I bought two 5000 BTU heat pumps. They were initially intended to be connected to a water heater and it would heat the water and blow out cold air. So I connected these two units in series and dropped the feed line and return in to our pool (they each have their own small circulation pump) and connected the air output to our furnace. In summer or as soon as we need cooling in the house I turn these on and they start to do their thing. In the middle of summer I can get by with running the central AC very little and the pool heater not at all. The cost of running the heat pumps are about the same as running AC alone, but instead of dumping the heat outside I am heating the pool. Since our pool is indoors and the surface is at a minimum of 6 feet below ground and deep end is about 25 feet below ground the pool needs heating year round, about the same all year actually.