The ZoneHandler is factory set to maintain about .3" (adjustable) static pressure. The end result is that when one or more zones close down, the FanHandler reduces the blower's speed so that the pressure in the ducts remains at the set point and the volume of air coming from the open zones remains steady. This eliminates the high-speed air noise problems associated with inefficient and hard to install dump zones or barometric damper short-circuits. AND, when there is no call for heat or cool, the blower's speed drops down to whatever (adjustable) minimum speed you set it at. Thus providing comfortable, continuous air circulation during the heat or cool off periods where the blower would normally be inactive and useless. This assures that filters, electronic air cleaners, UV (germicidal lamps) and any other accessory can continuously work at peak efficiency.