DV-80-10-2 -- System-Powered Variable Air Volume Terminals

Although lower pressure fan systems can be designed using a good duct design program and electronic or pneumatic controls, good systems can be designed using system powered controls if the engineer selects units using less than 0.5 in. wg (125 Pa) to operate the controls. Good duct design is critical because insufficient duct pressure may mean total loss of control instead of just insufficient cooling at design loads.

If normally open (N.O.) terminals are selected, extra precaution must be taken to assure sufficient control pressure on fan startup. The morning warmup feature available on most system powered terminal unit controls should be considered if night set back temperature of 55°F (13°C) is used. Heating interface with system powered controls is difficult because of the limitation to two-position control and generally limited power to precisely actuate the controls. Therefore, externally powered controls should be used if heating interface is required.