Abstract: A heat exchanger comprising a combustion chamber adapted to burn fuel having an inlet at one end for fuel and an outlet at the other end for exhaust gases and wherein a sheath surrounds the combustion chamber to form a chamber for heated air, the combustion chamber having a plurality of radial fins projecting into the heated air chamber. One of the ends of the sheath is axially spaced from the end of the combustion chamber having the fuel inlet and the other end of the sheath extends axially beyond the other end of the combustion chamber to an outlet for discharge of heated air. A second sheath surrounds the first sheath and encloses the combustion chamber and defines an annular space with the second sheath open to ambient air which flows through the annular space and then through the air chamber past the fins to the outlet for heated air.