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Another hot water in the cold water storage tank question...

Discussion in 'Plumbing and Central Heating' started by Giles Mann, 4 Jun 2021.

  1. Giles Mann

    Giles Mann

    30 Dec 2019
    Thanks Received:
    United Kingdom
    Hi all. I am fully aware that this question has been asked multiple times and I have trawled through a lot of threads on this.

    I am trying to diagnose an issue and was just hoping someone could confirm if I am on the right lines here.

    The set up:
    2x large storage tanks at same level in loft. One for cold water bathroom taps, one to feed hot water cylinders at ground floor.
    1x small tank (F&E for CH) @ higher level than other two
    3 flats being fed by this system (share of freehold/self managed block)

    The issue:
    Every so often the overflow pipe from the CWS tank (which feeds the HW cylinder) will run (with varying flow rates) for several days at a time. It may stop occasionally (I guess when someone is showering etc). Then the problem will go away for a while (months even) before restarting.

    The last time this happened, I isolated the mains feed into the building and it stopped the overflow almost immediately (opening stopcock made it start overflowing again). I went up into the loft and to my horror the water was pretty hot to touch. The vent pipe (presumably from HW cylinders) end was slightly submerged in the water (which it seems is wrong); however, if the water was only as high as the ballcock valve dictated it wouldn't have been submerged. I got my neighbour to drain the tank to correct level and watched. No water was coming out of vent pipe so it must be from the outlets at the bottom of the tank. AN hour later the overflow was going again.

    My thinking on potential issues:
    - Dodgy mixer tap (probably kitchen) where mains fed, pushing hot water back into tank
    - Failed thermostat causing overheating of water in cylinders
    - Punctured coil in HW cylinder - I feel like this is unlikely as the ballcock in the F&E tank wasn't running whilst the overflow was happening

    Any other ideas and things to look at? cheers
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  3. polesapart


    27 May 2011
    Thanks Received:
    United Kingdom
    A mixer tap of some kind backfeeding mains to hot would account for what you have described.
    • Thanks Thanks x 1
  4. DP


    10 Dec 2003
    Thanks Received:
    United Kingdom
    If that was the case, pipe connected to top of cylinder ( hot water supply pipe to your taps) will be cold
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    3 Sep 2019
    United Kingdom

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