Variable Air Volume (VAV) Units Information

Variable air volume (VAV) units are manufactured assemblies of air valves and dampers contained in a single enclosure. As HVAC system components, they regulate airflow to a room or zone in response to a thermostat or other signaling device.


Typically, VAV units or VAV boxes are installed inside HVAC ductwork, overhead pipes that route cool air to each room. These variable air volume (VAV) units regulate the volume of the cooled air to the zone by opening or closing the dampers, ventilation components that provide flow control. The inlet to the duct is from the air handling unit (AHU) and the outlet is the connection to the duct leading to the zone. Thermostats are installed in VAV zones to detect the current temperature and determine if conditioned air is required.

Types of Variable Air Volume (VAV) Units

There are several basic types of variable air volume (VAV) units. Fan-less or cooling-only VAV boxes are thermostat-controlled assemblies with an integral damper. Fan-powered VAV units feature this same design, but add a fan. When the room or VAV zone exceeds a pre-set temperature, the fan is triggered. This design circulates the air without conditioning, and can reduce the overall demand on an HVAC system. A third type of VAV unit, variable air volume with reheat, is similar to a fan-less VAV box but includes an electric coil. When the air in the damper reaches a specified temperature, the coil reheats the air. In this way, variable air volume units can provide zone-specific heating without adding additional heat to the entire building. Applications include office buildings and larger residential structures.

Box Sizing

Box sizing is an important consideration when selecting variable air volume (VAV) units. Parameters include inlet size, area and velocity; outlet size, area, and velocity; maximum flow and radiated noise criteria; and best, worst, and average turndown. Often, box sizing involves tradeoffs between competing factors such as space requirements, noise, and reheat energy. As a rule, total pressure is a more important factor than static pressure since total pressure is what HVAC fans must overcome. Typically, parallel fan-powered variable air volume (VAV) units are used in zones or rooms with relatively high heating requirements.

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