Chapter 4: System Configurations

4.1. Introduction

4.2. System Descriptions and Points Lists

4.2.1. Single Duct, Constant Volume, Single Zone

4.2.2. Single Duct, Constant Volume, Reheat

4.2.3. Single Duct, Constant Volume, Bypass VAV

4.2.4. Single Duct VAV and VAV with Reheat

4.2.5. Hybrid Constant and Variable Volume Systems

4.2.6. Constant Volume and Variable Volume Multizone

4.2.7. Texas Multizone

4.2.8. Three Deck Multizone

4.2.9. Dual Duct Constant Volume and Variable Volume

4.2.10. Dual Duct, Dual Conduit

4.2.11. Low Temperature Air

4.2.12. Natural Ventilation Cycle



Figure 4.1 Typical Single Duct, Constant Volume, Single Zone Air Handling System

Figure 4.2 Multizone Unit


4.1.  Introduction

Many different air handling system configurations may be tested using the procedures in the Functional Testing Guide. For each of the twelve system configurations presented, the following information is provided:

·       Description of function

·       Points list

·       Appropriate applications

·       Energy conservation control strategies

The points lists are intended to be used by designers and commissioning providers as starting points for their own points lists. In a future Design Guide revision, system diagrams may also be provided (similar to Figure 5.1) for each configuration as links to AutoCAD drawings that can be used by designers and commissioning providers as starting points for their own system diagrams.

Users may encounter a system that does not match any of the configurations shown in this chapter. Such a system will likely be a variation on one of the basic systems, and the user should be able to adapt the information to their specific application.

System Variations

The system configurations begin with a single zone constant volume 100% outside air configuration. All subsequent configurations are recirculating systems with economizers. Systems that use 100% outdoor air are possible in almost all of the configurations discussed in this chapter, but we focus on recirculation systems because they are more typically found in office applications. Although the points list for 100% outside air systems may be somewhat simpler, designers should remember that freeze protection and humidity control become even more critical in air handlers that use 100% outside air.

A number of the air handling systems presented in this chapter must reheat the supply air to offset perimeter loads. Alternatively, any of these systems could use hydronic heat for the perimeter loads instead of the ducts, dampers, and controls associated with an all-air reheat system. Using hydronic perimeter heating can often be more easily implemented and controlled than reheating air. Perimeter hydronic heating also has the potential to be more comfortable than warm air due to radiant heating effects. As a result, space temperatures tend to run lower for equivalent comfort, and less reheat is required. Eliminating the reheat element pressure drop also saves fan energy, but this may be balanced by the pump energy of the heating water system.

The systems configurations in this chapter may require a return or relief fan if they recirculate air, however, the addition of these fans does not affect the system configuration. The pressure drops associated with the return and relief paths determine if the fans are required to avoid over-pressurizing the occupied zone due to the restriction created by the return and relief system. Additional discussion of this topic can be found in Chapter 20 Return, Relief and Exhaust. The examples below will assume the return or relief fans are required in order to demonstrate their impact on the point lists.

Static Pressure Safety Points, Limit Switches, and Permissive Interlocks

Static pressure safety points and limit switches have more to do with the outdoor air and life safety requirements associated with a system than the HVAC process it provides. The following three systems are examples of how the system size and configuration can affect the safety points installed:

·       A 100% outdoor air constant volume system rated for capacities in excess of 15,000 cfm and equipped with a fan capable of developing significant static pressures. There would typically be an inlet damper that closed when the system was not operating to isolate it from the external environment. In turn, these dampers may cause the designer to consider some combination of limit switches, static pressure safety switches and permissive interlocks to prevent the fan from starting with the dampers closed and damaging the fan casing or duct system.

·       The above 100% outdoor air system that is rated for less than 15,000 cfm would most likely not require smoke isolation dampers. However, the designer still may apply safeties to protect the inlet system and fan casing from problems associated with the failure of the inlet damper to open.

·       For an economizer system rated for less than 15,000 cfm with return ducts and a return damper, the designer may rightly deem the system safe to operate without any static pressure related interlocks or safety systems.

In short, the safety point functions will tend to be independent of the HVAC process associated with the system. Since the point lists associated with this chapter are related to the HVAC process, they will not reflect every safety point associated with smoke isolation or 100% outdoor air configurations. To illustrate these concepts, the Single Duct, Single Zone, Constant Volume System point list includes the points associated with protecting the intake system and fan casing from excessive negative pressures due to a failure of the inlet damper. The Single Duct Variable Volume Reheat System has been configured to reflect a system with smoke isolation requirements and the associated safety interlocks.

4.2. System Descriptions and Points Lists

For each system configuration described in this chapter, there is a corresponding points list that can be used as a starting point for creating your own points lists. Information for the control contractor regarding application of the points can be found in the footnotes of the Points List Spreadsheet. The last column in the spreadsheet, Design Guide Supplementary Notes, contains a more detailed explanation of point application specifically for designers and commissioning providers. At the end of each system description in this section, the points list is compared to a previous list. Within the Points List Spreadsheet, these differences are listed in bold type. Table 4.1 gives an overview of the points lists provided in the link below:

Bevel: Points List Spreadsheet

Link to an Excel spreadsheet of the points lists indicated in Table 5.1 below.

Bevel: Points List Explanations

Link to a document that contains the Points List Explanation notes referenced in the last column of the Points List Spreadsheet.

Table 4.1 Points List Overview

System Configuration

Tab in Excel Spreadsheet

System Reference

Constant Volume, Single Zone, 100% Outside Air



Constant Volume, Single Zone with Economizer and Return Fan

CV SZ econo & return


Constant Volume with Reheat

CV Reheat


Constant Volume with Bypass Variable Volume

Bypass VAV


Variable Air Volume with Reheat

VAV Reheat


Hybrid Constant and Variable Volume Systems


VAV with reheat

Constant Volume Multizone



Variable Volume Multizone



Texas Multizone

Texas MZ


Three Deck Multizone

3Deck MZ


Dual Duct Constant Volume



Dual Duct Variable Volume



Dual Duct Dual Conduit


Building envelope loads system: VAV; Internal loads system: Constant Volume Reheat

Low Temperature Air


VAV with reheat

Natural Ventilation


No mechanical cooling for pure natural ventilation, various applications for mixed mode systems.

