowest.
Fig. 10.39 Water tank heat storage. The 870,000-ft2 (80,825-m2) transportation office building (Park Plaza, Boston), population 2000, uses a three-compartment insulated concrete tank storing 750,000 gal (2,838,990 L) of water. (a) At an outside air temperature of 40º to 50ºF (4.4 to 10ºC), the surplus internal heat is stored in the tanks rather than rejected as exhaust air. (b) By the time the outside air is about 50ºF (10ºC), the tanks are fully charged; up to an outside temperature of 60ºF (15.5ºC), the economizer cycle provides energy-conserving cooling. (c) At about 60ºF (15.5ºC) chillers must operate, but working all night when the outside air is cooler enables them to work less by day. Their nightly production is stored as cold water, available to help with the following day’s peak. Smaller machines and more efficient operation are the result. (Courtesy of Shooshanian Engineering Associates, Inc.)
Design guidelines for water storage are 0.5 to 1 gal/ft2 (20 to 40 L/m2) of conditioned space. Such large, heavy tanks are frequently located in basements and underground parking facilities. Water tanks require additional floor space for equipment, although they may allow somewhat smaller chillers to be installed. They may also contribute to lower fire insurance premiums because considerable water is stored and ready to use.
Ice storage tanks are another storage approach, as shown in Fig. 10.40. Because they take advantage of the latent heat of fusion (143.5 Btu/lb of water at 32ºF [334 kJ/kg at 0ºC)]), such units can store far more energy in a given-size tank than can water. Design guidelines for ice storage are 0.13 to 0.25 gal/ft2 (5 to 10 L/m2) of conditioned space. A comparison of the relative sizes of storage tanks needed for an 18-story office building is shown in Table 10.6. Also, with ice storage there is less undesired mixing of hot and cold water within the tank.
Fig. 10.40 Ice tank heat storage. Chilled water in an ice tank forms a layer of ice around the refrigerant pipe (supplied by the refrigeration machine). As chilled water at about 35ºF (1.7ºC) is removed from the tank, it enters a heat exchanger, so that its temperature is closer to 45ºF (7.2ºC) for distribution to cooling coils throughout the building.
TABLE 10.6 Cooling Storage Comparison

aBtu/lb based on t  15Fº for water storage and on t  20.5Fº in the melted ice water (added to 143.5 Btu/lb at fusion).

Ice can be made in several ways. A common method is to form ice as a layer around a pipe that carries refrigerant in a closed circuit through the tank. Control of the thickness of this layer is important, because if too little ice is made at night, too little cooling will be available the following day. In another method, ice is formed and thawed by circulating a brine through coils in cylindrical water tanks. Other methods form ice on plates, then harvest it in an insulated bin.
The Iowa Public Service Building (Fig. 10.41) uses ice storage in six tanks that occupy about half of the floor space in the ground floor mechanical room. This installation serves a five-story, 167,000-ft2 (15,515-m2) utility office building that utilizes solar collectors supplemented by small backup boilers. The six ice-making machines serve a 75,000-gal (283,900-L) ice storage pit with 90 million Btu (26,355 kWh) capacity. Winter heat exchange opportunities include reject heat from the ice-making machines, heat from the central toilet exhaust air, and heat from return air taken through luminaires. Solar collectors on the roof preheat the ventilation air, which is fed into the ceiling plenum at each floor. Fan-coil units then draw from this fresh air supply.
Fig. 10.41 The ground floor of the Iowa Public Service offices (Sioux City) contains six ice machines within the mechanical space adjacent to the loading dock. (Courtesy of Rossetti Associates and Foss, Engelstad, Heil Associates, joint venture architects.)
Water and ice storage tanks are sometimes located at the top of a building, especially when other related mechanical equipment is there also. Despite the weight of such tanks, they can become a structural advantage. The tuned mass damper method of reducing lateral vibration (or sway) in high-rise buildings utilizes a heavy, moving mass at the top; when the building begins to sway, the mass is moved in the opposite direction. In the Crystal Tower in Osaka, Japan, this mass is provided by ice storage tanks. This 515-ft (157-m), 37-story building has an ice thermal storage total of 25,400 ft3 (720 m3) divided into nine tanks. Six of the ice storage tanks provide the movable structural mass, suspended from roof girders, weighing 540 tons (489,880 kg), including steel framing. The chiller and condenser are also in the equipment penthouse.
