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...

  • The hot tank outlet (by which you mean the hot tank overflow outlet I presume) typically forms an inverted U over the edge of the cold water tank. There is no way for cold water to flow up the U and down into the cold water tank. Even if the hw overflow comes into the side of the cold tank, I still don't think much cold water could get into the hw tank; it is already full of hw, and although the cold is denser, the difference isn't that big. Commented Oct 24, 2019 at 12:43
  • Check the float valve, probably the float isn't air-tight any more and water entered it. Also add a pressure regulator to tank cold inlet: in the future, even in case of pressure spike, you will be safe.
    – DDS
    Commented Oct 24, 2019 at 13:37
  • @MartinBonner yeah I wasn't really sure that was a possibility, still not got a clue what caused it
    – MorkPork
    Commented Oct 25, 2019 at 14:11
  • @DDS I wondered about the float but once I dropped the water level back down it seems to be working fine (it was also up as high as it could be when I checked the tank initially and had shut off the water, it was just 5 inches under the surface) and when I checked it the next day it still appears to be ok
    – MorkPork
    Commented Oct 25, 2019 at 14:12

1 Answer 1


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.

  • I wondered about that but our shower is fed from the cold and hot water tanks and there is pump to up the pressure so I'm assuming it's not that 🤔
    – MorkPork
    Commented Oct 25, 2019 at 14:10

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