4.2.1. Single Duct, Constant Volume, Single Zone

Figure 4.1 illustrates a typical single duct, constant volume, single zone air handling system. This air handling system is probably one of simplest configurations possible since there is no terminal control equipment. Most residential air handling systems are of this configuration. In a sophisticated application, the system can still have some complex components. The preheat coil in the figure can see large quantities of subfreezing air, requiring that special design steps be taken to prevent it from freezing. The constant volume pumped coil approach shown is one of many solutions to this particular design issue. Depending on the capacity of the system and the use of an economizer, the return/exhaust/relief fan may not be necessary.

More information regarding this particular system configuration can be found in: 2000 ASHRAE Handbook, Heating, Ventilating, and Air Conditioning Systems and Equipment, Chapter 2, American Society of Heating Refrigerating and Air Conditioning Engineers, 1791 Tullie Circle, N.E., Atlanta Georgia 30329, 404-636-8400,



Figure 4.1 Typical Single Duct, Constant Volume, Single Zone Air Handling System

Single Duct, Constant Volume, Single Zone with Economizer

With an economizer, the single zone constant volume system requires the following additional points:

·       Return air temperature

·       Mixed air temperature

·       Return air damper command and damper feedback (proof of operation)

·       Relief air damper command and damper feedback (proof of operation)

·       Relief air control point (outdoor/return damper signal or building static pressure signal) For constant volume systems, having the relief air dampers track with the economizer dampers will usually work reasonably well unless the building is excessively leaky. For additional information on this topic see the related discussions in Chapter 9: Economizer and Mixed Air.

·       Minimum outdoor air damper command and feedback (if applicable)

·       Zone CO2 Level (if applicable)

·       Manual override capability for the unoccupied cycle. The owner may desire a simple way for the zone occupants to override the operating schedule regardless of whether or not a facility operator is on site. Refer to Section 3.4.2 Manual Override for details regarding how to implement this function.

Single Duct, Constant Volume, Single Zone with Return or Relief Fan

For systems with a return or relief fan, the single zone constant volume system can include the following additional points:

·       Return/relief fan start/stop command and feedback

·       Return/relief fan hours of operation

·       Smoke detector and/or fire alarm interlocks will be required for the return or relief fans in most instances

·       Return/relief fan capacity control command and feedback. In the event that the relief dampers are not controlled by the same signal as the economizer, the return fan will need capacity control.

4.2.2. Single Duct, Constant Volume, Reheat

A typical single duct, constant volume air handling system with reheat delivers a fixed volume of cool supply air to multiple zones and reheats this air as demanded by the thermostat in each zone. The supply air temperature is set low enough to meet the zone with the highest demand for cooling. Terminal equipment at each zone consists of a set of steam, hot water, or electric reheat coils controlled by the zone thermostat. Constant volume with reheat provides comfort control for zones with unequal loads and is often used for applications with close temperature and/or humidity tolerances. In some instances, recovered energy can be utilized for reheat to minimize the energy intensity of the process.

The energy intensity associated with this process can be reduced by setting the minimum outside air flow appropriately for all zones. Constant volume reheat systems tend to require reheat for all zones if the minimum outside air flow has been set too high.

Energy can also be minimized by incorporating a strategy to reset the system supply temperature based on the cooling demand of the zone with the highest load. In a properly operating resetting routine, the zones at or equal to the maximum demand for cooling will be the only zones that are not being reheated. To make sure that air temperature is reset as high as possible during cooling mode, compare reheat valve positions. At least one reheat valve should be nearly closed. Values of 95 to 98% closed are common when using this routine. If the discharge temperature was reset until a valve was fully closed, then there would be no way of knowing if the zones in that state were satisfied or actually starting to overheat and required additional capacity through lower supply temperatures. The bottom line is that this approach minimizes the energy burden associated with a constant volume reheat system by keeping at least one zone on the verge of running out of cooling capacity.

Even with the reset strategy, the reheat process results in high energy use. Supply air temperatures in the system are generally set based on the sensible heat ratio in the space and the associated humidity control requirements. Air volumes are then determined based on this supply air temperature and the design load in the space. Since the spaces seldom see the full design load, any excess capacity must be used up in the reheat process to avoid overcooling the zone. There are several energy implications to this reheat process.

·       The cooling plant provides cooling that is reheated, in addition to the cooling that is required to meet the load in the space. Thus, the cooling plant tends to run at a constant load factor even though the load is varying.

·       The fans must deliver a constant volume of air based on the design load condition at a temperature based on dehumidification requirements in the space on a design day.

·       The heating plant, including boilers, pumps, and any other related equipment must run any time the reheat air handling system is operating if temperature control is to be maintained. This is true even in the summer, because reheat energy is required to prevent the zones from overcooling.

·       In the winter months, for reheat zones that serve perimeter areas, the supply air must be reheated to make up for the initial cooling of the air and then heated further to meet the any heating loads.

In an efficient constant volume reheat system, the supply air temperature should be reset based on the maximum demand for cooling, but during moderate cooling loads, a raised supply air temperature could result in problems with humidity control. Therefore, the reset schedule needs to include an upper limit. A lower limit is also desirable to prevent an errant zone or inappropriate operator command from lowering the set point beyond what is necessary to properly dehumidify the air.

If the system under consideration is a true constant volume reheat system, then the distribution system pressure or flow requirements for different zones to will not usually vary.[1] As a result, the zone control system requires no flow regulation and simply consists of a thermostat and the reheat valve. The most cost effective way to provide this function is usually with a stand-alone pneumatic, electric, or electronic thermostat. However, installing a DDC controller to perform the zone control functions can have benefits that often justify the added costs. Benefits of DDC control include:

·       Zone valve position and temperature data become available from a central location. This provides the following benefits that are useful for commissioning and ongoing operations.

1    Faster response to comfort problems.

2   Mitigation of comfort problems in critical areas before they become issues by using alarm settings.

3   Documentation of performance for quality control purposes or to demonstrate compliance with specified zone temperature requirements through trending functions.

