A damper assembly for use in a round forced air duct feeding a register vent through a register boot includes a damper frame having a frame periphery with a first side and a second side that is at least substantially parallel with the first side, the first side and the second side each having a length that is greater than a distance between the first side and the second side. A damper blade is pivotable relative to the damper frame about an axis of rotation, at least substantially parallel with the first side and the second side, between an open position and a closed position. A flexible outer seal extends outwardly from the frame periphery of the damper frame and is at least substantially circular in order to seal against an inner surface of the round forced air duct.