Abstract: Combustion gas recirculating system for a gas-fired furnace of the type which includes a plenum with a cold air inlet duct and a warm air exhaust duct, a combustion chamber where gas is burned to form hot combustion gases, a heat exchanger in the plenum for receiving the hot combustion gases and including an outer surface configuration past which air can flow and be heated, an outlet in the heat exchanger for the hot combustion gases, and a blower for moving air through the plenum and past the heat exchanger. A recirculation duct connects the combustion gas outlet with the cold air inlet duct and a thermostatically controlled valve regulates the flow of hot combustion gases from the combustion gas outlet to the cold air inlet duct by opening when the hot combustion gases reach a predetermined temperature and closing when the hot combustion gases are below said predetermined temperature.