Abstract: As shown in an embodiment of the invention as depicted, the heat exchanger comprises a cylindrical chamber into which, and from which heated fluid, such as the combustion gas product from a furnace, is conducted. The chamber confines therewithin a plurality of axially-extending tubes, and ducting admits ambient air to and from the tubes. Accordingly, the heated fluid and the ambient air are indirectly heat-exchanged. The chamber is fixed within an enclosure or housing, except that an axially-extending portion of the chamber is left exposed, in order that the heated-fluid input and discharge can be effected directly to and from the chamber. The housing forms a compartment alongside of the chamber, and to a given side thereof, which compartment causes the ambient air to cycle or pump through the tubes and compartment, prior to any substantial air-discharge, to generate an improved draft or current flow in the heat exchanger.