Quote:
Originally Posted by Daox
Also, as the temperature of the water decreases, the efficiency would increase.
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True. This isn't immediately intuitive, but it makes sense. The furnace has a heat exchanger to move heat from the combustion gases which go up the flue, to the air that blows through the ducts (or water in the case of a boiler). A furnace causes a bigger rise in air temperature when the air intake temperature is low. Hence, a furnace is more efficient at heating up a cold house than it is at holding a temperature constant.
That's not to suggest you want to cultivate a cold basement or fail to seal your cold air return. To guess at some numbers, maybe your furnace causes a 60°F rise in temperature when it's drawing in 40°F air (output: 100°F), and a 58°F rise in 65°F air (output: 123°F).