While in California – the land of air conditioners and swimming pools – I noticed a relatively easy freebee for air conditioners and heat pumps. First. for comparison and perspective, consider that geothermal transfer systems are now used to replace the condenser/evaporator coil in cooling/heating systems. The ground, instead of the air, acts as the thermal reservoir for the system. Typically, [I believe that] hundreds of feet of heat conductive plastic pipe are buried on the property and then connected to the a/c unit. Since the ground is relatively cool in the summer, and relatively warm in cold weather, the system sees a smaller hill to climb for both heating and cooling; hence lower heating and cooling bills. It seems that with very little effort, the same could be done with swimming pools. The advantage wouldn’t be as great, but the cost of installation would be much less. It seems to me that the best system would incorporate a water supply hook-up as part of the condenser/evaporator unit. A fractional HP water pump would circulate water from the pool, over the heat exchanger, and back to the pool again. The circulation pump would turn on based on ambient conditions and the water temp. In many cases, say in the heat of summer, the pool water may be as much as forty degrees cooler than the ambient temp. Generally I think this would yield the greatest advantage during the most extreme weather - during the periods with the heaviest demand for energy. This could also elimate that noisy condenser coil fan! Note that we often heat the pool while cooling the house anyway.