Hi, My cold water cistern in the roof constantly overflows when the hot water is on. When I take the top off I can see hot water entering at the bottom from the pipe which should feed cold water to the bottom of the hot water cylinder. I have an in-direct two circuit system and when the hot water is on all four pipes to the hot water cylinder feel hot to touch. It's like there's too much hot water being produced, which is escaping back through both of the cold water feeds. Any ideas on what the problem is and how I fix it?