Abstract: An attachment for a building furnace which contains a combustion unit inside of an air plenum. The attachment is inserted as a replacement section of the smoke flue. The jacketed chamber acts as a heat exchanger, preheating incoming combustion air. The outer end lower portion of the jacketing chamber receives a fresh air duct from the outside of the building. A duct from the outer end upper portion of the heat exchange chamber leads into the lower portion of the combustion unit plenum at the burners. Attached to the smoke flue, and between it and the outer portion of the chamber, are longitudinally-oriented channeled elements for transferring sensible heat. A return duct from the heated living area leads into the furnace, terminating at the inlet of a blower in the bottom of the furnace. The outlet of the blower is upward into the space surrounding the combustion unit. Such space also joins above the latter to lead upwardly into the heated living area.