a heating assembly generally designated by the numeral 210 defined by an exterior chamber 211 enclosing an burner housing 212 to form a plenum chamber 215 therebetween with the combustion process contained within a burner cavity 213 then emitting its combustion products cp directly into the chimney stack cs like numbered parts functioning in like manner heating assembly 210 may be positioned adjacent the opening in fireplace fp in an alignment wherein the heat exchange flow if driven across coil 12 by the fan assembly 26 is either conveyed through a selectively articulated gate 221 into the plenum chamber 215 or is turned to bypass the plenum to merge with the air flow circulation if 2 in this latter mode all the heated air in the plenum chamber 215 is channeled into a return duct 217 to be directly conveyed into the chimney stack cs by shutting an exit gate 222 in the plenum enclosure thus during the cooling cycle both the combustion products cp and the air heated in the plenum around the burner housing 212 are conveyed directly into the chimney increasing the draft therein during the heating part of the cycle the exit gate 222 is opened thus letting heated.