If insulated I think the 45k would be plenty. I have a huge two stall and the 45k gets to 70 degrees from 35 degrees very fast (~15 minutes?).
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musky's reznor is the other of two units always mentioned as being the best when I crawled garage web sites prior to installing one
modine or reznor - can't go wrong
This is what he put in, the hot dawg 45,000. He being the 30 year expert stated the same thing you said #guywhocouldworkforaheatingandcoolingcompany, claimed it would be a bit quieter, a bit more efficient, and took up a bit less room. Ran it yesterday and last night, the garage is at 66*. I could hear it in my boys room last night very faintly when it would cycle. Of course its unusually warm right now so it didn't run often, but kept the temp at 66 even when I was moving stuff out of the garage with both overheads open, it only took a couple minutes to recover. He installed a digital thermostat that will go as low as 35* up to 80, so a guy can keep it low when out of town.
I know my former place with the electric one would take an entire day to heat up an additional 20*, and would take about 20 minutes or more for a cold recovery.