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.