Something else to consider about freezing temps. The filter gauge. Typically this is exposed to cold air, sticking out the side of the filter valve assembly, and is likely to freeze first as there is no real circulation through it. Once it freezes it's shot....but cheaply replaced. You can remove it and put in a pipe plug, or, as I do, wrap layers of old bedsheets, heavy cloth, or whatever over the top of the filter and valve assembly to utilize the mass of the filter to keep components from freezing. It takes both time and temperature to cause a freezing event that leads to damage.
Al