Variable Air Volume

Schematic diagram of a built-up VAV system served by chilled water and hot water coils. (Source: IBPSA-USA BEM Workshop)

A Variable Air Volume (VAV) system is an all-air HVAC system that varies the amount of supply air flowing to each zone to maintain comfort while generally keeping the supply temperature of the air fixed (or at least relatively constant). A modern VAV air handler has a fan using a variable frequency drive that reduces the airflow as needed, but historically other methods have been used, such as variable inlet vanes or an outlet damper. The air handler consists of a fan, an air filter, a cooling coil, often using chilled water, and an optional preheat coil, often using hot water. The chilled water and hot water are typically from the primary plant’s chiller and boiler. For smaller buildings, a direct-expansion cooling coil may be paired with a number of different types of preheat coils. Each zone is then served by a VAV terminal box that includes at least a damper controlled by a thermostat to reduce the amount of air coming into the zone for lower cooling loads. The VAV terminal unit, usually called a VAV box, often has a reheat coil to provide warmer air to the zone when it needs heat. Often the reheat is either by electric resistance or a hot water coil. Each VAV box often serves a number of different spaces, each of which gets a fraction of the airflow provided.

Variable air volume systems are a common system for many types of buildings and have been since the 1970s. While many different control systems and configuration options exist to make them more efficient, the VAV system is often considered the baseline system configuration for medium to large buildings. In this role, designs for alternative systems are often compared to VAV to justify their added benefits compared to cost differences.

How It Works

Example VAV system design. (Source: IBPSA-USA BEM Workshop)

Three components are key to any variable air volume (VAV) system:

• A fan that can modulate the amount of supply-air flow
• Terminal boxes that can modulate the amount of supply-air flow into a zone
• Controls for those components and others based on various sensors

These components together provide a simple system that is relatively low cost in installation, operation, and maintenance, can respond to changing loads from various zones throughout a building, is quite reliable, and can be installed in a wide variety of occupancies.

A typical variable air volume system is shown in the figure below:

Air returning from the zone is brought into the air handler by a return fan which is also able to modulate the amount of air, often by using a variable frequency drive. The air handler includes dampers that provide supply air by mixing outdoor air and return air in order to meet the outdoor air requirements for the zones being served. When outdoor temperatures are lower than the supply air temperature but not too low, greater outdoor air quantities are used in what is called an economizer mode. The supply air is then filtered and conditioned by a cooling coil and often a preheat coil. The cooling coil can either be hydronic, using water pumped from a central plant that includes a chiller, or can be a direct-expansion coil for a smaller VAV system, often located on a rooftop. The cooling coil provides dehumidification, and condensate is often collected under the coil and drained away from the air handling unit. The preheat coil is necessary for colder climates where even with minimal outdoor air, the supply air would be colder than desired if no heat was added.

The supply fan, which provides a variable volume of supply air to the supply duct, draws the air over the cooling coil and, when present, the preheat coil. The supply fan is typically a variable speed fan using a variable frequency drive or, for older systems, has inlet guide vanes or dampers or other means of reducing airflow. The supply air temperature in the duct is maintained at a constant temperature, usually near 55F, although it may be reset depending on the season or outdoor air temperature. The temperature is controlled by sensors in the duct and modulated by the cooling coil, preheat coil, and economizer operation.

Common VAV terminal unit types. (Source: IBPSA-USA BEM Workshop)

Photograph of an installed VAV box. (Image courtesy of Kolderup Consulting)

Variable air volume terminal boxes, or VAV boxes, come in a variety of configurations:

