Quote Originally Posted by CarlD View Post
I know that there is a constant temp of, I believe, 58 deg F underground, like 3 to 5 feet, depending on location.

But I've NEVER heard of a heat pump that used THAT as the source of heat! Wow--that's a great idea, if it can be made to work--a sufficiently large heatpump would work no matter HOW cold it got.
I'm in Central PA and I installed a Geothermal Heat pump for my home back in 1992. BELIEVE THIS!! It paid for itself in 4 years compared to the baseboard electric I had been using. There are Geothermal heat pumps for pool applications. If you are even thinking of a heater in PA I would recommend you look at Geothermal. The size is still determined by surface area and the temperature differential at which you intend to use it.

One other thing I have to mention is whether you asked yourself this question. "Am I going to use the pool when the air temperature is low?"

My wife and I don't go near the pool unless the air temp is above 75 - 80degrees even if the water is the same or higher. Maybe I'm chicken but it's cold out there when your all wet!! Any heater is expensive if you don't swim when the air is cool.