Vaillant boiler: works fine for radiator, but only sometimes for hot water

edited 14 February at 1:45AM in In my home (includes DIY) MoneySaving
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BrokenlynxBrokenlynx Forumite
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I've recently moved into a flat where I've got a Vaillant boiler installed in the kitchenette area.

The boiler appears to be reasonably old (but not ancient), having had another Vaillant boiler in my previous place its similar to the approximately 8 year old EcoTec boiler I had there but probably older as it doesn't have the EcoTec branding. 

What I'm finding is that firing of the radiators is reliable. I've had a nest thermostat installed' and it works fine with the boiler for warming the radiators. 

However I find the provision of hot water from the boiler to be much more intermittent. In the morning there was no hot water where as during the later evening hot water seemed to run fine. I'd run the tap for at least a few minutes without the water ever warming up during the morning, yet whilst I do this, the boiler visibly and audibly appears to fire up whilst the tap is on (making some noise and showing and orange light on the front). The same happened when I tried during the evening just that within about 20 seconds the water had reached a reasonably hot temperature.

Having googled similar issues I've heard that the boiler pressure can contribute to this, that said, during the morning when the boiler was not working pressure was at approximately 2.5bar, during the evening when it was working the pressure was lower, at around 1.75bar. The pressure meter shows the green area as being between 1-2bar.

I'm mainly trying to gather information before attempting to do anything to the boiler so Im curious about what else I can observe (or try) to be able to get a more consistent provision of hot water.

For reference here's shot of the pipes, valves and taps connected to the boiler, I haven't touched any of these since recently moving in.



Any help is appreciated, thanks in advance.

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  • ThisIsWeirdThisIsWeird Forumite
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    Good info and photo - there are some checks you can do by feeling these pipes. :smile:  It would help, tho', to have a full pic of the boiler so we can ID the exact model.

    Something that can be said - your pressure is too high, and that needs sorting before it reaches ~3bar, whereupon it could lose some system water. Could you take a note of its 'cold' reading, when it hasn't been used for a while? Eg, if you are out during the day and before you turn on the boiler on your return. And then note what the pressure does over the half-hour or more after it's fired up for CH.

    As for the DHW issue, since the boiler does fire up with a hot tap, it's not the water Flow Switch that's at fault. That leaves a sticking Diverter Valve as the most likely culprit, but that's just a guess. When we ID the CH 'flow' pipe, we can feel if it becomes hot instead of the hot water pipe - that would be an indication.

    We'll then be in the realms of a 'fixed-price' repair, usually around £300, but they will tend to sort out everything that's faulty/dodgy, so hopefully the pressure issue too, tho' I think some won't cover the Expansion Vessel - the 'pressure' device - for some reason.

    In short, tho', this isn't an issue you can DIY. Other than the high pressure IF it's a consistent HP, and not a fluctuating type. 
  • edited 14 February at 10:59AM
    BrokenlynxBrokenlynx Forumite
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    edited 14 February at 10:59AM
    Thanks for the detail, here's a further image from the front including the pressure reading in the morning after some hot water usage for a shower: 



    As you can see pressure on the high side (I think the lowest I've seen it is around 1.7, so slightly lower than that).

    As a sidenote this morning I've had no issues with hot water it worked fine.

    I'll give the cold test a try and also testing whether the central heating pipe heats up when I'm running hot water from the tap.

  • edited 14 February at 11:43AM
    ThisIsWeirdThisIsWeird Forumite
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    edited 14 February at 11:43AM
    A Turbomax Pro, by the looks of similar pics on t'next.

    Rather than look up installation instructions, I'm sure you can ID which pipe is the CH 'flow'; it'll be 22mm, and become hot when the CH is on... (The 'return' will be ditto, but heat up after, and to a lesser degree).

    For the 'test', the flow pipe would need to be cold. If you fire up the DHW by turning on a tap, see if the CH flow pipe becomes hot under the boiler, especially if the DHW supply doesn't. That would suggest a sticking Diverter valve, that hasn't moved properly to supply DHW, but instead sends it out the CH pipe. Obviously you'll need your boiler to be faulty for this. The 'stickiness' can be intermittent, so it might continue to work ok for a while.

