Combi boiler losing pressure

Discussion in 'Plumbers' Talk' started by Rebel49, Oct 13, 2014.

  1. Rebel49

    Rebel49 New Member

    I'm at my wits end with my 7-year-old Baxi 105HE combi. It's losing pressure quite rapidly, i.e. from 1 bar to nearly zero overnight.

    Here's a summary of how things are:
    1) New expansion vessel fitted 6 months ago.
    2) No problems over the summer (but no heating used).
    3) Since using heating again, pressure dropping constantly - I think slowly when heating not on, more quickly when heating on.
    4) Pressure rises at first about half a bar, from 1 to 1.5, when central heating starts - seems right - but then gradually drops.
    5) No obvious leaks from pressure relief valve.
    6) No obvious leaks from boiler.
    7) No obvious leaks in radiator system - not a drop of air to bleed anywhere.
    8) Have used leak sealant just in case - problem seemed to improve for a few days, now back to just as bad as before.
    9) Now here's a weird thing I've noticed (well, may just be normal or may be a clue) - if I try to top up the pressure when the heating is actually on, the pressure won't increase - it rises a little when it's filling but drops straight back down again. I can hear the water going into the boiler but it's just 'disappearing' somewhere. Where can that be going?

    Any bright ideas welcome.
    Many thanks.
  2. Blimey, a mystery indeed :eek:.

    If it's dropping from 1bar to zero virtually overnight, that indicates a sizeable leak - which should in theory be noticeable. Unless it's under the floor.

    Are these the same symptoms you had 6 months ago when the expansion vessel was replaced? And the diagnosis of a faulty vessel was made by who - you or the plumber?

    Anyways, you clearly have a decent knowledge of the system here, and as far as I know the pressure (ie - water) loss can surely only be due to a leak from a pipe, a rad or the PRV (for the latter, it's really worth tying a poly bag around the outside pipe to catch anything being chucked out.)

    Any faults within the boiler would surely only add to the pressure as the higher mains water would only go one way...

    Hmm, unless a leaking main heat exchanger? This could leak out through the condensate pipe? Would a test there to be - check pressure last thing at night, turn off all heating controls so's the system doesn't come on overnight (and create condensate) and tie collecting bag over condensate pipe?

    Other than that, I'm stumped.
  3. Rebel49

    Rebel49 New Member

    Thanks for that.

    The old expansion vessel was corroded and leaking, diagnosed by Baxi. The replacement is a genuine Baxi part and I'm sure that's no longer the problem.

    I'm also as sure as I can be that there are no leaks in the radiator system (there was a bit of weeping from a couple of taps but I've fixed those) - there are no pipes in solid floors, only between floorboards and plasterboard ceilings and I reckon there would be some evidence of that after several weeks.

    As for the PRV, unfortunately the outlet is high up above first floor level, too high for me to reach...but I've watched it for ages (with binoculars!) under all conditions, heating on & off, and can't see a drop.

    Now then, the idea of a leaking heat exchanger - I have noticed dripping from a joint in the condensate drainpipe outside. Would that cause the pressure loss? Would that happen only when the heating's on? If so, I'd better go out with my binoculars again!

    Anyway, if it is that, sounds like a big expensive job - and after £300 for a new expansion vessel maybe it's time to cut losses and get a new boiler?
  4. You've seen dripping from a joint in the condensate pipe? What about the end of the pipe?!

    Do bear in mind, of course, that when your CH and DHW is running, you will have condensate produced which will be coming out the end of that pipe in regular 'dribbles/slugs'. That's normal. What I was suggesting you do for a test is to note any emission from there during the night when your boiler is off, and your pressure falls.

    (Surely the end of your condensate pipe isn't way up your wall too? Unless it points into a gutter or hopper? Where does your safety discharge pipe point at - hopefully not over people's heads...)
  5. Rebel49

    Rebel49 New Member

    Ok, what I've discovered is that the end of the condensate outlet is just loosely fitted horizontally into the top 'T' of the condensate downpipe, which is connected to the bathroom waste downpipe. So the discharge I can see is dripping from that first 'open' joint. That is also at first floor level but I can see that when (and only when) the CH is running the condensate is dripping furiously, almost running, down the wall from that outlet - and presumably most of it is still going down the pipe, which suggests that there's a big volume of discharge there.

    Now I'm guessing that the overnight pressure loss probably occurs when the CH is still running at bedtime, and early morning before I'm up, and not actually during the night. I think that there's hardly any, if at all, pressure loss when the CH is off...but I'll double check that tonight.

    So I guess that this confirms a likely diagnosis of leaking heat exchanger ?
  6. The condensate is running down your wall...?! :eek:

    It won't do your wall much good - it's fairly acidic.

    Anyways, yes, that should be trickling as the boiler runs, especially on CH.

    Whether the exchanger is more likely to leak when hot - I dunno. Possibly. I chust dunno. Plumbers will advise on that one.

    It won't be quite as straight-forward as you think to check the system pressure, tho'; when the system cools down, that in itself can cause a drop in pressure if other things are fault. For example, if your new expansion vessel wasn't set up properly or has since leaked air, then the system water pressure will characteristically rise as the system heats up - often to the point of triggering the safety discharge valve - before sitting fairly high while the system runs, but then it'll fall dramatically when the system cools after turning off.

    So, if your vessel is misbehavin', it'll show wide pressure fluctuations which you can't necessarily put down to a normal system leak.

