Abstract: The present invention provides a novel heat exchanger which is adaptable for attachment to an exhaust stack of either an oil or natural gas furnace and includes a heat exchange chamber with a plurality of separate, closely packed tubular conduits arranged to transport hot exhaust gases therethrough, a fan assembly which is attached to a side wall of the heat exchanger and is selectively actuable to force ambient air around the plurality of tubular conduits to heat the ambient air and thereby provide an additional heat source, and a thermostat extending into the ambient air passageway for monitoring the ambient air temperature and selectively actuating the fan assembly when the ambient air reaches a pre-set temperature.