Abstract: A secondary heat exchanger is positioned within a cold air return plenum of a forced air gas or oil fired furnace to preheat cold air drawn into the furnace through the cold air plenum. A heat recovery flue pipe is connected between a primary flue pipe of the furnace and the secondary heat exchanger, and an exhaust pipe is connected between the secondary heat exchanger and the outside of a structure served by the furnace. A secondary blower coupled to the exhaust pipe draws hot combustion gases from the primary flue pipe through the heat exchanger and exhausts cooled combustion gases to the outside. The secondary heat exchanger comprises a combustion gas inlet plenum and a combustion gas outlet plenum, with a plurality of heat conducting pipes sealingly connected therebetween.