·       Many current technology DDC terminal unit controllers have the capacity beyond what is required for basic control functions. These spare points can be used to provide energy conservation and improved performance in the following ways:

1    Monitoring of the reheat coil discharge air temperature for commissioning and diagnostic purposes. For example, an alarm can be generated if the reheat valve is commanded fully closed but there is still a temperature rise across the reheat coil due to a leaking valve.[2]

2   For systems serving zones with radically different operating schedules, a spare output can be used to control two position dampers in the branch ducts serving the different areas to shut down the air flow when the zone is unoccupied. This function is similar to a two-position VAV system. To be effective, this feature needs to be combined with at least one of the following measures to prevent the flow eliminated in one area from simply moving to other zones.

·       A variable speed drive at the supply fan controlled to maintain a constant pressure at some point in the duct system. This is usually the most cost effective approach because it maximizes the fan energy savings at a minimum of cost.

·       If the fan can tolerate being pushed up its curve, installing constant volume regulators on the zones will allow them to control flow as well as temperature. This usually is more costly than installing a variable speed drive unless there are only a few zones. In addition, the ongoing maintenance of the flow control loops adds complexity to the system.

Single Duct, Constant Volume with Reheat - Additional Points compared to Single Duct, Constant Volume, Single Zone System with economizer and return fan:

·       Reheat valve command and feedback

·       Discharge air temperature per reheat valve

·       Alternate configuration (points not included in example points list below): Zone flow control to allow flow to portions of the areas served by the system to be shut down when they are unoccupied while other areas remain in operation. Usually, this feature requires some form of capacity control at the air-handling unit, or some sort of volume regulation at the zones as discussed previously. These requirements add some points in addition to the point controlling the shut down damper.

4.2.3. Single Duct, Constant Volume, Bypass VAV

The single duct constant volume with terminal unit bypass is a slight variation on the constant volume reheat system. Instead of terminal reheat coils, the constant supply flow is varied to meet the cooling load in the space by diverting some of the air directly to the return plenum or duct, thus bypassing the space. Injecting supply air directly into the plenum can pressurize the plenum and cause the return air to flow back into the space. For this reason, the return plenum in bypass systems must be kept at a lower pressure than the occupied space. Return fans may be necessary to attain this low plenum pressure. Constant volume bypass VAV systems can be more energy efficient in cooling and heating modes than constant volume reheat systems since the bypass air reduces the return air temperature during cooling mode and increases the return air temperature during heating mode. As a result, the cooling or heating at the air handler is reduced. However, since a constant volume of air is moved through the space, fan energy is not saved compared to a conventional VAV system, which can often be much more significant than the heating or cooling energy.

The terminal equipment applied with this system can be very similar or identical to the terminal equipment associated with a conventional VAV system and can include reheat capability. For additional information on these terminals refer to Section 4.2.4 Single Duct VAV with Reheat. The point requirements for VAV terminals applied in the bypass VAV system configuration will be similar to those described in Section 4.2.4.

Single Duct, Constant Volume, Bypass Variable Air Volume - Additional or replacement points compared to Single Duct, Constant Volume Reheat System:

·       Return plenum pressure differential to zone pressure. Several sensors may be required if the plenum is subdivided by smoke or fire separations due to the pressure drops created by flow through the transfer ducts.

·       Bypass air damper command and feedback. Some systems use a common bypass damper, while others bypass at the zone level with the same actuator that controls the terminal damper. To check against bypassing too much air, in cooling and heating modes, at least one bypass damper should always be fully closed. Otherwise the discharge air temperature set point could be increased (for cooling) or reduced (for heating).

Since the zones in this application are variable air volume zones, each zone requires all of the points associated with variable air volume operation.

·       The least point-intensive approach to VAV systems is to provide pressure-dependent VAV operation. These terminal units will require damper and reheat control (may need reheat in addition to the bypass during minimum flow) and a space temperature sensor as an input.

·       Pressure independent control requires the same points as pressure-independent control plus a flow input from a flow-measuring element on the terminal unit intake.

4.2.4. Single Duct VAV and VAV with Reheat

The single duct variable air volume (VAV) system controls temperature by varying the supply air flow rate to each zone. Air flow is varied by modulating dampers at the terminal units, or VAV boxes. Variable volume systems are more energy efficient than constant volume systems, especially when loads vary across zones. Since each zone is supplied with the minimum amount of flow necessary for cooling or ventilation, the total airflow demanded is greatly reduced compared to constant volume systems. A fan capacity control mechanism[3] reduces the supply air flow and saves energy.

If perimeter heating loads are addressed by an independent perimeter heating system, like finned tube radiation, fan coil units, or radiant slabs, then it is often possible to achieve temperature control simply by varying air flow. In situations where the perimeter heating loads must be served from the VAV system, or where the ventilation requirements (and thus, the terminal unit minimum flow settings) are high, it will probably be necessary to provide reheat coils on many of the terminal units and operate the reheat system all year. Zones with less demand for cooling limit the flow through their VAV boxes, then reheat the supply air if the minimum flow exceeds space cooling demand. The reheat process in a VAV system adds energy, but it is far less significant than the reheat associated with a constant volume system. Using recovered energy from the refrigeration system condenser circuit to serve the reheat requirements in the summer can mitigate this energy burden to some extent.[4]

New VAV boxes are commonly pressure independent, with a flow sensor at the inlet of the box that regulates the zone airflow based on the zone temperature. Pressure dependent boxes, an older VAV technology, operate without this flow sensor. Instead of controlling the damper based on the measured air flow, the damper in a pressure dependent VAV box modulates based directly on the zone temperature. In this case, the flow varies with duct static pressure and is more difficult to control. In all cases, the terminal units are set up to provide some minimum flow level required for ventilation purposes, even if the airflow is not needed to serve the load during lightly loaded conditions. For pressure dependent systems, minimum flow is achieved simply by a limit on the minimum damper position. For pressure independent systems, a minimum flow set point must be met.