(vii)     Air-Handling Equipment
A given HVAC system type may have many variations. In some variations, all the air is passed through one central equipment room. In others, air handling may be done in many separate and smaller rooms, whereas central heating and cooling require only one equipment room. Some detailed design guidelines for air-handling equipment for either case are shown in Fig. 10.42. Air-handling equipment room dimensions are shown in Table 10.7. Total air quantities (cfm) may be estimated from Table 10.4 or obtained more precisely from cooling load calculations.
Fig. 10.42 Some variations on a huge variety of HVAC equipment rooms. (a) Plan with floor-mounted air-conditioning unit. Outdoor air is drawn from an adjacent shaft; return air ducts are above the equipment, and a remote exhaust fan expels what is not recirculated. (b) Plan on an exterior wall. Outdoor air is taken through the wall, and a return air fan is overhead. (c) Section through plan b. (From AIA: Ramsey/Sleeper, Architectural Graphic Standards, 10th ed.; © 2000 by John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.)
TABLE 10.7 Air-Handling Equipment Rooms

Some common fan types include:
Panel: The most simple type on this list; for high-efficiency air delivery, the motor is generally mounted in the center of the propellers. It is likely the noisiest type. It is not designed for much static pressure from ductwork, filters, and so on.
Fixed pitch vane axial: Capable of working against somewhat more static pressure than the panel fan; more common in industrial applications.
Centrifugal: With an airfoil bladed wheel, it has high efficiency over a wide operating range and is quieter than the previous two. Major changes in pressure result in only minor changes in the volume of air delivered.
Vane axial adjustable pitch: Takes less space than centrifugal fans and can work against more static pressure. The pitch is adjustable occasionally, not continuously, for system balancing or seasonal changes in air volume.
Vane axial controllable pitch: The automatically controlled pitch responds to changes in temperature, humidity, airflow, and so on, depending on the sensors used. It is commonly used in VAV systems.

Filters and other methods of IAQ control were discussed in Chapter 5. There are numerous combinations of air handlers, filters, and coils for air heating/cooling, selected by the consulting engineer. For the architect, the primary issues include adequate space, adequate access (for present maintenance and future equipment replacement), and noise isolation.
(d)       CONTROLS
Although the most obvious HVAC control function is to maintain desired comfort conditions, controls also increase fuel economy by promoting optimum operation and act as safety devices, limiting or overriding mechanical equipment. They also eliminate human error: controls fall asleep only during a power failure.
Although precise control of temperature and humidity everywhere in a building may be a tempting thought, controls can usually maintain only a range of conditions, not a setpoint. This range (within which neither heating nor cooling is called for) is called the deadband. Temperature variations occur vertically within a space—the higher the space, the greater the variation. Variations between horizontal positions within a zone are highly likely, especially where one or more walls are exterior and where different rooms share a zone controlled by a single thermostat. Variations in time also occur: a building will always be warmer at the moment the heating system turns off than at the moment the system turns on.
Individual controls can be classified as follows: controllers, which measure, analyze, and initiate action; actuators, which are the controller’s servants and in turn become the masters of pieces of equipment; limit and safety controls, which may function only infrequently, preventing damage to equipment or buildings; and accessories, a miscellaneous collection.
Systems of controls can be classified by power source: electric (both analog and direct digital control); pneumatic (in which compressed air is the motivating force); and self-contained, including “passive” controls such as those motivated by thermal expansion of liquids or metals. Another way to classify control systems is by the motion of the controller equipment: two-position systems are of the simple on–off type; multiposition systems have several varieties of the on position, commonly used for separate operation of more than one machine; floating controls can assume any position in the range between minimum and maximum; central logic control systems can be programmed to integrate the many aspects of building control into one decision-making unit, and are now the prevalent approach to building management.
