Abstract: Heat exchange means for use in combination with flue means conducting heated gases from a combustion zone and for extraction of thermal energy from the heated gases. A casing means is provided defining an enclosure with a central axis, and having fresh air chambers at opposed ends of the casing, and with a heat exchange chamber disposed between the fresh air chambers. Heat exchange conduits couple the opposed fresh air chambers, with the heat exchange conduits extending through the heat exchange chamber about an annular zone spaced outwardly from the central axis. A pair of baffle plates are provided within the heat exchange chamber, with the plates being arranged at opposed ends of the chamber, and with one baffle plate being movable axially. The controllable positioning of the movable baffle plate defines the axial length of the heat exchange conduits exposed to heated flue gases moving through the heat exchange chamber.