Instead of changing the supply air temperature (the only temperature control mechanism for the constant volume system), the variable volume system maintains a fixed supply temperature for similar outdoor air conditions. Although the VAV system does not need to change supply air temperature, for energy savings and comfort control, the supply air temperature can be reset based on ambient conditions or some other indicator. When supply temperature reset is used, the supply temperature is decreased in the summer to meet high cooling and humidity loads and increased in the winter to match the lower cooling loads and reduce reheat energy.

However, supply temperature reset needs to be applied with caution on VAV systems since it can counteract the variable flow operation, resulting in reduced fan energy savings and often nearly constant volume operation. Increasing the supply temperature setpoint so that the fan does not reduce speed is undesirable since the fan energy savings usually outweigh the heating and cooling energy savings achieved by the temperature reset.

Heating can be accomplished through hot water, steam, or electric reheat coils (with or without a fan-powered VAV box), or by circulating warm return plenum air using fan-powered or induction VAV boxes. In some situations where the loads served have high internal gains and the ventilation requirements are relatively low, reheat is not necessary during the summer months if some discharge temperature reset is provided. In these situations, reheat can be accomplished by using the perimeter heating system during the winter, spring, and fall months. Eliminating the reheat coils and the piping network and controls associated with them has the following benefits:

·       Reduced first costs and operating costs

·       Reduced potential for energy waste via unnecessary reheat due to inappropriate settings or valve leakage.

·       Parasitic losses associated with operating the reheat system during warm weather are eliminated.

·       Fan energy is saved since the reheat coil pressure drop has been eliminated from the system.

More information regarding this particular system configuration can be found in ASHRAE Systems and Equipment Handbook, 2000, p. 2.10.

Single Duct Variable Volume with Reheat - Additional Points compared to Single Duct, Constant Volume System with Reheat:

·       Supply and return/relief fans motor speed command and feedback (compare for VFD diagnostics)

·       Drive selector switch status

·       VAV box flow

·       VAV box discharge air temperature

4.2.5. Hybrid Constant and Variable Volume Systems

In large systems or systems that have undergone renovation, it is not unusual to find a combination of constant volume and variable volume zones in a single air handler. Health care, laboratory, and process applications are particularly prone to this configuration. In general, the central system portion of these air handling arrangements will look schematically identical to a VAV system with similar point and control requirements. The zone portion of the systems will appear to be schematically identical to the VAV zones with the constant volume feature achieved by operating the VAV terminals at a fixed volume. Usually the fixed volume is accomplished by setting the maximum and minimum flow settings of the VAV box to the same value, but it can also be accomplished using mechanical flow regulators that have no external control connections.

Regulating the constant volume flow is necessary to ensure that the flow and pressure variations created in the system by the operation of the VAV terminals do not cause flow variations in the constant volume areas. Without the flow regulation, the flow variations produced in the constant volume zones will tend to be above design flow since the system was probably balanced for design flow at maximum capacity. As the VAV terminals reduce flow, excess flow would occur at the constant volume zones. These flow variations can cause several problems.

·       The excess flows in the constant volume zones lead to even greater reheat loads (and the related parasitic loads) than would be seen if the constant volume flow was regulated, thus energy is wasted.

·       The excess flows reflect fan energy that could be saved if the constant volume flows were regulated, thus energy is wasted.

·       The excess flows in the constant volume zones can create pressure relationship problems between the constant volume zones and their surroundings. Without regulation on the constant volume zones, pressure relationship problems can be difficult to control and diagnose since they vary with the operation of the VAV terminals. Even with regulation on the constant volume loads, there can be pressure relationship problems that occur as the VAV terminals in surrounding areas operate. However, regulating the constant volume zones minimizes this potential. Additionally, if the flow regulation is provided with DDC technology, then the potential exists to write control algorithms that modify the constant volume parameters based on the current state of the air handling system as a field solution to a pressure control problem that may occur down the road.

Point requirements for these systems will be similar to those associated with the single duct VAV and single duct VAV reheat systems, as described in Section 4.2.4 Single Duct VAV with Reheat.

4.2.6. Constant Volume and Variable Volume Multizone


The traditional multizone includes separate ducts for the heating coils and cooling coils, traditionally called the hot deck and cold deck, respectively. The discharge of the air handler is divided into a number of zones via a zone damper assembly. For each zone, the full cold deck, the full hot deck, or a mixture of the two airstreams can be supplied - as the cold deck damper closes, the hot deck damper opens. The dampers modulate to provide a supply air temperature that adequately serves each zone’s heating or cooling load. The air is distributed by a single dedicated duct to each zone.

Multizone systems are often served by packaged air handlers that supply only up to sixteen zones, but these systems are cost-effective compared to built-up air handling systems. The traditional two-deck multizone cools and heats air that is supplied to a single zone, which is a reheat function that is not allowed by many energy codes. This issue is addressed by the three-deck multizone discussed in Section 4.2.8.[5] Although traditional multizones do have some reheat, multizones (without precooling coils) do not cool all of the supply air, since some of the supply air is diverted to the heating coils. As a result, multizone systems are more thermally efficient than constant volume reheat systems, which cool all of the supply air before reheating it as necessary. However, multizone systems are not as flexible as constant volume reheat systems for future reconfigurations since the zone configuration is based on the air handling unit configuration at the zone control dampers.

Multizone systems can be a low first-cost method of serving different loads in a number of zones and have the advantage of placing all of the zone equipment at the air handling unit location (except for the Texas Multizone, discussed in Section 4.2.7 Texas Multizone). This configuration lends itself to applications with a limited number of multiple zones where the zone requirements and configurations are unlikely to change over time.

The constant volume multizone supplies multiple zones with different loads using a constant volume fan. In most instances, this is accomplished with a single fan, but there are designs that utilize a fan for each deck for energy conservation purposes. In many ways, these designs are schematically similar to double duct systems.

The energy efficiency of a multizone system can be improved by adding a variable volume feature. The zone dampers can be delinked with independent actuators for the hot and cold deck dampers associated with each zone. The zones are then controlled to modulate the cold deck damper to a minimum position that will guarantee the required ventilation flow prior to opening the hot deck dampers. If the flow at the minimum cold deck position results in the space being overcooled, then the hot deck damper is allowed to modulate open as required to maintain temperature.

