Discussion in 'Plumbers' Talk' started by MARC888, Jan 24, 2018.

  1. MARC888

    MARC888 New Member


    Hope someone can help. Have a two year old Worcester Bosch boiler that is losing pressure.

    Had WB come out and replace PRV under warranty but still got pressure drop.

    However outside there are TWO overflows from the boiler....one copper pipe from the PRV and one white plastic from somewhere else within the boiler. When the system is cold / not under pressure (heating and water off), and pressure is topped up to around 1.5 bar, the pressure drops slowly to zero over around a three hour period with no visible water from the overflow outside. However if I then turn the heating and water on, after around 10 to 15 minutes water can be seen gushing out of PLASTIC overflow and then stops. Then again after around 10 to 15 minutes it happens again. Anyone any idea what this problem is ? Faulty / split expansion tank ? Faulty / split heat exchanger ?

    Thanks in advance
  2. Dr Bodgit

    Dr Bodgit Well-Known Member

    The plastic pipe is the condensate outlet, its perfectly normal for this to be "leaking" as its discharging the condensate from the boiler.

    You must have a leak somewhere in the system, may be its the expansion tank, hard to tell. I would expect there to be water leaking.
    MARC888 likes this.
  3. If there is no water coming out the 15mm copper pipe outside - the safety discharge pipe - then your PRV is ok.

    The larger plastic pipe is your condensate pipe, and - as you sussed out - this is fed from the combustion chamber. When the boiler is actually running, condensate is formed in the combustion chamber and this falls in to a syphonic trap which 'glugs' it out the pipe when it reaches a certain level. So, during normal use, you can expect a 'glug' of 'water' out that pipe every - I dunno - every few minutes or so? But gushing? No. It all depends on what 'gushing' means!

    So it's hard for us to tell on here whether it's normal. What I would suggest is, if the pressure drops that much when the boiler is cold, then that water must be coming out somewhere so I would have expected to see it coming out that pipe during this time - it wouldn't 'wait' for the boiler to be turned on again (unless your syphonic trap is partially blocked with debris and it took the force of combustion and fan-power to drive out the accumulated water).

    The amount of water involved in that pressure drop varies, but I reckon it'll be 2 or 3 cupfuls. If this happens when the boiler is cold, that amount of water would, surely, have come pouring out the condensate pipe.

    So, your careful observations still don;t paint a clear picture.

    Something else you can try - turn off the boiler at the main switch, look under the boiler and ID the 22mm copper flow and return pipes to the rads. You will find a screwdriver slot on the valves where the pipes join the boiler - turn these through 90o and this will isolate the boiler from your radiators. Repressurise to 1.5bar (that's fine for these tests, but for general use you can keep this lower - say 1 to 1.2 bar) and monitor what happens.

    If the pressure still falls, then the leak is from within your boiler. If no water comes out the PRV pipe, then it must be coming out the condensate one instead - and that means a failed main exchanger ('heat engine'...).

    If, however, the pressure remains constant but then falls as soon as you reopen one of these valves, then that indicates a leak in your rads/pipework.

    When done, open both valves and switch boiler back on.
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  4. MARC888

    MARC888 New Member

    Hi. Thanks for the response. The WB engineer did carry out a pressure test and the boiler did hold pressure when isolated from the 'system' and there was a slight pressure drop when he opened up the system which could indicate a leak in the system somewhere (rads etc.) but I cannot see any visible signs of a leak either from ceilings or rads / TRV's. Is there any way of detecting and locating a leak in the system other than visual ?
  5. Dr Bodgit

    Dr Bodgit Well-Known Member

    Do you have pipes underneath a suspended ground floor, for example, where you can't see it? A slow leak there wouldn't show until a fair bit of water had leaked out.
  6. MARC888

    MARC888 New Member

    Yes about half the ground floor is suspended and the other half concrete / screed which was done over 10 years ago. I did have a similar issue of pressure dropping and traced it back to a dodgy flexi pipe / push fit seal on a ground floor radiator (which is boxed in). In that instance no pressure was holding at all on boiler and I could hear the water gushing out. That would have been around four years ago. Think this may be a good place to start ?
  7. Dr Bodgit

    Dr Bodgit Well-Known Member

    Its as good a place as any...you have a leak and it has to be somewhere. Where you can't see is a fine place to start!
    Deleted member 33931 and MARC888 like this.
  8. MARC888

    MARC888 New Member

    Quick update but no solution as yet.

