Here in Vermont we design heating systems to perform at -10 to -20 degrees. Depending upon how tight the doors and windows are, it may if your temperatures don't drop below 20 degrees. It may take a while to heat up and I would not expect any temperature over 62 degrees inside the garage. Maybe a 50,000 Btu heater? Remember the output may be 20% less than the rated input of the heater.
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Here in Vermont we design heating systems to perform at -10 to -20 degrees. Depending upon how tight the doors and windows are, it may if your temperatures don't drop below 20 degrees. It may take a while to heat up and I would not expect any temperature over 62 degrees inside the garage. Maybe a 50,000 Btu heater? Remember the output may be 20% less than the rated input of the heater.