ABSTRACT
A constant air volume system varies the air temperature into the conditioned space (CS) while the airflow remains constant. A variable air volume (VAV) system varies the airflow into the CS while the temperature remains constant. VAV systems may be pressure independent or pressure dependent. For pressure-independent systems, in addition to the CS thermostat, there is a velocity sensor and velocity controller at the terminal box to maintain the maximum and minimum air volume to the CS. If the VAV terminal box and air damper or air valve are mechanically operating properly, then it falls to the controls to maintain the system. Balancing a pressure-independent dual duct VAV system requires setting both hot and cold duct volume controllers. A fan powered box system takes a standard VAV, constant temperature, primary system and changes it to constant air volume, variable temperature, secondary system which supplies air into the CS.
