Abstract: Heat exchanger means for extracting heat from the flue gases of a warm air furnace is in the form of a tube wound spirally to define a first passageway wherein ambient air is heated by the flue gases passing through a second passageway between segments of the first passageway for the ambient air, the tube being provided with a deflector directing the flue gases through the second passageway and a shroud for controlling exhaust gases passing through and leaving the convolute tube.