In HVAC, VAV (pronounced "V - A - V", three syllables) refers to a system type where the airflow varies and the supply temperature remains constant to meet the cooling load. In heating, VAV systems are generall constant volume. Most VAV systems have a minimum airflow rate, so that when heating or cooling is not required, airflow is still provided. This is generally 30% of the design airflow, or .4 cfm/sq ft in many buildings. The value of a VAV system is controllability of each zone, and fan energy savings. However, during hours of low cooling load, the supply air may over-cool a system and reheat is required (see Supply air temperature reset)
Example:
Medium to Large office buildings often use VAV systems. One air handling unit (AHU) supplies air to a number of zones. At each zone, the airflow is controlled by a VAV box, which often contains a reheat coil that provides heat during heating hours, as well as reheat during hours where the vav minimum stop is reached