A sliding damper plate is positioned between the blower and heat exchanger of a down draft gun-type furance for mobile home usage and includes cam means at opposite ends thereof for actuating associated switches coupled to the furnace and air conditioning control circuits. When the damper is closed to cut off the flow path through the furnace to permit a separate air conditioner unit to utilize the existing heating ductwork in a mobile home, one of the switches is actuated to permit operation of the air conditioning unit. Alternatively, when the damper is moved to an open position for heating operation, the air conditioning unit is inactivated and a second switch closed by the cam to permit furnace operation. If the damper is partially open or closed, both the furnace and air conditioning unit are deactivated.