• Throttle without reheat - the box modulates the amount of air supplied to the zone by using an air valve or damper to reduce the supply flow. The amount of air is modulated in response to the zone air temperature thermostat or sensor. Most are pressure independent so that the flow control is in response to the zone air temperature and not the supply duct pressure. Without reheat, a zone can be over-cooled, so these are only appropriate for zones that are not expecting heating, such as some core zones.
• Throttle with reheat - the addition of a reheat coil allows these terminal boxes to be applied to all zones, including perimeter zones. The reheat coil is either a hydronic coil that is supplied hot water from a central boiler or an electric resistance coil. The control of the air volume during cooling and heating operation is described more below.
• Induction - the box pulls air from the room before supplying it back to the room, enabling a more consistent airflow from the supply diffuser and also allowing for a lower temperature supply temperature which reduces the airflow required from the supply fan. Induction units often also require higher inlet pressure which reduces the energy savings from the supply fan. These units are not as commonly specified anymore, due to the many advantages of fan-powered boxes.
• Parallel fan-powered or parallel powered induction unit (PIU) - a fan is located in the box, not in the primary supply air stream, and allows continuous air circulation in the zone. Pulling air from a common return plenum can also help circulate outdoor air between zones. The fan can be controlled to run intermittently and is usually fixed speed. During unoccupied hours, the primary system supply fan can remain off, and the box fan can run as needed along with the reheat coil to keep the zone above the unoccupied set point temperature.
• Series fan-powered or series powered induction unit (PIU) - a variable speed fan is located in series with the primary supply airstream with the fan running continuously when the zone is occupied. The operation is similar to a parallel fan power box. Both types of fan-powered boxes often recover heat from lights in the zone and minimize or eliminate the need for reheat.

Many different strategies can be employed in VAV systems to optimize the operation, and some control sequences are now required by energy codes and standards. Variable air volume systems do need to be carefully designed since a single zone can often drive the behavior of the entire system. The zone that needs the highest fraction of outdoor may set that fraction for all zones served by the system.

Information Needed for the Model

Many different design and control parameters are needed for most simulation tools related to variable air volume (VAV) systems. In addition, most building energy simulation tools group parameters such as those specified at the system level different than the zone level, and these groupings may be different depending on the exact nature of the VAV system selected (hydronic versus direct expansion, for example) so it is always good to have a full understanding of the system being modeled prior to beginning the modeling process.

The parameters typically needed to model a VAV system include the following:

• Supply and return fan minimum and maximum flow rate and characteristics of the fans, including motor and impeller efficiency, pressure drop characteristics, and part load performance
• Cooling coil type and cooling system efficiency and performance (if direct expansion)
• Central preheat coil type and availability schedule and efficiency and parasitics if gas fired
• Supply set-point schedule (both daily and seasonal), reset strategy, and humidity controls
• Minimum and maximum outdoor air flow rate, economizer control type, and temperature or enthalpy limits
• VAV box sizing parameters related to maximum airflow rate, required outdoor airflow rate, and minimum air flow rate
• VAV box reheat coil type and related parameters
• VAV box flow control parameters for normal operation and during reheating (including dual maximum if specified)
• For fan-powered VAV boxes, the type of flow (parallel or series), the source and volume of the recirculation flow, the fan performance, and control parameters
• Zone set-point, humidity requirements, and outdoor air requirements
• Various other control parameters, such as night-time operation

Because of the many options and variations available with VAV systems, building energy simulation tools often have dozens of inputs related to their sizing, operation, and control.

Common Measures

Several broad categories of measures are common for VAV systems:

• Efficiency - measures that improve the performance of a system by improving the efficiency of the components, usually the fans and coils.
• Design - measures that reduce the volume of air by using different temperatures or pressures and result in reducing the size of the duct work and major components, including the fan. For VAV boxes allowing even larger turn-down ratios enable even lower flows to be achieved, especially if the boxes are right-sized rather than oversized. Greater use of preheat instead of reheat can be beneficial depending on their relative efficiency.
• Control - measures that use more sophisticated control sequences, sometimes with additional sensors, in order to improve performance at different operating conditions, including resetting supply temperature. Better economizer controls can also reduce active cooling, and ventilation optimization can reduce the amount of outdoor air. Optimal start and fan-pressure optimization can also be implemented. Demand-controlled ventilation can be coupled with VAV to reduce outdoor air provided to each zone.
• Fault - measures that, when implemented, help building operators diagnose and rectify problems with the operation of the system - these are often difficult to model in most simulation programs.

