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Its that time of year (and getting hotter) when we require both hot water and cooling from our ASHP. Hot water works fine, cooling works fine, however, today the buffer tank was 36C although it had not been heated for weeks. This suggests to me that the heat from the hot water tank is migrating to the buffer tank. This also coincides with my thoughts about what may be happening on cool nights in winter when the bungalow heating doesn't come on over night and the hot water tank seems to lose a lot of its heat. ( it would be reasonable to loose a couple of kW's from the hot water and it be enough to warm the bungalow on a cool night) The primary pipework is all 28mm and very insulated. The primary pipework only goes to the hot water tank and the buffer tank. The emitters are on a secondary circuit circulating through the buffer tank. The 3 port valve only has no mid position, so either supplying the buffer or the hot water, not both. Three port valve working correctly. The top of the ASHP is about 1.4 meters above the top of the buffer tank. The top of the buffer tank is about 0.7 meters above the top of the hot water tank. A circuit of the longest part of the primary pipework is about 11 meters. So, my thoughts are: Can a 900mm long 28 mm copper pipe full of static water carry 2kW of heat from the hot water tank to the buffer tank (noting that the port valve would stop the possibility of the water circulating through the pipe) within 8 hours? If this is not the case what is happening. If this is the case what safe options can I use to stop this. Your thoughts please. Thank you. Marvin