Installing and Commissioning Multizone Systems

Through the process of mixing hot and cold air streams at the air handler for each zone, thermal efficiency is lost due to significant heat transfer between the hot and cold decks at the air handler and leakage through the deck dampers. Over time, damper linkages must be well maintained to minimize this leakage. For larger building areas, extensive ductwork is required to supply air to remote spaces. Duct leakage along this distance should be minimized, since the leakage wastes energy and may result in uncomfortable conditions as remote zones are supplied a reduced amount of air.

Since the entering condition to both decks of the multizone is the mixed air condition for recirculating systems or the preheat coil leaving condition for 100% outdoor air systems, coordinating the economizer control or preheat control with the hot and cold deck control is important from an energy efficiency and performance standpoint. For a recirculating system operating on an economizer cycle, the depressed mixed air temperature (relative to the return air temperature) places an extra heating load on the system for any air that serves the hot deck, compared to serving the hot deck directly with return air. Thus, maintaining an economizer set point that is as warm as possible, perhaps based on the cooling demand, will minimize the reheat penalty at the hot deck.

Multizone units with high percentages of outdoor air which are located in humid environments often use a precool coil upstream of the hot and cold decks. Without a cooling coil upstream of the hot deck, all air passing through the hot deck to the zones will be at a specific humidity determined by the mix point for the return air and outdoor air. For a 100% outdoor air system or a system in economizing mode, the hot deck can be 100% outdoor air. Integrating the economizer cycle with the operation of the cold deck capacity control needs to be used with caution since the air going out to the loads through the hot deck may not pass through a cooling coil and thus may not be dehumidified. Similarly, on a 100% outdoor air multizone unit, the preheat function should be coordinated with the control of the cold deck so that the preheat coil does not heat air any higher than the requirement of the cold deck.

Deck temperature reset routines targeted at maximizing the cold deck temperature and minimizing the hot deck temperature can be particularly important for multizone units from an energy conservation standpoint for several reasons:

·       These routines will minimize the heating energy in the hot deck and cooling energy in the cold deck.

·       The closer the deck temperatures are, the lower the thermal losses will be from the air from a zone at full cold deck flowing past the fully closed, but warm hot deck damper blades as the air passes through the zone dampers. Similar effects will be minimized on zones demanding full hot deck.

·       The thermal losses through the hot deck casing will be minimized.

For multizone systems serving internal zones, it may be possible to reset the hot deck temperature as low as the return temperature since there is no need to heat the internal zones. Multizone units serving perimeter zones can also use return air in the hot deck once the outdoor conditions rise to the point where the zone is experiencing a net heat gain. In most buildings, this occurs somewhere between 60 and 70°F outdoor air temperature unless there are extensive areas of glass. Perimeter zones that have an independent perimeter heating system can also use this approach to hot deck reset.

In order to have a reasonable control response, the pressure drop through the hot deck and cold deck needs to be nearly identical. This is often accomplished by using a smaller coil in the hot deck section since the hot deck coil will typically be shallower than the cold deck coil and will never be wet. Some systems also have a baffle plate in series with the coil to tune this pressure drop relationship. These arrangements can produce high static pressure losses that translate to a constant fan energy penalty.

Figure 4.2 Multizone Unit 

Constant volume multizone - Additional or replacement points compared to Single Duct, Constant Volume, Single Zone System with Economizer and Return Fan:

·       Preheat coil leaving air temperature is the hot deck temperature

·       Preheat coil leaving water temperature is the hot deck coil leaving water temperature

·       Cooling coil leaving air temperature is the cold deck temperature

·       A unit with a high percentage of outdoor air may require a preheat coil and/or a precool coil ahead of the fan. (Add a preheat leaving air temperature sensor, valve command, and feedback and/or a precool leaving air temperature sensor, valve command, and feedback) These configurations are not included in the points list below.

·       Mixing damper position command and feedback for each zone

·       Zone temperature feed back (to zone dampers)

Variable Volume Multizone - Additional or replacement points compared to Constant Volume Multizone

·       For each zone, a hot deck damper command and feedback independent from a cold deck damper command and feedback

4.2.7. Texas Multizone

The Texas multizone, designed to serve hot and humid climates, is a modification of the traditional two deck multizone unit. In hot and humid climates, the cold deck air typically must be overcooled for adequate dehumidification. Return air (typically called the neutral deck) is used instead of an actual hot deck to provide some reheat for each zone. The use of return air as the first stage of reheat saves energy compared to the traditional multizone configuration. Additional heating is provided by independent reheat coils in the individual zone ducts, often at a location near the zone they serve. The zone reheat valve is modulated open after the zone dampers are in the full return air position. Placing the reheat coils in the individual zone ducts saves energy by ensuring that only the air for the zone with a heating demand that cannot be met by the return air will use additional reheat.

Since there is not heating coil in the hot deck, all air flow resistance required for creating an equivalent pressure drop through the hot deck compared to the cold deck is provided by a baffle plate.

Additional or replacement points compared to Constant volume multizone:

·       Hot deck temperature is replaced by mixed air temperature

·       Perimeter zone reheat valve command and feedback

·       Leaving air temperature from zone reheat coil. Use for commissioning and troubleshooting.

4.2.8. Three Deck Multizone

The three deck multizone adds a neutral deck (mixed air) between the cold deck and hot deck. Through the zone damper configuration, cold deck and hot deck air are not allowed to mix. The neutral deck air is mixed with the cold deck or hot deck air to meet the space requirements. In this way, there is no reheat energy used. Full neutral deck air can also be supplied for low heating loads. Like the Texas multizone, using return air for reheat is a heat recovery function, which makes this strategy more energy efficient than the traditional multizone.

The neutral mixed air condition can be much different than neutral return air, especially if the unit requires a high outside air percentage and is located in a hot and humid climate. In these cases, the humidity of the neutral and hot decks may be too high to meet space conditions. The cold deck can be overcooled to reduce its humidity further, or the outdoor air can be pre-cooled to reduce the latent load of the mixture of outside and return air that supplies the hot, neutral, and cold decks. In this way, the neutral deck can be used to temper the cold deck air to meet the space condition required at each zone without increasing humidity to inappropriate levels. This precooling configuration adds a reheat function to the three-deck multizone if the hot deck is utilized during times that pre-cooling occurs.

