Variable air volume (VAV) systems work by fluctuating the air flow rate needed to meet the thermal requirements of a conditioned space. VAV systems may be used for greater energy efficiency by incorporating variable frequency drives (VFDs) to control fan speed in a building’s HVAC system to more accurately reflect the needs of the building’s occupants. For instance, a VAV system may reduce the ventilation rate for a particular space when that space is unoccupied. Variable air volume systems work with a building’s centralized HVAC system.