Abstract: A heat exchanger with a tubular body having helical ribs about its periphery constituting expanded interior and exterior conductive heating surfaces. A clamp is captively held to the tubular body for mounting the heat exchanger to the exhaust duct of a freestanding heating unit. Hot gases passing through the exhaust duct conductively heat is the expanded surface of the tubular body and air therein. A thermostatically controlled blower directs the heated air within the tubular body towards a heated surface of the freestanding heating unit for convectively heating the area about the unit.