Water overflowing from F&E tank in loft

Discussion in 'Plumbers' Talk' started by misskiti, Sep 13, 2021.

  1. misskiti

    misskiti New Member


    Pleas can you assist? Water has been dripping out of the overflow pipe from the F&E tank in the loft for a couple of weeks and is steadily worsening.

    We have a very old oil boiler and central heating system and have not lived here long. There is a gravity fed tank for cold water to upstairs and the F&E tank for central heating. The issue is not the ball valve as this has been replaced on both tanks.

    Have been advised that it might be sludge build up, is this correct? If not what else May it be please?

    We had not been running the heating when the issue started, only the hot water during this time. The drip is pretty much constant, not just when boiler is on. I have noticed it stops dripping for a few minutes when the boiler fires up but then carries on.

    Any help appreciated, thank you.
  2. exbg

    exbg Active Member

    Almost certainly not sludge. Most likely problem, and not good news, is a holed coil in the cylinder. Fairly expensive job, I'm afraid

    EDIT: can you see which tank is overflowing? If it is a hole in the coil it will be showing via the big tank (CWS). Normally, it will be worse in the morning until you draw water when upon the level will drop to below the overflow. It may still drip if the OF does not have a great "fall".
    If it is from the little tank, it could be "pumping over", which has a few causes. Or it may be as simple as the ball valve being set too high. As the heating expands the water, it rises, so there should be a reasonable gap between the normal water level and the OF outlet.
    Last edited: Sep 13, 2021
  3. TW2

    TW2 New Member

    If it were the hot water cylinder coil that had perforated, would not the overflow come from the small F&E tank, as this would normally be at a lower level than the main big tank, so that the big tank would feed into the hot water cylinder, then through the perforation into the CH circuit and thus back feed up to the F&E before overflowing from it?
    Check that the ball valve has been adjusted for the right height for the tank overflow and also that the "new" ball valves are actually working properly.
    Tie up or otherwise prevent the F&E tank being filled and see if this cures the problem.
    Only if the F&E continues to be filled would it indicate a HWC coil hole.
  4. exbg

    exbg Active Member

    Absolutely correct, in that the lower of the tanks will "present" the problem. My post did not explain it at all well.
    However, many F@E's are higher than the CWS, and, generally- certainly in my area, the CWS are either lower or the bases of both are at the same height. Obviously, the latter will result in the CWS water level being higher, thus leading to your described result.
  5. misskiti

    misskiti New Member

    Thank you for your responses. It’s the smaller F&E tank which is overflowing, the CWS tank is fine. The F&E tank is at a lower level than the CWS.

    Will check the ball valve again as suggested whilst crossing fingers and toes! We bought the house from someone we know and I didn’t think the hot water cylinder was replaced that long ago.

    Would the upstairs taps be running brown coloured water if it was a coil leak? Water is clear currently, though the water in the F&E tank is a bit rust coloured.
  6. TW2

    TW2 New Member

    The flow would be from the tap supply water into the CH system, so the amount of discolouration coming "backwards" would be too tiny to be noticable at the taps, unless there were a major coil tear, and in that case water would be pouring from the overflow.
    I reckon it's the ball cock and valve.
    Have you isolated the F&E tank supply, yet?
  7. dcox

    dcox Screwfix Select

    I agree with TW2 above. In my experience, the most common cause is the ball valve itself not shutting off completely - and that’s a relatively easy and cheap thing to fix.
  8. terrymac

    terrymac Screwfix Select

    When your system is cold ( central heating and domestic hot water ,boiler not active for a few hours) check the level of water in the f&e tank ,it should only be a couple of inches above the tanks outlet. If the water level is higher than this ,remove the excess and re set the float arm so it closes the ball cock valve ,to achieve the lower water level.
    If the ball valve is closing completely ,and water level continues to rise to the overflow level ,then water is entering the central heating system via the coil.

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