Three deck multizone - Additional or replacement points compared to Constant volume multizone

·       Mixing damper position for each zone (one damper linkage for three decks)

·       Neutral deck temperature

4.2.9. Dual Duct Constant Volume and Variable Volume


Dual duct systems consist of a hot supply air duct and a cold supply air duct that run throughout the building to each terminal unit. This configuration is similar to a multizone, except mixing does not occur at the air handler, but at a mixing box at each zone location. Dual duct constant volume systems maintain a constant volume of air to each zone terminal unit, and vary space temperature by changing the fraction of hot and cold air that is mixed. Since the hot and cold decks may have different static pressures, controls must be used to maintain a constant flow through each terminal unit. The temperature of the hot deck can be reset higher in the winter and lower in the summer to save reheat energy. If heating loads are low enough and the system has two fans, return air can be utilized in the hot deck instead of the heating coils. Similar to traditional constant volume multizone systems, dual duct constant volume systems lose thermal efficiency due to the cooling and reheating process that occurs to maintain space temperature. Dual duct constant volume systems are an improvement over constant volume reheat systems since not all air passes through both the cooling and heating coils, thus some reheat energy is saved.

Dual duct variable volume systems are more efficient than dual duct constant volume systems because the cold deck and hot deck airflow can be modulated at each zone based on the zone thermostat. As a result, fan power and reheat are reduced, since the volume of air to the space can be reduced to a minimum before the hot deck air is introduced.

Both constant volume and variable volume systems can utilize a single fan for both the hot and cold ducts or two fans, one for each duct. When a single fan serves both ducts, the mixed air (mix of return and minimum outdoor air for ventilation) is sent through both the heating and cooling coils. Thus, the ventilation air can lead to increased hot deck reheat energy compared to full return air, and the single fan configuration may not be conducive to economizer use. With separate hot deck and cold deck fans, the hot deck can draw from the return airflow directly, while the cold deck can draw from the outside air as necessary for economizing. The thermal energy savings can make up for the potential increase in fan power from the two fans.

Installing and Commissioning Dual Duct Systems

In this section, the challenges in installing and commissioning dual duct systems for energy efficient operation are discussed.

In order to use the hot deck for only reheat and avoid sizing the hot deck for perimeter loads, most dual duct systems have reheat coils at the perimeter zones to handle the envelope losses. In the dual fan case, the minimum flow for ventilation must come from the cold deck since the hot deck is pure return air. In this way, for zones with heating loads, the reheat is similar to a VAV or constant volume reheat system.

The dual duct system has nearly twice as much sheet metal as you would for other approaches since you end up running two ducts, each one of which must be sized for more than 50% of the air flow, and generally end up being the same size. Even if the ducts were sized for 50% air flow each, there would still be more metal used in the duct system at an equivalent friction rate due to perimeter vs. cross-section issues. (For instance, two 12 x 12 ducts have 8 feet of perimeter and can not carry as much air as one 12 x 24 duct, which has 6 feet of perimeter.)

All of this extra ductwork creates congestion in the ceiling cavity. When tapping the ducts for a terminal unit, one duct will cross the other; i.e. if you connect on the cold deck side, then the hot deck connection has to cross over or under the cold deck to get to the terminal unit. A solution to this problem is to put the supply ducts up high and the terminals down low and tap the bottom of the duct. However, most buildings do not have enough ceiling space to do this.

From a control standpoint, the dual duct system is more complex, especially with the dual fan. For instance, you have one return fan that has to track with two VFD supply fans. In addition, the hot deck fan must never move more air than the return fan; otherwise, it will pull mixed air into the hot deck portion of the system. Designing the controls to make the system work under all operating modes is not easy.

At the terminal unit, similar control problems can exist. Two dampers must be controlled for correct minimum and maximum air flow and space temperature. Dual duct systems can waste energy when hot and cold deck dampers at each zone do not fully seal, creating a false load and unnecessary heating or cooling. Commissioning and maintaining the damper close-off positions on possibly hundreds of dual duct terminal units can be a difficult task.

Single Fan Dual Duct Constant Volume: Additional or replacement points compared to Constant Volume Multizone system

·       Mixing damper command (at the zone instead of at the air handler)

·       For any terminal units with supplemental perimeter heating with reheat coils, similar to constant volume reheat system (not shown in points list)

Dual Fan Dual Duct Constant Volume: Additional or replacement points compared to Single Fan Dual Duct Constant Volume

·       Start/stop command output and proof of operation input for both hot deck and cold deck fans.

Dual duct variable volume: Additional or replacement points compared to Variable Volume Multizone system

·       Motor speed command and feedback (compare for VFD diagnostics)

·       VAV box flow

·       VAV box discharge air temperature

4.2.10. Dual Duct, Dual Conduit


The dual duct, dual conduit system is a subtle variation of the dual duct dual fan approach that provides supply air for building envelope loads separately from internal loads with two separate supply conduits (supply ducts). The hot deck in a dual duct, dual fan system may use mixed air or full return air, and therefore is not decoupled from the outside air. The dual duct, dual conduit configuration creates a separate system that can be tailored to the needs of the perimeter loads, which will be a heating load part of the year. The second system can be tailored to the needs of the interior, which will generally have a year round cooling load. These systems have frequently been high velocity systems that used induction units[6] for the terminal devices and served high-rise buildings.

Since the perimeter load condition varies with the season, the temperature of the supply air for the envelope loads usually is varied based on ambient conditions. In many applications, independent perimeter systems are provided for each face of the building. This configuration allows the supply air temperature for each face to be tailored to the current load conditions. For instance, a building with a lot of glass on a sunny winter day may actually require cooling on the perimeter on the South face for a portion of the day, but require heating on the perimeter on the North face at the same time. In some arrangements, the perimeter terminal equipment can be an all-air system. In other arrangements, the perimeter terminals consist of reheat coils or chilled-hot water coils. Systems with chilled-hot water coils could be operated in many modes including:

·       Supply cold air to the terminals with chilled water in the coils for supplemental sensible cooling on peak days

·       Supply cold air to the terminals with hot water in the coils for operation more along the lines of a constant volume reheat system for days when some zones, but not all zones might require some heat while others were a net cooling load.