For more measures, please see the Additional Resources section.

Variable air volume systems are so prevalent in design that they are frequently used as the basis of comparison for other systems able to serve the same purpose. Other systems that may be considered instead of variable air columns systems include

• Variable refrigerant flow (VRF)
• Water source heat pumps
• Radiant systems,
• Chilled beam,
• Systems using DOAS
• Displacement ventilation

Common Control Options

A wide range of control options is available for VAV systems, although not all of them are typically modelable in modern simulation software tools.

Fan Control Options

Fan control options impact how the decrease in flow results in a decrease in energy. Modern systems use variable speed drives, but older systems may use inlet vanes or discharge dampers. These fan control options are often reflected in an energy simulation model by the part load characteristics of the fan at different fan fractions.

Reheat Control

The behavior of the VAV box in heating is a very important parameter for any simulation. For many simulation programs, the thermostat or heating “action” can include the “normal” mode when the amount of supply air flow does not increase with increasing demand for heat but stays at the VAV box minimum.  For “reverse” action, as the heating load increases, so does the flow volume. First, the hot water flow for the reheat coil is increased until it reaches the maximum at the minimum airflow box position, and then when that is not sufficient, the airflow rate through the box is increased until fully open. A variation on this is the “dual maximum” control would have a separate maximum airflow rate specified for the heating airflow. Note that many modern building energy standards, including 90.1 and Title 24, require the dual maximum control logic for VAV boxes. The amount of time the system spends at lower supply air flows is increased substantially using the dual maximum approach, resulting in fan energy savings.

• Comparison of dual maximum flow control to older (less efficient) VAV box control. (Source: IBPSA-USA BEM Workshop)
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Static Pressure Control

Impact of static pressure reset control on VAV system fan energy. (Source: IBPSA-USA BEM Workshop)

Typically VAV systems need to provide adequate pressure in the duct to supply air to all the boxes. Higher pressure increases the energy used by the central fan, so methods to reduce this pressure have direct energy benefits. The most common approach is to have a single pressure sensor in the duct that represents the system. Choosing the actual location of this sensor can be based on the designer’s previous experience and often following a rule of thumb, such as two-thirds or more down the main duct. This pressure control is not always optimal since all boxes may be partially closed, and the pressure is still maintained at the same set point. If at least one terminal box is fully open and the other boxes are partially open with a lower duct pressure, the same flow to the boxes is possible with lower overall pressure on the central supply fan. This is a control system called static pressure set point reset and can be accomplished by a DDC system that keeps track of all the terminal box flow rates. Several different approaches can be used, including “trim and respond” as described in Taylor November 2015 and in the ASHRAE Handbook Applications Chapter 48. Unfortunately, not many building simulation software programs can adequately model the detail necessary to show this control scheme.

Supply Air Temperature Control

While supply air temperature being fixed is a common configuration for VAV systems, it is rarely the optimal control option. For cooler seasons of the year, the supply air temperature can be reset warmer and result in energy savings. This reduces the demand on the reheat coils in the VAV boxes and extends the use of economizer operation at the air handler. During warmer weather, the supply temperature being low reduces the fan volume required and, thus, the fan energy. Some simulation programs offer ways to modify the supply air temperature based on outside air temperature or other factors.

Additional Controls

Like any all-air system, VAV has control options such as:

• Economizer type and limits
• Unoccupied operations, often called night-cycle control
• Optimal start
• Heat recovery
• Demand controlled ventilation

Many of these are available as options in simulation software.

