in the art a substantial portion of the thermal energy contained in the exhaust gas can be recovered such that the exhaust gas discharged to the exterior ambient surroundings 46 is at a very low temperature preferably below the temperature desired in the heated space 48 such as at or below 60 f 16 c for example in many applications furthermore because of the relatively low temperature of the exhaust gas the exhaust gas conduit 44 can advantageously be constructed of relatively common conduit materials including common copper tubing for example in many applications the gaseous fuel supply sub system 20 as illustrated in fig1 wherein a gaseous fuel is withdrawn from a gas source 102 which can consist of a conventional natural gas supply system or other gaseous fuel sources well known in the art the gaseous fuel is conveyed through a safety valve 104 which is preferably adapted to be automatically closed or to automatically fail in a closed condition in the event of a malfunction in the heating system 10 the gaseous fuel is then conveyed through the gas conduit 103 into the intake or suction side 106 of a gaseous fuel compressor 108 which raises the pressure of the incoming gaseous fuel to a predetermined pressure level substantially equal to that of the compressed combustion air described above the compressed gaseous fuel is then expelled through the discharge side 110 of the gaseous fuel compressor 108 and conveyed by way of a gas conduit 112 to the above described adjustable venturi device 32 wherein it is intermixed at predetermined proportions with the compressed combustion air before being ignited by the ignition device 40 and injected into the combustion.