·       Supply warm air to the terminals with hot water to the coils for peak heating loads.

Many times, the hot water system supply temperature is operated on a reset schedule based on outdoor ambient conditions.

The supply air system that serves internal cooling loads and ventilation loads for dual conduit system is cool year-round since, in theory, an internal space will never see a net energy loss if it is surrounded on all sides by conditioned spaces. In the past, induction terminals that mimic current technology VAV and VAV reheat systems served the interior zones. Many of the older versions of this system have been converted to current technology VAV and VAV reheat systems to reduce the operating static requirements.

In a variation on this system, one conduit or duct system is heating-only, utilizing return air and a heating coil while the other is capable of cooling and handles the conditioning of the ventilation air. In this configuration, the envelope and the internal zones that require reheat terminate in a dual duct mixing box and the system tends to operate like a dual duct system.

Installing and Commissioning Dual Duct, Dual Conduit Systems

There are some operational and commissioning issues that are unique to dual conduit systems. Terminal units that were equipped with chilled water coils typically had a small drip pan that was intended to collect any minor condensation that occurred when chilled water was in the coil. Generally, these pans were not piped to drain (hence the name drip pan vs. drain pan). While in theory this is reasonable, since the ambient air dew point should have been reduced by the central system, infiltration through the envelope on the perimeter can cause locally high dew points resulting in condensation problems that overwhelm the drip pans and cause water damage. This problem is most prevalent in older high-rises with leaky envelopes located in humid environments. It can be especially pronounced if the building is operated on a schedule and pressurization is not maintained overnight. When this occurs, the stack effect or other phenomenon that can make the building negative, like a continuously operating exhaust fan with no positive make-up air source, fills the building with humid air during the shutdown period. Then, when the systems are restarted, they must deal with a dehumidification load, and, if chilled water is introduced into the perimeter system coils, the dehumidification occurs there, often with disastrous results in terms of water damage. This usually terminates any efforts at operating the building on a schedule, much to the detriment of its energy efficiency.

This problem usually can be mitigated in the following manner. If not all of the central systems are shut down and the cooling plant remains in operation, then the systems that stay on line will hold the dew point of the building at the intended level, eliminating the start-up dehumidification load and associated condensation problems. In most cases, the systems can be operated in a pure recirculation mode (no minimum outdoor air) to conserve energy. However, introducing some outdoor air will help pressurize the building and may prove beneficial in terms of avoiding localized condensation at start-up due to localize infiltration of the envelope.

From an energy approach, this method of unoccupied operation will use more energy than simply shutting everything down because it requires operating enough air handling capacity to adequately circulate some air through most of the building in some manner. But this can often be accomplished by running only a few of the systems because the real issue is controlling the vapor pressure in the building, not the temperature, and the water vapor will migrate to the area of lower vapor pressure even if there is no active air flow forcing it in that direction. It also requires operating the cooling plant at a low load condition. But, experience has shown that operating in this manner will prevent the water damage problems if the systems that remain on line are carefully selected. And since the energy used to keep these systems running with the chiller plant at low load is often less than the pull down load that occurs if the building fills with humid air during the off cycle.[7] Thus, as an alternative to not shutting anything down during hot and humid weather, it offers an attractive method to achieve some of the fan energy savings available via scheduled operation without risking damage to the building finishes and the subsequent termination of any scheduled operation. Savings can still be significant since fan energy is a major constituent of the overall energy consumption pattern of many buildings, often several times the consumption of the cooling plant.

Building envelope loads system: Single duct VAV

Internal loads system: Constant volume with reheat

4.2.11. Low Temperature Air

Low temperature air systems have the opportunity to reduce costs – both annual operating costs and first costs. The principle behind this system is that by supplying air between 45°F and 48°F instead of the typical 55°F supply air, the volume of air supplied can be greatly reduced. Low temperature systems can reduce supply air volume by 30%-40%. The fan energy savings and other benefits must be weighed against any increases in energy and other drawbacks to decide if a low temperature air system is appropriate for a given application.

The reduction in air flow requirements translates into a number of related benefits:

·       Reduced fan power A smaller fan motor can be selected, and/or the VFD will have greater turn down. It also may be possible to eliminate the return fan. Lower fan horsepower also reduces fan heat in the supply air stream.

·       Smaller air handlers Smaller duct sizes and reduced coil face area will decrease the footprint of the air handler. The area needed for vertical air shafts is also reduced.

·       Smaller ductwork In cramped spaces above the ceiling, smaller ductwork makes installation easier. The space savings from smaller ductwork can also lead to shorter floor-to-floor height.

·       Smaller terminal units Smaller terminal units are cheaper and save space above the ceiling.

Lower supply air temperatures result in lower room relative humidity, which has the following benefits:

·       Indoor air quality Less potential for growth of mildew and mold due to the reduction of condensation. To achieve this, the building envelope needs to have low infiltration. Slightly positive pressurization is an effective way to achieve this; rather than humid air infiltrating, dry air exfiltrates.

·       Longevity Building materials and finishes are likely to last longer with reduced humidity.

·       Raise space temperature setpoint Lower humidity allows a higher temperature setpoint for equivalent comfort. To avoid dumping cold air on occupants, low temperature air systems typically use high-aspiration diffusers that increase room air mixing and diffuser throw. Fan-powered VAV terminal units can also be used to improve mixing with constant fan operation during occupied hours.

While the overall system often saves energy, a number of aspects of the low temperature air system reduce this total savings. Designers should be aware of the following energy increases in their calculations of system energy consumption:

·       Chilled water system efficiency Low temperature air systems rely on low flow (high delta T) chilled water systems, which tend to increase energy chiller energy slightly. Producing colder water (leaving chilled water temperatures of 42°F to 38°F) can reduce chiller efficiency by 6%-10%. Efficiency can be improved by selecting a chiller precisely for the desired chilled water temperature. To further optimize the chillers, the actual load profile must be used to optimize the chiller part-load efficiency. This actual load can be difficult to predict during design. If actual loads are variable, there may be a mismatch between the load profile that the chillers have been optimized for and the load profile that the chillers will actually see.

