so I have a normal non combi boiler and in my loft a cold water tank, then in my airing cupboard I have a hot water tank with an overflow/pressure release pipe that runs up and to the cold water tank for when the water expands (the hot tank is fed from the cold water tank).
The other day I got a call from the neighbour saying my overflow pipe was gushing, and it was really going for it, not just a trickle but like there was a tap on directly connected to the outside!
Wife came home and turned off the mains water and I came home soon as I could a couple hours later to take a look. I looked at the water tank in the loft and the level was a good 4-5 inches up past the overflow pipe! I ran the water for a while to get the water level to drop, turned on mains again and it refilled without any issues… Hasn't happened since, as yet…
I cannot for the life of me work out how this might have occurred, the only thing I can think of is that the water mains pressure was somehow so high that it forced past the float valve somehow?
I noticed that the hot water tank was also cold, I didn't check at the time but there's a chance the water had risen so high it had started pouring into the hot water tank from the hot tank outlet possibly, but I don't think it's related to how the cold water tank got full in the first place…
Anyone ever had anything like this? Could the valve just had seated a bit dodgy and not sealed off properly perhaps? If so how could I check this, it seems fine now checked it earlier today and at most there is a very irregular tiny drop coming from it that's it…
Best Answer
If you have a shower mixer that lets through it is possible for mains pressure cold water to backflow through the mixer and into the hot water tank system.