I just checked my furnace in a 1000 sqft bungalow, it's 60 000 BTU. In -20 it's running fairly often, and if I turn it down to 16 when out, it needs a good hour to heat up the house. So with only 34 000 btu at -20, I suspect it could not keep up. So I can't see it working in a larger house, and in mine I suspect I'd need supplemental heating below -10.
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