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Pressure Control with VAV Diffusers

Posted by Price Industries on December 31, 2019 at 9:13 AM
Price Industries

Consider Various Pressure Control Methods in a VAV System

All diffusers become noisier with higher pressures at their inlet and this is true for VAV diffusers.  Pressure is often reduced by the use of a balancing damper or terminal unit, but VAV diffusers are unique as they have a built in VAV damper that can close down to a minimum flow. 

Prodigy VAV Diffuser with LCD Thermostat

A comment sometimes made about VAV diffusers is that they become noisy when they close.  This is a direct indication that the pressure has been allowed to build up when the diffuser closes.  A method of pressure control must be considered in all VAV diffuser systems and a variety of pressure control options are offered for both the Prodigy and Varitherm VAV diffusers.

Fan speed control with an all low pressure duct design is the most energy efficient method as all pressure drops are reduced to a minimum.  The size of the ducts can limit the practical use of this approach.

For larger systems, air may be distributed with medium pressure and then reduced to low pressure with a Pressure Control Valve (PCV) for each zone of VAV diffusers.  The PCV is a damper and controller that modulates to provide pressure control.  Fan speed control is used in the medium pressure ducts to capture the energy savings when the air is not needed.

Pressure Control Valve

For constant volume systems a bypass is required.  The PCV may be used as a bypass, or, when there is a plenum return system, the Pressure Relief Collar (PRC) provides a local bypass on the inlet of each Prodigy or Varitherm diffuser.

Pressure Relief Collar

If only a few VAV diffusers are to be used as part of a larger system, and no less than 30% of the total air flow is supplied by the VAV diffuser(s)the pressure increase should be negligible and pressure control should not be required.

For more information about the Prodigy and Varitherm VAV diffusers, or for design assistance with VAV diffusers please contact an Application Engineer at

Topics: Diffusers, Pressure Drop, HVAC Fundamentals, HVAC, Design Engineering, Tech Tip

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