Exceptions of use: Vari-Flow VAV diffusers

With the use of Vari-Flow VAV diffusers, we do NOT recommend the use of balancing dampers, tapered take offs, or extractors. These are all designed for constant volume systems using constant volume diffusers. We recommend the use of plain, straight, inexpensive, start collars.

Balancing dampers are a waste of money when used with Vari-Flow diffusers. Here's why. A balancing damper is set and balanced for maximum design condition volume. Most systems operate at maximum design volume less than 5% of their operating time. When used with a Vari-Flow diffuser, the balancing damper will only function at full flow. When this happens there will be a pressure drop across the balancing damper preventing more than maximum design volume from flowing to the diffuser. Again, this occurs less than 5% of the system operating time. The rest of the operating time the Vari-Flow VAV diffuser will require less than full flow to maintain its temperature set point.

Lets consider what happens to the balancing damper at reduced flow conditions. As the Vari-Flow VAV diffuser closes, the volume is reduced at the Vari-Flow diffuser causing less volume to flow across the balancing damper. With less than full volume flowing across the balancing damper, the pressure drop causing reduced flow occurs at the Vari-Flow diffuser damper not the balancing damper. The balancing damper will become ineffective and virtually non-existent in function as the volume is reduced below it's full flow.

Therefore, it will have no function 95% of the operating time of the system. What will happen the 5% of the time when the system does operate at full volume without the balancing dampers? When this occurs the Vari-Flow diffuser damper will be full open in response to temperatures above the set point temperatures of the VAV Vari-Flow diffuser. The Vari-Flow diffuser may deliver more than maximum rated volume on start up. The additional air will rapidly cool the zone the Vari-Flow diffuser serves causing the zone temperature will rapidly drop. The zone temperature will rapidly approach the Vari-Flow diffuser temperature set-point. The Vari-Flow diffuser damper will start to modulate closed to maintain the set point temperature. Once in control, the excess air will be diverted to zones with greater demand. As all of this is occurring at the Vari-Flow diffusers, the control damper(s) will be modulating to maintain their static pressure set-points in the run-outs to the Vari-Flow diffusers in direct response to required Vari-Flow diffuser volumes

When designing regain run-outs, we recommend that all elbows be specified with radius throat, radius heal, with turning veins. This will minimize and frictional loss in the elbow. The purpose is to conserve the velocity head so that it is available for it's intended purpose of static pressure regain.