These air terminal units are known as Variable Air Volume (VAV) and Constant Air Volume (CAV) boxes for use on HVAC system. VAVs vary the airflow at a constant or more precise control of temperature, while CAVs supply a constant airflow at a variable temperature. There are 4 basic types: Single Duct, Dual Duct, By-Pass, and Fan Powered. They are equipped with or without controllers, actuators, thermostats, and auxiliary equipment such as electric & water coils, sound attenuators, air filters, etc.