Common Applications

Variable air volume systems have been applied to almost all types of commercial buildings that require both heating and cooling but are most appropriate for buildings with many discrete zones with individual thermostat controls. They are well suited for applications that can take advantage of outdoor air economizer modes and heat recovery from an exhaust system. They are not often applied to single-zone buildings or buildings that could be served easily by rooftop units. They are also rarely applied to residential buildings.

Model Output Checks

For variable air volume systems, timestep (often hourly) outputs should be examined during several different operating scenarios:

• Near design conditions for cooling
• Near design conditions for heating
• Time with some boxes only partially open for cooling
• Time with reheat in some boxes (multiple states depending on the control options)
• Time with boxes operating near minimum for cooling
• Night time with heating demand
• During different economizer modes

For each of these times, the following output variables should be checked to make sure they match the expected operation:

• Central fan airflow volume
• Return, intake, and mixed air temperatures and enthalpies
• Cooling coil exit temperature
• Preheat coil exit temperature
• VAV box flow rates
• VAV box delivered air temperature
• VAV reheat coil operation
• VAV box fan volume (if applicable for fan-powered terminal units)

The VAV box values should be reviewed at several different VAV boxes in the model that do not have the same operating characteristics, such as on different faces of the building and for a core zone. Especially when multiple simultaneous control options exist in the model, it is important to gain an understanding of what is actually happening in the simulation rather than just what the modeler expects to occur. This checking does not only provide an understanding of the specific building and system being modeled but almost always increases the understanding the modeler will have of the overall simulation program.

Related Energy Code Requirements

ASHRAE 90.1-2019 Energy Standard for Buildings Except Low-Rise Residential Buildings includes a number of requirements specifically mentioning variable volume systems, including:

6.5.3.2.2 VAV Static Pressure Sensor Location

Static pressure sensors used to control VAV fans shall be located such that the controller set point is no greater than 1.2 in. of water. If this results in the sensor being located downstream of major duct splits, sensors shall be installed in each major branch to ensure that static pressure can be maintained in each.

Exception to 6.5.3.2.2 Systems complying with Section 6.5.3.2.3.

6.5.3.2.3 VAV Set-Point Reset

For multiple-zone VAV systems having a total fan system motor nameplate horsepower exceeding 5 hp with DDC of individual zones reporting to the central control panel, static pressure set point shall be reset based on the zone requiring the most pressure; i.e., the set point is reset lower until one zone damper is nearly wide open. Controls shall provide the following:

a. Monitor zone damper positions or other indicator of need for static pressure.

b. Automatically detect those zones that may be excessively driving the reset logic and generate an alarm to the system operator.

c. Readily allow operator removal of zones from the reset algorithm.

6.5.3.3 Multiple-Zone VAV System Ventilation Optimization Control

Multiple-zone VAV systems with DDC of individual zone boxes reporting to a central control panel shall include means to automatically reduce outdoor air intake flow below design rates in response to changes in system ventilation efficiency as defined by Appendix A of ASHRAE Standard 62.1.

Exceptions to 6.5.3.3

1. VAV systems with zonal transfer fans that recirculate air from other zones without directly mixing it with outdoor air, dual-duct dual-fan VAV systems, and VAV systems with fan powered terminal units.

2. Systems where total design exhaust airflow is more than 70% of total design outdoor air intake flow requirements.

6.5.3.4 Parallel-Flow Fan-Powered VAV Air Terminal Control

Parallel-flow fan-powered VAV air terminals shall have automatic controls configured to

a. turn off the terminal fan except when space heating is required or if required for ventilation;

b. turn on the terminal fan as the first stage of heating before the heating coil is activated; and

c. during heating for warm-up or setback temperature control, either

1. operate the terminal fan and heating coil without primary air or

2. reverse the terminal damper logic and provide heating from the central air handler through primary air.

