Consider various pressure control methods in a VAV system. All diffusers become noisier with higher pressures at their inlet, which is true for VAV diffusers.
Pressure is often reduced using 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.
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 Smart Electronic and Thermal VAV diffusers.
Fan speed control with a 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 Independence Module (PIM) for each zone of VAV diffusers. The PIM is a damper and controller that modulates to provide pressure control. Fan speed control is used in the medium-pressure ducts to capture energy savings when the air is not needed.
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 (R-Ring) provides a local bypass on the inlet of each Smart Electronic or Thermal VAV diffuser.
If only a few VAV diffusers are to be used as part of a larger system, and no less than 30% of the total airflow is supplied by the VAV diffuser(s), the pressure increase should be negligible and pressure control should not be required.
A VAV diffuser design should be based on a ‘Pressure Dependent System’ approach and not a ‘Pressure Independent System’ which is commonly used for VAV Terminal (Box) designs.