Control diagrams for two common HVAC applications are shown in Fig. 10.43. Single-duct VAV systems (Fig. 10.43a) with the constantly varying flow rate, the fan must be regulated so as to maintain the minimum pressure (and therefore, flow) needed at the most demanding outlet. This outlet may be either the one most remote from the fan or the one needing the greatest flow (because it has the highest gain) at the moment. Economizer cycles (Fig. 10.43b) compare outside to inside positions, and vary the proportion of fresh (outdoor) air to return air, to provide “free” cooling (see again Fig. 10.36).
Fig. 10.43 Controls for some common HVAC applications. (a) Single-duct VAV. (b) Economizer cycle.
Most of today’s large buildings are regulated by central logic control systems, usually called building management systems (BMS). The goal is a productive, cost-effective environment achieved by optimizing the interrelationships among the building’s structure, systems, services, and management. The HVAC system is centrally regulated and interconnected with lighting, electric power (such as load shedding), elevators, service hot water, access control and security, telecommunications, and information management. Such systems not only maintain comfort with energy conservation, they also sound the alarm about malfunctions, can learn from past practice, and keep records of performance.
This integration and automation of the many building control systems is made possible through direct digital control (DDC), which can be applied to a wide variety of elements. They provide “dynamic control” in the case of HVAC, anticipating time-based changes in heat flow patterns or in occupancy schedules. These actions depend on direct digital microcontrollers located on each piece of regulated equipment throughout the building. Three building types show applications of this comprehensive automated control opportunity.
Laboratories have proven to be especially difficult HVAC control problems due to their fume hoods. Conditioned air is provided from the central HVAC system, often by a VAV supply. Whenever a fume hood is exhausting air (frequently in huge quantities), the VAV supply and return systems are affected. Complicating this relationship is the nature of the laboratory work; where the experiment could be damaged by outside contaminants, the lab should be positively pressurized to minimize infiltration (and the VAV must therefore supply and return slightly more air than the fume hood exhausts). However, where the experiments involve diseased, toxic, or other hazardous substances, the lab must be negatively pressurized (and the VAV must supply and return slightly less air than the fume hood exhausts). DDCs tied to a central system can balance energy conservation, lab worker safety, and safety for the nonlab environment.
Hotel rooms can present serious energy loss problems from heating/cooling either an unoccupied room or a room with open windows. With DDCs inter-tied with the registration desk, an “unoccupied” mode of operating can be remotely controlled, with bare-minimum heating or cooling. When the room is occupied, the supply of either hot water heating or cooled air can be throttled back whenever the window is open. Also, a “purge” mode can enable a new arrival (or the front desk, in anticipation) to select a greatly increased flow of outdoor air for a limited time period to dilute cigarette smoke or other odors. Chapter 30 presents more detailed information on intelligent buildings.
Offices might be provided not only with DDC for the VAV supply units, but also with inter-tied DDCs for an interconnected ventilating window (preventing simultaneous open windows and treated forced-air delivery), daylight reflectors (mini-light shelves), venetian blinds, a radiant heater valve, an electric light switch, and an insulating shade. A control panel or “dashboard” gives the worker an opportunity to interact with the central control in operating these devices.
Office buildings that use more passive strategies also benefit from BMS. The British Research Establishment building for fire research, “Building 16” (Fig. 10.44), depends upon cross- and stack ventilation for its cooling, and upon movable louvers on the south windows for sunshading and daylight penetration. A common network links this building’s control systems; each worker has a TV-like controller that regulates lights and can override the programmed settings of the nearest high-level windows (for ventilation) and south-window louvers (for sun control and daylighting). This innovative office building uses a ground heat pump (borehole type) to supplement solar and internal winter heat gains or to provide supplementary cooling to the night ventilation system. Note the provisions for cross-ventilation for individually enclosed offices on the ground floor, utilizing a cavity above the ceiling to carry the ventilation air on to exhaust on the other side of the building.
