Variable-Air-Volume Units

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A variable-air-volume unit, or VAV box, as they are commonly called, is part of an air conditioning system. It is a device installed in the supply ductwork which regulates the volume of air being delivered to a zone. A zone is a defined space that is governed by a single thermostat or sensor, and for which there is a main supply duct that subdivides into as many ducts as are required to fully air condition the zone’s spaces. The device itself regulates the air passing through it by opening and closing a damper, and it can vary the quantity of air which passes through it. As the thermostat senses the need for more air conditioning, it signals the VAV to open the damper, and vice versa.

Variable-Air-Volume Units

Topic Summary

There are many different configurations of VAV boxes, but they usually include: an air entrance, air exit, controls, a damper, an actuator, and a method of reheating the supply air.

In both the winter and the summer, certain zones within some buildings always require cooling. A windowless interior space is an example of this; it is surrounded by conditioned spaces on all sides, as well as above and below, so its load calculation is solely dependent upon the people in the zone, what activities they are doing, and the number of lights in the zone. In addition to mitigating the heat produced by people, lights, and machines, HVAC systems must provide for fresh air requirements. This means that even in the winter, when a building's perimeter zones typically require heat, the interior zones may need air conditioning.

VAV units typically use less energy than other systems to condition the air, as the need for conditioned air is not constant throughout every space in the building. The volume of air required is varied according to the need. Frequently, a VAV system is associated with a variable frequency drive for the air handler. Varying the frequency varies the speed of the fan in the air handler, and thus produces less volume. The air volume requirement reduction and the variable frequency drive directly impact the energy costs for a building.

While VAV units typically use less energy than a constant volume system, higher first costs are associated with the system as it requires additional components and installation time.  A substitute for the higher cost VAV system is to use VAV diffusers. These are often referred to as "a poor man’s VAV," because they cost less to install and still vary the air; however, there is no reheat ability, so the savings are nullified.

Last modified on Tue, Aug 24, 2010
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