Hello, I'm new here but it seems I have a huge problem with my water tank - or according to the last plumber my boiler. My home setup is: I've a water tank in the loft, a hot water tank at floor level and a shower pump, that pumps both hot and cold water to the bathroom tap, shower and toilet. It all started when the pump started leaking, apparently, it was leaking for a while before going noticed. Got it replaced, for that all water from the tanks were flushed out. Just after the change, I noticed the pump going on and off, first every couple of minutes to the point it was going on and off every 5 seconds during the night. I mentioned to the plumber it asked to wait how it would behave during the weekend and he would return next week if it stills. - this was a Friday. I kept it off during most of the weekend because the noise was too loud, also started noticing water coming from the bathtub overflow drain, also mentioned this to him. On Sunday night I got home after a few hours out just to see that the tank upstairs has overflown, and water was everywhere. I called the insurance, the emergency plumber came, replaced the ball cock - said it was the cause. The pump guy also came back to double-check why the pump was going on and off - at this point, I thought the overflow problem was fixed. He did loads of check, suggested I should put a non-return valve in the pump and that would fix it. Not convinced I start monitoring the cold water tank level, and saw it raising, and raising, above the overflow level. The weird thing is that there was no water coming from the ball cock valve, also I only noticed it after turning the boiler to heat water. With a flashlight I could see a stream of water moving inside the tank through the exit pipe, the pipe was warm and by touching the water it was hot too. I called the insurance again, saying the first guy didn't' actually fix the problem, and I was still without running water at my place. The second insurance guy came and when I explained to him the situation there was a lot of "this doesn't make sense", "I've never seen this before". I showed him videos and was able to reproduce the problem by turning the boiler on. (I've uploaded the video here , but it's very hard to see in the video) My theory was, somehow the hot water tank was overflowing, and some part of the water was going back to the cold water tank, and some part going to the pump, causing it to run eventually. He called a colleague that told him he saw this many times before, and it was caused by a boiler valve malfunction - saying I need actually a heating engineer. That the boiler was overheating the water making it go everywhere. Giving all this griefed report, have anyone here seen this before? Should I trust that this is a boiler problem? Everyone that came here was systematically wrong about the issue, which made me paranoid now about it. Many thanks in advance.