Note also that turning fuel into heat may be only 60% efficient with older burners that let a good portion of the heat go away with the CO2 through the exhaust pipe, instead of into the heat transfer fluid (the water for the radiators in a house). You need recent "recondensing" boilers to reach >90% efficiency (they cool the exhaust so much that steam condenses and must be drained: if the boiler has a tiny pipe to the sewer, it is one of those). Also, tap-water boilers with a permanent candle were using ~50% of the gas just to stay online... Pierre
