I had the same question. I thought I could run a hose from the drain on the home 50 gallon nat gas water heater and leave in on slow(maybe 1/4) flow for several days into the pool. That way the tank will never drain, but will run constantly. after the water in the pool gets too high, pump some out off the bottom(to get the coldest water). I realize this would only be possible because I live in georgia. All I need to do is gain a few degrees in the spring, that is all. I do not need heat in the fall or summer. 28000 gallon pool, water is currently 74 degrees and frigid to most of us older folks.