Abstract: A hot air furnace includes a cylindrical combustion chamber with enclosed upper and lower ends. A plurality of apertures are formed in the lower end of the combustion chamber wall through which flame jets are radially discharged into a flue chamber defined by the space between the combustion chamber wall and a flue chamber wall circumscribing and spaced from the combustion chamber wall. The end of the flue chamber wall nearest the apertured end of the combustion chamber is closed with a domed panel and the opposite end is closed with a streamlined closure member. A plurality of vertically extending fins extend a substantial distance between the combustion chamber wall and flue chamber wall along a portion of the length of said flue chamber wall spaced from the apertures in the combustion chamber wall. An air chamber wall circumscribes and is spaced from the flue chamber wall which includes a plurality of vertically extending fins disposed in the air chamber so formed to surround the flue chamber wall.