Cold water tank overflowing with hot water

Discussion in 'Plumbers' Talk' started by Tiago, Oct 8, 2019.

  1. Tiago

    Tiago Member

    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.
  2. kiaora

    kiaora Screwfix Select

    Sounds like it’s the pump installation, it’s pumping water back up ?

    have a good look, feel the pipes ...

    good luck

  3. Mike83

    Mike83 Screwfix Select

    It shouldn’t be a boiler or valve issue.
    If it was the water would leave via the expansion pipe.
    It could be a faulty mixer valve as the storage tank is back filling?
    Heat likes this.
  4. Heat

    Heat Screwfix Select

    I think I would turn electric to pump off and isolate the water supplies to it, then see if tank stops overflowing.
  5. Mike83

    Mike83 Screwfix Select

    Water is getting pushed back up the feed that goes from the cylinder to the storage tank.
    For this to happen water is coming in from the draw off at the top of the cylinder.
    This could only be caused by a pump or a water main.
  6. Tiago

    Tiago Member

    Thanks everyone for commenting, just to add some comments to this:

    - I'm not sure if the pump is to blame, because I can see the stream coming in the tank even when it's off (it's usually off most of the time, it starts going on more often when the water tank is way over the overflow mark).
    - I'm going to run an experiment, and leave the mains open - but no boiler in action, and pump off - to see if it will still go above the overflow mark - I suspect it won't, but I'm not sure if I ran that scenario, it usually takes about 30 mins - 1 hour to be visible.

    Also not sure if worth mentioning, but the last plumber was amazed by the fact that the whole pipe was warm, from the hot water tank to the cold one.

    My biggest fear is that they are going to send me a boiler engineer, and this he will either not fix it or suggest some hack (like a non-return valve another plumber suggested). and I end up with a bigger problem over time.
    Last edited: Oct 8, 2019
  7. Heat

    Heat Screwfix Select

    Any mixer tap or shower mixer can be culprit letting water pass through them if one side is higher pressure - like mains cold water to a tap with hot tank fed gravity water.
    Turn your mains stopcock off to see if water in tank stops rising.

    The heating coil inside copper cylinders can also crack and cause whatever cold water tank with highest water level to overflow the lower level tank.
  8. Tiago

    Tiago Member


    So 1 of my assumptions was wrong, it's not only the hot water that is going up.
    If I open the mains and leave the boiler (and pump) off, it still overflows through the exit valve, I can see the stream going in and after 30 minutes it was above the overflow mark.

    I'm in full despair at this moment.
  9. Mike83

    Mike83 Screwfix Select

    Sounds like a faulty mixer valve.
    It will be a mixer that contains a main.
    Something like a kitchen sink tap.
  10. Mike83

    Mike83 Screwfix Select

    Can you isolate the kitchen cold tap only.
    It may have a valve under the sink.
  11. Tiago

    Tiago Member

    Thanks @Mike83
    If you look at the attached picture, would twisting those 2 screws isolate it from the system?
    I'm going to try it tomorrow morning if it is.

    Attached Files:

  12. terrymac

    terrymac Screwfix Select

    Yes ,turn till slot is vertical (1/4 turn) and precise.
  13. Tiago

    Tiago Member

    more updates:

    I ended up testing it tonight. I isolated the kitchen sink, opened the mains, and the overflow water was still getting into the tank!!!
    Could this be any other mixer in the house? (only 2 left, at bathroom: tap and bathtub).

    Also, maybe this is helpful information: when the pump is on, it seems I've more hot water than cold water pressure on them.
    Also, when the pump is off, the cold water is almost zero from them (even if there's a bunch of water in the cold tank) - while the hot one is quite alright (the difference is the hot water tank is at the same level of the sink, and the cold one is in the loft above).

    I really would like to thank everyone me helping out here, I'm feeling more and more confident this has a plausible explanation in the end.
    Last edited: Oct 8, 2019
  14. Tiago

    Tiago Member

    Also, would a broken mixer be a viable explanation of the pump going on and off randomly?
  15. Mike83

    Mike83 Screwfix Select

    I’m not sure about back filling bringing the pump on.
    But the fault you describe I’ve witnessed plenty times and it was due to faulty mixers.
  16. terrymac

    terrymac Screwfix Select

    The shower pump can't raise the level of water in the loft tank ,as the tank is the reservoir of water that supplies the pump and hot water cylinder. All that would happen is the pump would circulate ,the volume of water can not increase.
    You either have mains water getting into the gravity system via a mixer ,or if your f&e tank is higher than the cold water storage tank ,a pin hole leak in the cylinders coil.
  17. Tiago

    Tiago Member

    I see, I'm going to test the kitchen sink again, just to be absolutely sure. Thanks again for replying.

    I had a new plumber in today but he didn't focus much on this when I tried to explain him, instead he thinks it's caused by bad workmanship on the pipes around the hot water tank.
    I'm including a picture, maybe you guys can say if this makes sense or not - or he's clueless.


    He thinks the water is being going back to the cylinder by the pump pipes, following that arrow. For that he thinks that the non-return valve there is broken. He also said that connection should never exist.
    Which takes us to the picture bellow, where he thinks that T connection should just be an L, and should not be connected to the escape pipe (that according to him is where steam is released when pressure builds, but also connects water back to the cylinder.


    Would this also be a viable explanation?
  18. Mike83

    Mike83 Screwfix Select

  19. Mike83

    Mike83 Screwfix Select

    Are these 2 red lines on the same pipe.
    That’s an embarrassment that plumbing.
  20. Mike83

    Mike83 Screwfix Select

    The non return valve could be faulty.
    I’m also trying to work out if having the 2 hot connections on the pump incorrectly fitted would cause an issue.

    Connections actually look fine.

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