Low Pressure VAV

Low pressure VAV diffuser offer significant advantages of traditional VAV which include:

  • Individual zoning

  • VAV turndown to 10% while maintaining adequate throw
  • Energy savings of low pressure systems (Studies have indicated 15-47% savings over traditional VAV systems

Designing Modular VAV Systems

The Therma-Fuser diffuser is a simple stand-alone device that provides VAV control when supplied with air in a suitable range of temperature and pressure.

Supply Air Temperature

When supplied with cool air, the Therma-Fuser dampers modulate open on a rise in room temperature. When supplied with warm air, the dampers open on a room temperature drop. Cool air should be a constant temperature not less than 50°F/10°C (40°F/4.5°C for model LT-HC) and warm air at a constant temperature of not more than 120°F/49°C. A constant supply air temperatures is best achieved by using a discharge air sensor/thermostat to control the source of cooled or heated air.

Changeover from cooling to heating occurs as the supply air rises from 76°F/24.5°C to 80°F/26.5°C and change back from heating to cooling occurs when the supply air drops from 72°F/22°C to 68°F/20°C. During changeover the Therma-Fuser diffuser is either closed or in partial control. Therma-Fuser diffusers do not cause the supply air temperature to be at some level, they respond to the air temperature supplied. Changeover from cooling to heating is best achieved by using a room temperature sensor/thermostat to enable the heating in the source of heated air.

Static Pressure

Static pressure at the inlet of the Therma-Fuser diffuser must be high enough (.05″wg/12 Pa or more) to obtain the required air volume. Above .25″wg / 62 Pa the sound level becomes noticeable (NC35) and around .40″wg/100 Pa the dampers begin to leak. When the static pressure is held constant, the sound level will decrease as the Therma-Fuser dampers close. A constant static pressure is best achieved through the use of fan speed control, modulating zone dampers, modulating bypass dampers, or a combination of methods.

Sizing and Location of Therma-Fuser Diffusers

Therma-Fuser diffusers are sized for the design or maximum air volume required using the published performance guide. Use a larger inlet size at a lower static pressure when lower sound or NC levels are required and when Therma-Fuser diffusers are at the end of a duct run where less static pressure is available.

Therma-Fuser diffusers may be oversized. They will simply turndown air flow to match the space load. Unlike fixed diffusers used with VAV boxes, Therma-Fuser diffusers won’t dump when turned down.

Sizing Ducts for Therma-Fuser diffuser systems

Duct work should be sized to limit the maximum static pressure at the inlets of all Therma-Fuser diffusers to .25”wg/62Pa or below at both design and turndown conditions and to maintain static pressure at the diffusers especially those further away from the fan, at least .05”wg/12Pa or enough to provide design air volumes. The best way to achieve this is to determine the maximum pressure drop allowable between the first takeoff and last diffuser and determine the equivalent length of the duct work. Then choose one of the duct sizing methods and assign duct sizes accordingly.

Designing Modular VAV Systems

Essentials of a Modular VAV System

Energy Study by Bassett Consulting Engineers

Energy Study by C.A. Pieper Consulting Engineers

Energy Study by Rumsey Engineers


Acutherm Low Pressure VAV Diffusers