Fig. 10.44 Natural ventilation and daylight strategies dominate the north–south section (a) of the British Research Establishment Building 16 at Garston. (1) Stack ventilation (hot, calm); (2) clerestory BMS-controlled ventilation; (3) night ventilation through slab, BMS-controlled; (4) cross-ventilation bypass over enclosed offices; (5) enclosed office single-sided ventilation; (6) corridor cross-over zone; (7) manually operated lower-level windows; (8) high-level BMS-controlled windows; (9) motorized external shading louvers, also BMS-controlled. (b) East–west section detail of precast floor structure. (1) Luminaire with integral photosensors; (2) heated/cooled screed using a geothermal source; (3) raised access floor for wiring; (4) cross-ventilation duct (night ventilation and/or cross-over ventilation for enclosed offices); (5) waveform precast concrete ceiling with poured-in-place topping slab. (Courtesy of Feilden Clegg Architects, Bath, England.)
Rapid changes in centralized control systems are continuing. Initially, DDC systems were developed by individual companies, with little or no opportunity for communication between systems. Such proprietary systems left designers frustrated by an inability to specify a wide variety of products, all controlled by a single BMS. In the late 1990s, two competing systems had emerged that promised integration of products and systems from different manufacturers. BACnet was developed by ASHRAE committee members as a nonproprietary communication standard. LonMark was developed by the LonMark Interoperability Association, a user-funded organization of building owners, specifiers, system integrators, and product suppliers. Open control system architecture is an opportunity for a BMS in which components from several vendors interoperate over a BACnet-adapted Ethernet LAN (local area network). It makes possible a system combining BACnet, LonMark, and proprietary subsystems (Fig. 10.45).
Fig. 10.45 Building Management System (BMS) that is open to various protocol standards. (Courtesy of Honeywell, Inc.)

11.10 DISTRICT HEATING AND COOLING
Often, large projects made up of many large buildings are well served by one central station heating/cooling plant. The familiar economies of scale apply here; very large, efficient, and well-maintained boilers and chillers encourage energy recovery through heat exchange, reduce air pollution, and remove the noise and other nuisances associated with heating and cooling from the other buildings. This approach is called district heating and cooling.
District heating for residences and small commercial buildings is common in northern Europe; district steam systems serve the central areas of many U.S. cities. Most often, electrical generating plants are the heat (or steam) source; cogeneration (discussed in Section 10.9) allows the waste heat from the generating plant to be put to use either as space heating or steam (indirect-fired) absorption cooling. Ironically, the trend toward better-insulated buildings has reduced the market for distributed heat, making the installation less cost-effective. This discourages suppliers both from investing in a district system and from encouraging technical innovations in energy conservation by their customers. Smaller, densely built-up central heating districts tend to be more successful than large, sprawling ones; lengthy lines cost more to install and maintain and lose more energy.
(a)       High-Temperature Water and Chilled Water
Long-distance steam distribution has been used for more than a century; the development of high-temperature water (HTW) and chilled water distribution among buildings is a more recent development. Offering many advantages (although steam is still frequently chosen for city distribution), circulated high-temperature, high-pressure hot water and chilled water in closed systems are widely used in U.S. Air Force bases and airports, and for groups of buildings such as hospital complexes and college campuses. Increased efforts are now being made to install new district heating/cooling networks served by existing fossil-fueled electricity generating plants. Such plants waste more than half of their fossil fuel input, and district heating/cooling could intercept much of that waste; see also the next section on cogeneration.
Water will not flash into steam if kept at sufficiently high pressure. It may then be circulated by pumps through supply and return mains and through branches to heat exchangers, which operate conventional low-pressure hot water systems, generate steam, and perform numerous other thermal tasks. Pressures are on the order of 400 psig (pounds per square inch, gauge) and temperatures are about 300ºF. During its circuit, the water will sometimes lose up to 150Fº and 60 psig in pressure. The section shown in Fig. 10.66 illustrates a common arrangement.
Fig. 10.66 Typical arrangement of a high-temperature water system. (Reprinted from High-Temperature Water Systems. Industrial Press. By courtesy of author Owen S. Lieberg, consulting engineer.)
High-temperature water has a number of advantages over steam for certain installations. It is a two-pipe system, and the temperature drop in the supply main is often as little as 10Fº. With reasonably high water velocities, mains can be reduced to almost half the size of those required for steam distribution, with no need for steam traps and pressure-reducing valves. The pipes need not pitch to low points, as in the case of steam (to accommodate condensation), but can follow the contours of the ground. Although installation costs are greater, operational costs are less than those for steam. Feed water treatment is negligible and corrosion is minimal. Underground problems of expansion and insulation are the same as in other subterranean systems. Large sweep-type loops accommodate expansion between fixed points, and underground piping is embedded in special thermally efficient insulative fill.
