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Erratic hot water supply from Combi

Discussion in 'Plumbing and Central Heating' started by MisterBoy, 5 Jun 2021.

  1. MisterBoy

    MisterBoy

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    We've been having issues running a bath since the winter where sometimes our hot water is very hot, other times merely lukewarm. We started off thinking it was simply down to the cold water being very cold during January but it is still doing it in June and our cold water isn't freezing anymore.

    We've already tried the obvious "don't run the tap as much" and it doesn't seem to make much difference. When the hot water is hot, we can run the tap on full - when it's having problems even backing it right off doesn't help much.

    We have had our diverter valve replaced in the past and that seemed an obvious thing to check but again, our heating is really not on much the last week or two and I've tried forcing the heating off to see it didn't seem to help.

    The weird thing is I have noticed sometimes if I turn the tap of entirely on the bath, and turn it back on, and now the water is too hot to touch. Or if I flush the loo while the bath is running lukewarm, the hot water suddenly gets hot. I don't just mean while the cistern is re-filling and the flow is lower; the hot water flow will drop and get hot while the toilet fills but once it finishes filling, the flow goes back to normal and stays hot.
    We've noticed this a few times but I cannot think of any explanation that would make sense, almost like it 'resets' the hot water... which seems crazy.
    Can any professionals tell me if this is possible or suggest what might be going on?

    Edit: Baxi Duo-tec combi 40 HE A
     
    Last edited: 5 Jun 2021
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  3. CBW

    CBW

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    An idea of what make and model combi boiler you have installed, as some have slightly different ways of operating.
     
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  4. MisterBoy

    MisterBoy

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    Sure, it's a Baxi Duo-tec combi 40 HE A.

    To clarify we're not getting 'kettling' or water cycling hot/warm. Sometimes it just comes out lukewarm. Once we get it flowing hot, it stays hot.
     
  5. CBW

    CBW

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    It could be the flow regulator or cold inlet filter partially blocked.
     
  6. DP

    DP

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    Check what the tap is delivering in 15 seconds times 4
    Then check if the sum is equal to hot water delivery for that boiler.
    I would be tempted to carry out full check on the boiler as gas line often undersized and having other gas appliances running at the same time may effect boiler operation
     
  7. ianmcd

    ianmcd

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    Run a HW tap and when the boiler fires feel the CH pipes below the boiler, see if one is heating up
     
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  8. MisterBoy

    MisterBoy

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    We don't have other gas appliances except our gas hob (not is use at the time, nothing else using hot water either) and we actually recently had the main gas pipe improved when we had to move part of it - but it didn't help. The boiler has been regularly serviced.

    I should add that a couple of years ago, I could run the hot tap on full blast. It used to be the case that in summer, I'd run the tap on full and the bath was too hot unless cold water was added while in winter we'd just run full hot. Now, I never do anything but hot water and it's a case of how much I back the tap off. Until now when even that doesn't seem to help.

    We've had all sorts of problems with this boiler the last 2 years - both the hot-cold heat exchange, the primary heat exchange and the diverter have all failed. Our plumber reckons a combi on a house our size is a bad option anyway but a new tanked boiler is a pricey way to address the problem... starting to think it might be worth it!
     
  9. MisterBoy

    MisterBoy

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    Thanks I need to check which pipes are which and that I can get to them but I'll give it a go.
     
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  11. DP

    DP

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    Get another engineer who can diagnose the defect instead of throwing parts at the boiler. If the fault is there then should be easy to fix
     
  12. MisterBoy

    MisterBoy

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    He didn't 'throw parts at it' those parts all failed, and it wasn't the same engineer in all cases:
    • We had no hot water at all but heating was working, the heat exchanger was replaced - it was totally gunged up on the CH side - and things worked again (we had the CH cleansed too)
    • Later, we had no hot water or CH and the boiler was kettling - this time the primary exchanger got replaced (never knew there were two) and things were OK
    • Later again, we had problems culminating in the boiler turning off and dripping water everywhere - the diverter valve (if that's the right term) had failed and lost all pressure. I think the guy managed to repair it rather than a full replacement.
     
  13. ianmcd

    ianmcd

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    The furthermost left copper pipe and the furthermost right copper pipe they should both be 22mm, you should easily get to them unless it has been boxed in, if one of these is heating up it will narrow down the fault for us
     
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  14. MisterBoy

    MisterBoy

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    Thanks @ianmcd. Managed to test this last night as the heating was totally off and we hadn't run hot water, so everything was cool to start with.
    Here's the bottom of the boiler:
    upload_2021-6-8_11-12-58.png

    1 & 5 are the 22mm pipes, the rest are all smaller (15mm presumably). I'm guessing 1 is the CH out, 5 is the CH return, 2 is the hot water out? No idea about 3, 4 or 6!

    Anyway all was cool then I ran down and start the bath running and came back up to see what happened.

    Pipe 2 got warm but could be touched quite comfortably.
    Pipe 1 on the other hand became red hot, too hot to hold onto certainly.
    Nothing else seemed to change though I only waited about 5 minutes. The hot water was hot-ish but I ran the bath using only hot water and it was easy to get into which shouldn't be the case!

    Here's the front of the boiler, sorry for the bad photo but you can see the hot water is set to max and it's maintaining 64C (not sure how hot that is in real terms).

    upload_2021-6-8_11-21-26.png

    Does both pipes 1 and 2 heating up suggest something is wrong?
     
  15. CBW

    CBW

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    1 CH flow (out)
    2 Dhw (hot water) outlet
    3 Gas
    4 Cold inlet
    5 CH return
    6 Safety discharge pipe (when over pressure occurs)

    if pipe 1 is too hot, then it suggests to me your diverter has failed.
     
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  16. MisterBoy

    MisterBoy

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    Thanks @CBW . THe CH flow pipe was certainly far hotter than the hot water outlet. But to fail twice in a year or two? Bad luck or sign of something else?
     
  17. CBW

    CBW

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    It could be a blocked plate heat exchanger, but unlikely. Isolate the flow pipe underneath, run the hot water and if you get hot water then it’s the Diverter, if it’s still the same you have a blocked plate. If the diverter has failed, it could just be a mucky system.
     
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