Abstract: The damper is a hollow sheet metal duct-like device of generally rectangular cross section, and is adapted to fit over the combustion air intake of a furnace blower so that the furnace must obtain its combustion air through said damper. A trap door type valve in the lower surface of device automatically opens in response to the suction of the blower to admit combustion air, and automatically closes due to gravity when said blower is de-energized, to cut off the air supply to the furnace combustion chamber when the furnace is not operating.