District chilled water systems also offer advantages. The remote central chillers are likely candidates to use waste heat in a non-CFC absorption cooling cycle. Natural sources are possible; the Toronto (Canada) District Heating Corporation uses Lake Ontario water, drawn from a 1.6-mile (2.6-km) intake at a depth of 200 ft (61 m) with a year-round temperature of 40ºF (4.5ºC). Passed through a heat exchanger (with the district chilled water) and then treated, the lake water then joins the city water supply.
With district heating/cooling, all facilities except air handling and ducts are located together in a remote central plant. This frees the buildings from the space requirements and visible impacts of stacks, boilers, fuel storage, water chillers, and cooling towers; the associated heat, humidity, fouled air, and noise are as remote as the central plant. When such a system serves individual customers, the heated or chilled water is metered. When it is owned by the group of buildings served (such as college campuses), it is usually not metered; this can become a problem when efforts to identify building energy waste and subsequent savings are being investigated.

11.11 COGENERATION

In the preceding chapters, electricity has repeatedly been called a high-grade source in reference to the high temperatures needed to produce electricity by conventional (fossil-fuel) means and to the large amount of waste heat (often twice the fraction of electricity) produced in the process. Cogeneration (also called total energy) is an attempt to recover some of the otherwise wasted lower-grade heat that accompanies the generation of electricity by steam turbines. Industrial cogeneration facilities are especially cost-effective in the pulp and paper, petroleum, and chemical industries. Building-scale cogeneration is a less obvious energy bargain.
(a)       Electrical Power Generation at the Site
Where conditions are favorable, electricity for power and light can be generated economically by a system that also supplies the building with heating in winter and cooling in summer. Such a system utilizes a fuel such as gas or oil and is often supplemental to the local electric utility company. Although installation costs are greater than those for the more conventional systems that use separate services of electricity and fuel, the savings in annual operating costs can sometimes pay for the excess installation cost in a reasonable time. Operational savings continue thereafter. This approach was developed largely in the 1960s and is used in hundreds of commercial and industrial buildings and in many schools. Cogeneration is particularly attractive for district heating/cooling plants.
(b)       Early On-Site Power Generation
Before 1900 and for several years thereafter, nearly all large buildings and groups of buildings were supplied with direct current generated on or near the premises. The motive power was usually in the form of steam-driven reciprocating engines with belt connections to direct-current generators. Direct current cannot be transformed to different voltages and must be generated and distributed at the voltage used in the building. At these relatively low voltages, power loss in the distribution system is great, and distance adds greatly to the loss. This tended to keep electricity usage within a building.
With the development and use of alternating-current machines, utility companies were able to establish central power stations from which electricity could be transmitted great distances to the user at high voltage. There it was transformed down to domestic voltages for use. Because, at high voltage, power losses are very low, this system became universal. During the 1920s and 1930s, owners removed their private power generators and accepted utility service, with its savings in operating expense.
(c)       How Cogeneration Developed
Older buildings used steam produced from coal-burning boilers from which little or no energy salvage was possible. Today’s fuels, used directly in reciprocating engines or turbines (including those that generate electricity), have residual heat value that can be recovered for purposes of heating or cooling. For cogeneration to be successful, there should be a reasonably steady demand in the building for the power generated and also for the heat recovered. Lighting and the demand for power by computers, electrical business machines, and other devices can create nearly constant demand for power throughout the year. Similarly, the exhaust heat recovery from the engines or turbines that power the generators is in demand for either heating or indirect-fired absorption cooling at most times of the year.
(d)       Turbines and Reciprocating Engines
Figure 10.67 shows two principal systems for total energy. In both systems—one using a turbine and the other a reciprocating engine to operate the generator that supplies electric power—heat is reclaimed to produce steam or hot water. The steam or hot water is then used for heating or, by use of an absorption chiller, for cooling. When a turbine is used, the fuels are natural gas or fuel oil. The heat is recovered by passing the hot turbine exhaust through a waste heat boiler to produce steam. Fuel for a reciprocating engine is natural gas or diesel fuel. Both the jacket cooling water and the hot engine exhaust are passed through heat exchangers that utilize the heat to produce steam or hot water for heating or cooling. In both systems, an auxiliary boiler, fired directly by gas or oil, stands ready to help maintain a balance in the system.
