Hi, We have a solar energy project in the south of France (http://smartfrance.blogspot.com) We have just got the evacuated tube solar system going and its great when the sun shines (which is quite often). When it doesn't shine I have been playing with the idea of circulating brine through tubes in the base of the swimming pool. The pool is covered and today was 11c (5C outside). When we built the pool we embedded 100 m of 16mm pex tube in the base of it and 100m of 25mm pex in the ground under the pool. The pool is insulated with 8cm of polystyrene on the base and 15cm on the walls. Could we circulate glycol solution through these loops and use a heat pump to extract heat and push that water into our underfloor heating circuit. Any engineers out there? The issue is that the pool could get hotter than 11C and so the input to the heat pump may vary from the 8C they normally see. We currently use reversible aircon to get heat on cloudy days but a water water heat pump would be a better solution in the long term..We have a 1500 lt tank as well as the 45 m3 (10,000 gallon) pool. As the pool has a polycarbonate cover it cools slowly and warms up quickly when the sun comes out. This is our first year with it so not sure how cool it will go left to its own devices. Obviously in a normal year the solar panels would have been pumping heat into the pool and the store all summer and autumn so we should be warmer than we are now. My guess is that the storage system/heat pump will be needed for 2 months a year. So the pool is a battery that stores heat. We also have the 100 m 25mm pex under the insulated pool which will be able to drag heat out of the ground. So I was hoping that between the 2 and the solar panels we could muddle through. My worry was that the heat pump would not cope with the varying temperature of the supply from the pool. I have spoken to heating engineers and they were unhappy about the possibility of a variable input temperature. Say from 20C to 3C. Is this really a worry with a heat pump? Any comments gratefully received. Thanks Bob T.