Think about how a heat pump works. For cooling, it takes your shop heat, compresses it even hotter, and then moves the heat to the hot outside air. You are transferring, say, heat from 200F coils to 100F outside air. For heating, it takes heat out of the cold air and moves it into your shop. When the air that you are taking the heat out of becomes more and more really cold, it becomes more and more difficult to compress the Freon enough to make it hot enough to efficiently transfer heat into your shop. Yes, they will work at low temperatures, but at lower and lower BTU output (and efficiency) as the outside temperature declines. It is a function of how much heat is in the outside air, and how much the freon is compressed, which becomes problematic for affordable and reliable compressors. In cold country, heat pumps usually work better when using ground water to add heat to the freon. It might be "good" down to -10 degrees F, but does not put out as much heat, an efficiency and comfort issue.