·       Cooling energy With a low temperature air system, significant dehumidification occurs that is not necessary to meet a typical space design condition. The lower humidity levels mean larger cooling loads for an equivalent zone temperature setpoint. Less air is being conditioned to this lower humidity, so the fan energy savings can offset or partially offset this increased cooling and dehumidification energy.

·       Reheat Compared to a traditional 55°F supply air system, the low temperature system will cause zone reheat to increase if the minimum air for ventilation has greater cooling capacity than the load in the space requires; zones reheat 41°F air rather than 55°F air. Resetting the amount of ventilation air based on actual ventilation requirements can minimize the reheat loads, although this strategy requires a means of sensing and controlling outdoor air flow and a method of detecting occupancy (typically CO2 sensors). These items add complexity to the system and must be well maintained for persistence of savings.

Even if the spaces do not run at minimum flow with reheat in the summer months due to the loads on them, they will run at minimum flow in the heating mode. If chilled water was not being used for cooling in any of these zones, then the supply air temperature could be raised to avoid excess reheat. Raising the supply temperature may be difficult because different zones will transition into the heating mode at different times depending on their exposure and loading.

·       Economizer cooling hours Using low temperature supply air means that the chillers have to run until the outdoor air temperature is below 41°F instead of 55°F. In moderate climates, this difference can translate into thousands of hours per year of additional chiller plant operation.

·       Condensation In moderate climates, buildings do not typically need to be kept pressurized or dehumidified overnight[8]. In these moderate climates, the outside air dew point can be greater than the low temperature system supply air temperature. If the building is shut down overnight or over the weekend, the stack effect and the difference between the vapor pressure inside and outside can fill the building with outside air. Since this outside air has a higher dew point temperature than the supply air temperature, condensation problems can occur during start-up. The cold supply air can cool the diffusers and other objects below the dew point of the outside air that infiltrated into the building, with resulting condensation damage and indoor air quality problems. Operable windows also represent a path for outside air to enter the building and cause condensation problems upon start-up.

4.2.12. Natural Ventilation Cycle

The systems previously described consist of mechanical equipment for heating, cooling, and ventilation for commercial buildings. Buildings can also be designed to provide comfortable thermal conditions without using air handling equipment, or only using the equipment for part of the year. Natural ventilation is a low-energy approach to fully meet ventilation requirements and fully or partially meet cooling requirements. Airflow in a natural ventilation cycle may rely on the stack effect, operable windows, wind pressure, wind driven ventilators, or other pressure effects. When the outdoor air is cooler than the air in the building, the stack effect causes warm air inside the building to rise and exit at the top levels and colder, more dense outdoor air to enter at the lower level. The neutral plane can be raised to the top of the building by creating a venturi as the air exits the top floor. Natural ventilation is a site-specific way to bring in outside air for cooling and ventilation without using mechanical energy.

If the natural ventilation cycle cannot meet cooling requirements, an air handler can take over. When natural ventilation and mechanical ventilation systems are implemented together, it is often referred to as a mixed mode system. Due to their highly customized nature, natural ventilation and mixed mode strategies are not specifically covered in this Guide, but commissioning the components of these systems, such as dampers and actuators, are covered. Thus, a commissioning practitioner faced with a project that includes such a system should be able to develop a test plan by assembling the necessary components from the information presented.


[1]   This should be very carefully considered during the design process. Installing unnecessary constant volume regulators adds an on going energy burden to the system due to their static pressure requirement. It also adds complexity and first cost due to the flow regulation and control requirement associated with it. This added complexity will ripple out into the operating life of the building as a higher maintenance cost.

[2]   Measuring the temperature across the reheat coil is not as simple as it sounds, so judicious application to the larger zones may be more viable than application to all zones. To be effective, the initial and ongoing commissioning process needs to perform a relative calibration between these sensors and the main discharge air sensor to ensure that an indication of a temperature rise is truly a temperature rise and not a false indication due to sensor calibration accuracy issues. In addition, duct temperature rise needs to be accounted for in some manner. And if the discharge sensor is located ahead of the fan, the fan heat also needs to be accounted for.

[3]   Typically, the fan capacity is controlled using a variable frequency drive, inlet guide vanes, variable blade pitch, or discharge dampers. All of these approaches will save the fan energy associated with the flow reduction, although some strategies save more energy than others. For smaller systems with low peaks on their fan curves, the complexity associated with these fan capacity control approaches may not be warranted when compared to simply allowing the VAV terminals push the fan operating point up the curve. Pushing the fan operating point up its curve is also the process provided by a discharge damper, the least desirable of the capacity control mechanisms due to the low fan energy savings potential and the impact the damper can have on the fan’s performance due to system effect.

[4]   Sellers, David and Tom Stewart, “Making Energy Intensive HVAC Processes More Sustainable via Low Temperature Heat Recovery”, Proceedings of ACEEE 2002.

[5]   An interesting and unusual variation on this approach has been observed where a chilled-hot water coil was installed in the traditional cold deck position, and the hot deck was simply a bypass of the cold deck. When outdoor conditions were not suitable for cooling, the chilled-hot water coil was served with chilled water and the temperature control function was achieved by mixing air discharged from the cooling coil with air that bypassed it. In this mode, the bypass air was very near the return temperature since the system was recirculating and only bringing in the minimum outdoor air required for ventilation. When outdoor conditions were suitable for economizer cooling, the chilled-hot water coil was served with low temperature hot water, the action of the thermostats was reversed and the traditional cold deck became the hot deck and the bypass deck became the cold deck, served by economizer cooling.

[6] Induction units are discussed in more detail in Chapter 19: Terminal Equipment.

[7]   When the building fills with humid air, the moisture content of the moisture absorbing materials in the building increases. To bring the building back under control, all of this moisture needs to be pulled back out of the materials and shows up as a part of the pull down load, just like the thermal energy that ends up being stored in the building elements when they are allowed to warm-up over night.

[8] In hot and humid climates, the building must be kept dehumidified and slightly pressurized during all hours of the cooling season to guard against condensation problems.