RickB,
I would think something is wrong with your thermostat, like it's not closing, or there isn't one in there. I would check it out.
cars need to run at a certain temperature. I don't think your thermostat is staying closed on cold days, I think it's stuck open, or you don't have a thermostat installed. an open thermostat will not allow your engine to reach operating temperature, and would also affect the heat output of your heating system.
the thermostat should stay closed until your engine reaches the proper temperature, then open and close to maintain that temperature.
just my take . . .