Based on a search through the ASHRAE Standard 90.1-2019 user manual, many other HVAC requirements in ASHRAE Standard 90.1 can impact VAV systems even if they do not explicitly mention VAV in the standard itself, including:

• Section 6.4.3.1.2 Deadband Controls
• Section 6.4.3.3.4 Zone Isolation
• Section 6.4.3.6 Humidification and Dehumidification Controls
• Section 6.4.3.10 Direct Digital Control Requirements
• Section 6.5.1.1.2 Economizer Control Signal
• Section 6.5.1.1.3 High-Limit Shutoff
• Section 6.5.1.3 Economizer Integration
• Section 6.5.1.4 Economizer Heating System Impact
• Section 6.5.2.1 Zone Controls (under Simultaneous Heating and Cooling Limitation), which allows “Dual Maximum” control (see exception 2) and reduces the use of reheat. Dual maximum controls are described previously on this web page.
• Section 6.5.3.1 Fan Power and Efficiency
• Section 6.5.3.1.3 Fan Efficiency
• Section 6.5.3.6 Fractional Horsepower Fan Motors
• Section 6.5.4.7 Chilled-Water Coil Selection
• Section 6.5.7.3 Laboratory Exhaust Systems
• Section 6.5.8.2 Heating Enclosed Spaces
• Section 6.5.9 Hot-Gas Bypass Limitation
• Section 6.7.3.1 Record Documents

In addition, Appendix G Performance Rating Method and Section 11 Energy Cost Budget Method of Standard 90.1-2019 both include reference systems that use VAV and many specific requirements for modeling VAV systems. For Section 11, four of the 11 systems are VAV, as shown in Table 11.5.2-1 (shown without notes below)

For Appendix G, five of the 13 systems are based on VAV, as shown in Table G3.1.1-4 (shown without notes below)

In Appendix G, additional references to these system numbers appear throughout the appendix.

Other ASHRAE standards also include references to VAV, including:

• ASHRAE Standard 62.1-2019 Ventilation for Acceptable Indoor Air Quality
• ASHRAE Guideline 36 High-Performance Sequences of Operation for HVAC Systems - which describes the control algorithms for various sequences, including the dual maximum control sequence.

Given the long history of the use of VAV systems in the HVAC industry, a number of other standards have been developed that impact design.

Similar or Related Systems

Several different systems are similar to or related to variable air volume systems

• Single zone VAV - these are systems, typically rooftop systems, that have a variable speed fan but only supply a single zone, so no VAV box is used.
• Variable volume and temperature systems (VVT) - can operate similarly to a VAV system but vary the temperature on a regular basis as well as the flow rate. Some will rotate between heating and cooling modes and have the appropriate terminal boxes open or shut at those times.
• Dual duct VAV - provide two supply ducts with either one or two different supply fans, and each duct provides either cool or warm air.

Additional Resources

• 2020 ASHRAE Handbook - Heating, Ventilating, and Air Conditioning Systems and Equipment. Chapter 4 - “Air Handling and Distribution.”

• 2019 ASHRAE Handbook - Heating, Ventilating, and Air Conditioning Applications. Chapter 48 - “Design and Application of Controls”

• Advanced Variable Air Volume System Design Guide. Energy Design Resources. December 2009.
• Cheng, Hwakong. Advanced Building Automation Systems Best Practices Guide. Version 1.0 June 13, 2022.
• Taylor, Steve. Dual Maximum VAV Box Control Logic. ASHRAE Journal. December 2012.
• Taylor, Steve. Making VAV Great Again. ASHRAE Journal. August 2018.

Links to external websites are provided as a convenience for further research, but do not imply any endorsement of the content or the operator of the external site, as detailed in BEMcyclopedia's general disclaimers.

References

Bannister, Paul. Energy Efficiency and the Control and Simulation of VAV Systems. 2008 ACEEE Summer Study on Energy Efficiency in Buildings. 

ENERGY STAR Buildings Manual Chapter 8 Air Distribution Systems

Dodd, Martyn. Comparing Energy Savings of Different VAV Systems. March 2012.