The closing of an outlet damper adds to the resistance that the Blower is working against. This moves the operating point to the left of the initial rating point. The savings in horsepower depends on the relative position on the fan Cone and is usually much less than offered by other methods. Outlet dampers are typically the least expensive option and should be considered when infrequent operation at lesser capacity is desired or when handling hot, humid or particulate laden air.
There are two types of outlet dampers: parallel blade and opposed blade. Parallel blade dampers are recommended for systems where air volume is modulated between full-open to about 75% of open. Opposed blade dampers cost about 10% more and are recommended for systems where volume is modulated over the entire range. Opposed blades reduce air volume in a closer relationship to the control arm movement.