    Had an engineer in today to do thermal imaging and moisture testing and found nothing untoward. Going to do a gas trace detection once I’ve had the system power flushed to get rid of leak sealer

    Anyway there is a pattern to the issue and any help would be appreciated.

    When the boiler is at rest / cold and charged to around 1 bar the system holds the pressure. When the CH is switched on the pressure rises to around 1.3 bar and holds pressure until the CH is switched off and then it goes to zero bar. Any ideas ? Cheers.
  9. Looks like 1.3bar is the 'triggering' pressure!

    Most such cases are open and shut - ie obvious. But I have seen some on here which are weird.

    I think one was ultimately found to have been a cracked or seam-split main exchanger which managed to maintain the water pressure when cold. As it heated up, this crack opened enough to allow the leakage, and then it pretty much closed up again when the boiler cooled down.

    (I'm sure I read this - perhaps I dreamt it...?)

    In your case, the pressure increases to 1.3 with the boiler hot - which is fair enough - but, unbeknown to you, this leak has begun inside and your boiler is slowly releasing water via the exchanger and out the condensate pipe (along with normal condensate). The pressure is maintained at 1.3 bar 'cos that's the job of the exp vessel, but you are constantly losing water.

    When the boiler cools down, that loss then becomes obvious as the pressure drops.

    AAAAARGH! Hang on - you've already pretty much worked out that the leak in in your rad/pipe system? In which case, possibly a poor copper solder joint could be doing the same thing - leaking more when hot?

    I dunno :(
    MARC888 likes this.
  10. Dave does Gas

    Dave does Gas Well-Known Member

    Think that you have the answer right there DA
    MARC888 likes this.
  11. MARC888

    MARC888 New Member

    Thanks for the info. The only reason I suspect it is a system leak rather than an issue with the boiler is that having closed the supply and return valves on the boiler the pressure remains constant; it held this pressure for around 1.5 hours. I then opened the valves up and the pressure dropped about 0.25 bar. However I did notice a couple of drops of water just below the boiler once I opened the valves.....could this have come from the heat exchanger, prv (or somewhere else) due to the sudden pressure increase ? If so would this mean there is a split in the exchanger ? Cheers
  12. koolpc

    koolpc Well-Known Member

    Water means one thing. Leak
    MARC888 likes this.
  13. MARC888

    MARC888 New Member

    Yeah.....but it’s important to know the reason and cause as the cost implications are vast depending on the cause itself.
    Dr Bodgit likes this.
  14. Dr Bodgit

    Dr Bodgit Well-Known Member

    I doubt its the pipes or a dodgy copper/solder joint. If it were, you'd be having a bigger problem with continual leaking. It sure is strange through and I can't think or anything else.

    Do you have any plastic/push fit fittings? This is more symptomatic of a compression fitting that's got a rubbery seal. It leaks when the pressure drops rather than when the pressure is high.
    MARC888 and retiredsparks like this.
  15. Back to basics, Marc - what is the cause and where is it located?

    Kools has answered the first for us ( how's you doin' Kools? :) ) and now for the tricky second bit.

    You have tried isolating the boiler for 1.5 hours with seemingly 'interesting' results, but also seemingly not quite 'conclusive'.

    So, do it again - except this time leave the bludy thing isolated all day (with the boiler switched orf, of course).

    I'd be tempted to 'push' the issue a wee bitty more and pressurise the system up to - ooh - 1.8 bar beforehand, then close the two valves fully.

    See what happens.
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  16. MARC888

    MARC888 New Member

    Thanks. I’ll give it a try. Appreciated.
  17. MARC888

    MARC888 New Member

    Yeah I have push fit at one radiator point. I’m getting this altered to copper too.
  18. MARC888

    MARC888 New Member

    Well found the leak after the whole system decided to dump itself in the house ...........somewhere. Took hours to find after pulling carpet up in downstairs bedroom, laminate floor in hallway and dining room, box sections in cloakrooms, and finally found the problem in concrete slab in kitchen area. Had to pull all laminate up in there and basically use jackhammer to channel floor out and re-pipe. Going to be a big cost unfortunately but at least the problem itself is sorted (hopefully). One lesson I've learnt.....if you can't hear or see the leak it's probably in the slab. What a nightmare............ leak1.jpg leak2.jpg
    Deleted member 33931 likes this.
  19. koolpc

    koolpc Well-Known Member

    Pipes in concrete! Yikes!
    MARC888 likes this.
  20. Wow!

    Well done.

    (Not an insurance job?)

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