Variable air volume (VAV) is a type of heating, ventilating, and/or air-conditioning (HVAC) system. The simplest VAV system incorporates one supply duct that, when in cooling mode, distributes approximately 55 °F (13 °C) supply air. Because the supply air temperature, in this simplest of VAV systems, is constant, the air flow rate must vary to meet the rising and falling heat gains or losses within the thermal zone served.
There are two primary advantages to VAV systems. The fan capacity control, especially with modern electronic variable-speed drives, reduces the energy consumed by fans, which can be a substantial part of the total cooling energy requirements of a building. Dehumidification is greater with VAV systems than it is with constant-volume system, which modulate the discharge air temperature to attain part load cooling capacity.
The air blower's flow rate is variable. For a single VAV air handler that serves multiple thermal zones, the flow rate to each zone must be varied as well.
A VAV terminal unit[1], often called a VAV box, is the zone-level flow control device. It is basically a quality, calibrated air damper with an automatic actuator. The VAV terminal unit is connected to either a local or a central control system. Historically, pneumatic control was commonplace, but electronic direct digital control systems are popular especially for mid-to-large size applications. Hybrid control, for example having pneumatic actuators with digital data collection, is popular as well.
A common commercial application is shown in this diagram. This VAV system consists of a VAV box, ductwork, and four air terminals.
Control of the system's fan capacity is critical in VAV systems. Without proper and rapid flow rate control, the system's ductwork, or its sealing, can easily be damaged by overpressurization. In the cooling mode of operation, as the temperature in the space is satisfied, a VAV box closes to limit the flow of cool air into the space. As the temperature increases in the space, the box opens to bring the temperature back down. The fan maintains a constant static pressure in the discharge duct regardless of the position of the VAV box. Therefore, as the boxes close, the fan slows down or restricts the amount of air going into the supply duct. As the boxes open, the fan speeds up and allows more air flow into the duct, maintaining a constant static pressure.