I don't know about that. It sounds like it could be a good idea to me. Ambient air temperature varies 25 degrees or so from night to day. The pool is going t

I don't know about that. It sounds like it could be a good idea to me. Ambient air temperature varies 25 degrees or so from night to day. The pool is going to tend to gravitate towards a temperature between the ground temperature and the average air temperature throughout the day. If you want to heat the pool in the Fall and Spring, just circulate the water through an air-source heat exchanger in the afternoon, and turn it off when the temperature starts to drop. Voila, warm pool. In some places, pools get annoyingly warm in the summer. Use the same device, at 3am, and the problem is solved. Don't want to use your pool in the fall and spring, but want to save on your heating and cooling bills? Use the pool as a heat storage device, and save the desirable temperature (afternoon or night, depending) in the pool, then run the heat exchanger in the house to enjoy the cool night air during the day.