    As for the pressure, it should be at its lowest level when it's all cold, and an ideal setting would be at around 1bar. Unless you have a 3+ storey house, that should be fine; it's less stress on everything, and also allows a good 'green headroom' for any increase. Some increase is fine and normal, but I wouldn't like to see it go above, say, 1.5 without good reason. Much over 2 would suggest it needs some attention. 

    So, whenever it's next fully cold, check the pressure. To reduce it to ~1bar is done by bleeding a radiator. Note down the new base reading, and monitor it for a few days.


  • BUFFBUFF Forumite
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    somewhere on the boiler (possibly behind a flap) will be a data label/plate giving the model no.
    If it is a Turbomax Pro as ThisIsWeird posits it is non-condensing with a SAP rating of just under 80% when new.
  • EctophileEctophile Forumite
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    Rather than look up installation instructions, I'm sure you can ID which pipe is the CH 'flow'; it'll be 22mm, and become hot when the CH is on... (The 'return' will be ditto, but heat up after, and to a lesser degree).


    Even easier than that, the first photo the OP posted has a handy label on the bottom of the boiler telling you which pipe is which.

    If it sticks, force it.
    If it breaks, well it wasn't working right anyway.
  • plumb1_2plumb1_2 Forumite
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    Ectophile said:

    Rather than look up installation instructions, I'm sure you can ID which pipe is the CH 'flow'; it'll be 22mm, and become hot when the CH is on... (The 'return' will be ditto, but heat up after, and to a lesser degree).


    Even easier than that, the first photo the OP posted has a handy label on the bottom of the boiler telling you which pipe is which.

    And also show’s it’s not a condensing boiler 
    A thankyou is payment enough .
  • ThisIsWeirdThisIsWeird Forumite
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    Ectophile said:

    Rather than look up installation instructions, I'm sure you can ID which pipe is the CH 'flow'; it'll be 22mm, and become hot when the CH is on... (The 'return' will be ditto, but heat up after, and to a lesser degree).


    Even easier than that, the first photo the OP posted has a handy label on the bottom of the boiler telling you which pipe is which.


    D'oh! :-)
  • brisbris Forumite
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    Plate heat exchanger probably needs changed or flushed. Its the usual suspect for intermittent hot water.
  • ThisIsWeirdThisIsWeird Forumite
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    bris said:
    Plate heat exchanger probably needs changed or flushed. Its the usual suspect for intermittent hot water.
    Very possibly.
    That usually causes a distinctive 'cycling' DHW output, repeatedly fluctuating hot and cold. And once these symptoms begin, they usually become progressively worse - with fewer 'hots'. But, yes, it is also a possibility. 
  • BrokenlynxBrokenlynx Forumite
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    Thanks for the helpful responses to this thread originally. 

    So I had an initial servicing done with British Gas Homecare and it threw up some issues. 

    • The position of the boiler is above a kitchen work surface so unfortunately the engineer said they couldn't complete a '1st visit servicing' of the boiler for health and safety reasons. Health and safety essentially doesn't allow him to stand on the work surface and the space doesn't allow for usage of a ladder with 'three points of contact' to be able to reach the boiler.
    • I gave the engineer the detail in the initial post of this thread. The engineer believed based on this alongside a short test of the hot water whilst reviewing the boiler panel the issue is indeed as bris said:
      Plate heat exchanger probably needs changed or flushed. Its the usual suspect for intermittent hot water
    • Given the age of the boiler and the issues mentioned the engineer suggested either replacing the heat exchanger, at a cost in the hundreds of pounds but with the difficulty of securing discontinued parts or replacing the boiler entirely (with the associated cost of bringing the boiler down and closer to the work surface such that it can be serviced in future.
    As it stands I'm able to live with the boiler as is as it still supplies hot water 90% of the time from the tap whilst works 100% of the time for the radiator. But I'm aware I have what is essentially a maintenance time-bomb, either replacement (my preferred of the options) or repair will be needed at some point in the future. 

    With this in mind, I'd like to ask what my options might be? And what options there might be for managing the cost/aggro of the potential replacement? Also does the advice offered by the British Gas engineer sound correct? Is there anything I can ask of British Gas to help (admittedly unlikely as they're not my Gas provider and I've only been signed up for 2 months of servicing that it sounds like I'll be unable to use anyway and should probably cancel).

    Sorry if the above is a little vague or unfocussed in terms of the questions but in short, I'm looking for what I should be researching in the time between now and a potential boiler failure. 
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