[wingnut60]

Should not be a check valve in the line from the water tank unless someone has put it in--the OEM setup relies on the internal check valve in the pump itself.
If you have been surviving without the furnace, or minimum use, you are a way tougher guy than me. I have always used the furnace + space heaters when in really cold temps, no way could I have kept the basement and the interior livable. Altho propane use is expensive, I have found it the only way to survive even with 50amp service. I had a 3rd 40# tank to swap with the outboard tank, but this assumes you have a propane supply within decent driving distance.