Fig. 10.67 Cogeneration (total energy) systems. (a) Using a turbine. (b) Using a reciprocating engine. (Reprinted by permission from Total Energy, Educational Facilities Laboratories.)
Cogeneration offers a degree of electrical independence and a way to use otherwise wasted heat. The latter is essentially free energy because it would otherwise have to be purchased and paid for separately in the form of electricity or other fuels.
(e)       Cogeneration for Housing
The Harbortown apartment and townhouse complex in Detroit, Michigan, utilizes a year-round natural gas–fired primary energy and cogeneration/waste recovery system (Fig. 10.68). Each apartment has one or two stackable, upright fan-coil units using either chilled or heated water. This water is provided by two direct-fired chiller–heaters: each incorporates a natural gas boiler and a two-stage absorption chiller in the same unit, taking less space in the mechanical room. The natural gas–driven cogeneration electric plant serves rather constant loads such as corridor and outdoor lighting. The generator’s waste heat is passed to a storage tank, where it preheats domestic hot water for the tenants. This meets about 87% of the hot water energy usage.
Fig. 10.68 Harbortown is a 120,000-ft2 (11,150-m2) apartment and townhouse complex (a) in Detroit that uses natural gas as a primary fuel year-round. (Photo © 1989, William Kildow.) Chiller–heaters (Hitachi) combine a boiler and a two-stage absorption chiller in one compact unit. (b) Comparison of headspace and (c) floor space required by chiller–heaters versus conventional units of similar capacities. (d) Cogeneration plant sends its waste heat to preheat domestic hot water. (Courtesy of Skidmore, Owings and Merrill, Architects-Engineers, Chicago.)
(f)        Fuel Cells
We first encountered fuel cells in connection with the explorations of outer space. Spaceships use stored oxygen and hydrogen to feed the fuel cell, yielding—seemingly miraculously—electricity, heat, and pure water. In effect, this reverses the process of electrolysis: using electricity to split water into its two components, hydrogen and oxygen. This theory dates back to Sir William Grove in 1839, long before the needs of space travel.
A fuel cell generates direct-current power by converting the chemical energy of hydrogen and oxygen into electricity and heat. Because no combustion is involved, nitrogen oxides and carbon monoxide are nearly eliminated. For a building’s fuel cell power plant, several components are added: first, a fuel processor (or fuel reformer) to prepare a hydrogen-rich stream of fuel, then the fuel cell stack, then a power conditioner (or inverter) to convert the fuel cell’s direct current to alternating current.
Fuel cells are not, by themselves, that much more efficient than combustion processes; they are about 40% efficient for power production. When combined with a way to use that 60% waste heat, they climb toward a total of 90% efficiency, with very few harmful emissions.
Hydrogen is the ultimate fuel; already hydrogen-rich, it needs no processor and produces the least environmental impacts from the fuel cell. Although hydrogen supply networks are not yet commonplace, scenarios exist for a “hydrogen economy” in which surplus electricity from windpower is used to split water into hydrogen and oxygen atoms and the hydrogen is stored for later distribution. Windpower’s extreme variability often presents power grid distribution challenges; storage solves this problem. Solar energy or biomass is other renewable sources for electrical generation. However, other hydrocarbon fuels also can feed fuel cells: natural gas (most common for building fuel cell power plants), propane, methanol, and, with additional pretreatment, methane from landfills and anaerobic digester gas from sewage treatment plants. Even with these less hydrogen-rich fuels, the emissions from fuel cells are well below those of combustion processes; the fuel processor emits carbon dioxide and a trace of carbon monoxide.
