Abstract: A flue gas closure system for a furnace which includes a burner and a heat exchanger through which flue gas flows. The system includes a closure member which in an open position permits the flow of flue gas through the heat exchanger and in a closed position prevents the flow of air through the heat exchanger and means for causing said closure member to be in an open position when the burners are on and to be in a closed position when the burners are off. The closure member includes a closure plate which is slidably connected to the heat exchanger. An actuator is connected to said closure member for moving said closure member to at least one of the open and closed positions.