    So, I guess you'd really need the whole CH system to be cool a while before bedtime so's you can note the 'cold' system pressure then, and again in the morning - before the system fires up again!

    Do you have an alternative source of heat for one evening so's you can leave your CH off and check this?

    Unless plumbers have an alternative idea?

    (Might still be worth doing that quick check of the vessel - do you have a tyre pressure gauge? Unscrew the shrader cap, and press your gauge on the valve - see what it reads. And listen for any water squirting oot...
  7. CraigMcK

    CraigMcK Screwfix Select

    funnily enough mines just started playing up, doing virtually the same thing. I'd tended to go for a heat X failure and when the burner is running the water is basically evaporated off, so there is no obvious signs of water. The problems is I don't really want to spend £200+ & labour on a heat X for a 12 year old boiler, so hunting around now for an alternative.

    Had almost settled for a Viessmann floor standing unit with unvented water cylinder in the bottom, but wife does not fancy the upheaval in the utility room, so back to drawing board at present... and no heating :(
    I can't believe the number of bad reviews almost every boiler manufacturer gets, makes it difficult to pick one
  8. Rebel49

    Rebel49 New Member

    Thanks again D's there's no big pressure fluctuation, if it starts at 1 bar it only goes up to about 1.2 bar and then starts dropping.
    Anyway, I'll do those checks tonight. Don't worry, we live in Yorkshire so it's lovely and warm here...and we have blankets, they're cheaper than gas :)
  9. Och, you hardy Yorkies, you... :)
  10. Rebel49

    Rebel49 New Member

    Another day...latest bulletin...

    I turned the CH off last evening and repressurised the boiler to 1.0 bar at bedtime. (What I should have done also, but forgot, was to turn off the DHW preheat, so the boiler fired up as usual a few times during the night...but anyway the boiler was cold this morning). The pressure had dropped during the night to about 0.65 bar.

    The heating was then on for about an hour this morning - the pressure rose to about 0.9 bar and, now the heating is off again, the pressure has dropped back to 0.6 bar. Something erratic there, better than usual, as I expected it to drop to zero as previously.

    So, none the wiser really. Weighing up a possible £500 repair against a new boiler...
  11. Hmm. I guess it makes some sense that the water loss will be greater when the boiler is running - the pressure is a bit higher, and expansion would likely tend to open any leaking gaps?

    That's all guesswork for me, tho'.

    I guess what Craig says makes sense too - any leakage from the exchanger would either be evaporated offski, or else end up with the condensate and be dumped without notice?

    The slightly smaller drop last night might just be down to the lower pressure you charged the system up to before going to bed - 1bar. And also it wasn't run that much overnight and no CH at all.

    Then you had it running for an hour this morn, and it seemingly lost a half-bar in that time, and might even fall slightly more as the system cools completely.

    Worth looking at BG fixed-price repairs? I know they have a supplement for exchangers, but even then it might be a lot less than £500?

    A new, good-name boiler is less than £1k. I know there's fitting on top, but I think I'd go that way rather than fork out £500 of just a 'part'. If you can get the exchanger done for, say, £300, then it might be worth it? (Assuming you do have the exchanger diagnosed as the culprit first, of course :rolleyes:
  12. Rebel49

    Rebel49 New Member

    I think that Baxi do a fixed price repair for £300 but maybe that doesn't include an exchanger. I'll give them a call. I've still got to weigh up what else is likely to go wrong in the next year or two. Modern boilers are like modern cars. I wish we could have kept our old regular boiler, it was built like a tank...
  13. Your last car was a tank? :eek:
  14. Rebel49

    Rebel49 New Member

    Not my last car but my first - a 1951 Ford Anglia :D
  15. Rebel49

    Rebel49 New Member

    PS re: a new boiler, I obviously wouldn't go for Baxi again, maybe a Vaillant or an Intergas look like good options?
  16. Dave does Gas

    Dave does Gas Screwfix Select

    If you go for the Valiant get an installer from their website, he/she will be accredited to give you an extended 7 year warranty.
    If you want a good mid priced boiler with a 5year warranty go for a glowworm again using their accredited installers, Glowworms are made by Vaillant so you are in effect getting a Vaillant boiler at less cost.
    Personal opinion now, avoid a Worcester, whist being a decent boiler when it comes to replacing parts they are badly designed.
  17. Rebel49

    Rebel49 New Member

    Thanks for that advice. I've heard only good things about Vaillant. As for Baxi I reckon they must be like Worcester - when we had the expansion vessel replaced it could only be done by taking the whole boiler off the wall, a hell of a job, what kind of ridiculous design is that?!
  18. Jitender

    Jitender Screwfix Select

    We had a Vail-ant EcoTEC plus 831 fitted at one rental. Which replaced a 14+ year old Ariston Eurocombi (faulty diverter valve).

    It was fitted by a registered installer so we have 7 years cover providing it is serviced by them annually (£50). Had to have supplementary bonding due to 16th ed. board. They didn't do a good job of the wiring so I fitted a new DP isolator and tidied up the wiring + the earth clamps were lose. Used 5-core flex.

    Quoted from BG to install a Worcester 30Si for nearly double what was paid for the Vailant, they were going to increase the 22mm gas to 28mm.



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    Last edited: Oct 14, 2014
  19. Where abouts are you, Rebel?
  20. Rebel49

    Rebel49 New Member

    Haworth, Keighley

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