If you were heating your house with room temperature air, I totally agree. But I think because you're heating air to well above the target temperature, you're losing a lot of efficiency.

Argh. I'd really like to do some math and figure this out, but I just don't have the time right now. Things that may need to be taken into account are the amount of energy it takes to heat air vs. walls and temperature differences between outside, inside, and heated air.
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Bitt Faulk