Abstract: A warm air furnace including a plenum, a blower, heat exchanger disposed within the plenum and an air distribution chamber. The heat exchanger includes an encasement surrounding a plurality of vertically extending heat exchanger tubes. The tubes are in communication with the plenum and with the air distribution chamber. A fibrous refractory inorganic tubular flame chamber is positioned within the encasement and is partially surrounded by the vertically extending tubes. The tubes are positioned in a generally U-shaped pattern and one of the tubes contacts the flame chamber. The tubes along the sides of the pattern are interconnected by upper and lower side baffles. The baffles are vertically spaced from each other to define exhaust outlets. A rear baffle extends between the tubes along the base of the pattern. A flue extends from the encasement for exhaust of combustion gases.