The proton-exchange membrane (PEM) fuel cells (Fig. 10.69) are promising for transportation when zero-emission vehicles are the objective and hydrogen is available as the fuel. This process has building applications as well. It works at a temperature below the boiling point of water. Relative to other fuel cell types, it has a smaller size, lighter weight, and lower noise levels. The PEM fuel cell stack involves several subsystems, typically including water purifiers and pumps, air compressors, and heat rejection components (coolant pumps and a radiator). This complicates performance at subfreezing temperatures and adds maintenance requirements. But the smaller size promises smaller building applications in the future.
Fig. 10.69 Schematic diagram of a proton-exchange membrane (PEM) fuel cell with a description of its operation. (Courtesy of Nucleus, Fall 1994; © Union of Concerned Scientists, Cambridge, MA.)
The phosphoric acid fuel cell power plant (Fig. 10.70) is at work in (or outside) a number of buildings and municipalities around the world. The ONSI Corporation’s 200-kW fuel cell power plant measures 10 ft ´ 18 ft, is 10 ft high (3 m ´ 5.5 m, 3 m high), weighs 40,000 lb (18,144 kg), and needs a clearance of 8 ft (2.5 m) around the module for maintenance. It also needs a cooling module (or a building’s cooling tower) for discharge of heat in excess of that recovered for building use. The acid electrolyte in the fuel cell stack works at about 390ºF (200ºC). At its rated power, it also produces 700,000 Btu/h (205 kW) to heat a water stream to 140ºF (60ºC). The rated sound level is 62 dBA at 30 ft (9 m) from the module.
Fig. 10.70 A fuel cell power plant using natural gas (or other hydrocarbon fuel). The fuel processor (reformer) converts the natural gas to a hydrogen-rich stream that enters the fuel cell power section (or stack). The output is electricity (dc), water, carbon dioxide, and considerable heat. Some heat is used by the fuel processor and some is rejected, but most is usable in a building or industry. A power conditioner (inverter) converts dc to ac power. (Courtesy of ONSI Corporation.)
The Durst Organization’s high-rise office building at 4 Times Square in New York City (Fig. 10.71) is a demonstration of several future-oriented strategies: PV cells integral to the façade, direct-fired (natural gas) absorption chillers, increased fresh air for IAQ, a dedicated exhaust air shaft (for smoking and other polluting activities), and waste chutes to facilitate recycling, among others. It also utilizes two 200-kW fuel cell power packages fed by natural gas. The electricity is about 80% destined for external lighting by night; huge electrical signs will be a prominent part of the building’s Times Square façades. By day, about 80% of the power goes to the building’s base load. The hot water will be used in winter for perimeter heating. In summer, the water is wasted. Although there was hope that one of the absorption chillers could be fed by this water, a variety of considerations precluded this: the fuel cells are located on the fourth floor (accessible to maintenance and close to the huge signs), whereas the chillers are located in the penthouse.
Fig. 10.71 The 48-story office building at 4 Times Square, New York City (a), shown under construction in 1998. (b) Many environmentally friendly features are included, such as façade-integrated PV and two fuel cell power plants like those shown in Fig. 10.70. Fed by natural gas, the fuel cells are expected to work without maintenance interruptions for up to 5 years and provide considerable hot water as well as electricity. (The fuel cells are actually installed at the fourth-floor level rather than at the penthouse.) (Courtesy of Fox and Fowle, Architects, P.C., New York; Consentini Associates, Mechanical Engineers; the Durst Organization, Developer.)
The 200-kW fuel cell in Fig. 10.70 is usually fed by natural gas, but other examples include hydrogen fuel (Hamburg, Germany: electricity to the grid, hot water to heat an apartment building), anaerobic digester gas (Yonkers, New York: wastewater treatment plant; electricity and hot water used within the facility), and landfill gas (Groton, Connecticut: closed landfill; electricity to the grid, plans for a hydroponic tomato–growing greenhouse to utilize the hot water). Plentiful hot water is a by-product of the fuel cell; it seems that finding a year-round use for this water is often a problem of excess supply.
Other fuel cell types include alkaline (operates at 480ºF [250ºC]), molten carbonate (works faster at 1110–1300ºF [600–700ºC]), and solid oxide (1200 to 1830ºF [650 to 1000ºC]) with a higher efficiency, and waste heat sufficient to run gas turbines. These higher-temperature processes are more likely at central station power plants.
Clearly, cogeneration and fuel cells have a promising future.

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