An improved heat exchanger (74) for use with a heating furnace (72) is disclosed. The exchanger (74) includes a first plurality of tubes (84) which conduct hot exhaust gases from the furnace (72). Tubes (98) carrying outside air for combustion in the furnace (72) pass proximate the first group of tubes (84) within a heat exchange chamber (90) wherein heat is transferred to the combustion air via a heat transfer medium. Similarly, ambient air in the building passes through the heat exchange chamber (90) of the exchanger (74) through a third plurality of tubes (118). These tubes (118) also pass proximate the tubes (84) carrying hot exhaust gases. In a preferred embodiment, a conduit (80), through which the exhaust gases transit from the furnace (72) to the first plurality of tubes (84) in the heat exchanger (74), can be routed through plenums (110, 126) through which combustion air and ambient air circulate after exiting the second and third pluralities of tubes (98, 118).