Damper is located underneath furnace and fits inside above duct connector. Opens automatically when furnace blower comes on. Shuts when furnace blower shuts off. When the furnace is not used, the damper keeps air from ductwork from coming back up inside the furnace. Damper is often used when separate a/c units don’t require use of the furnace blower. A 12″ x 12″ minimum opening in duct needed to use damper. Total size is 17.5″ x 18″.