Aprilaire 6506 6" RD CLOSED DAMPER Aprilaire 6-Inch Diameter Round Normally Closed Damper, A standard heating and cooling system delivers air to the entire conditioned space when the thermostat calls. This can result in certain areas of the space being over or under conditioned, while it wastes energy by heating or cooling unoccupied areas. The Aprilaire Zoned Comfort Control system helps to eliminate these problems. It works by directing conditioned air only to the area(s) of the home/building that need it. This is done through the selective opening and closing of air control dampers located in the supply ducts of the HVAC system. Aprilaire dampers feature patented technology that eliminates the components most prone to failure with other manufacturers dampers. Aprilaires zone dampers can be installed in a variety of duct sizes.