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Vaillant TurboMax plus boiler - HW fine, CH intermittent

Discussion in 'Plumbing and Central Heating' started by fathertobe, 29 Mar 2006.

  1. fathertobe

    fathertobe

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    Hi,

    I have a 2-3 yo Vaillant TurboMax Plus combi boiler.

    HW behaves fine, but I'm getting some odd behaviour from the CH. I think this behaviour started last week - when we had all rads replaced and TRV fitted to the new ones but I'm not completely sure.

    Anyway, here's the problem:
    I switch the heating on to continuous (or it starts via the timer) and the boiler lights. The yellow middle light goes on. However, if I then turn the HW on, the solid middle yellow light goes out. The green light on the right starts flashing (indicating HW demand) and then the yellow light comes on. HW comes out of the tap.

    Then if I switch the HW off, about 30 seconds later, the CH starts up again. Well, most of the time. Sometimes it fails to restart. And it can't be anything to do with the rads as they're not up to temperature.

    Really appreciate help.

    nick
     
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  3. fathertobe

    fathertobe

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    As a postscript to this, it also seems that the CH is switching itself off intermittently... no clear reason why. The switch is on continuous but the yellow light goes out.

    I have the service manual if anyone has any idea how I could easily diagnose what is going on.

    Thanks again,

    n
     
  4. chrishutt

    chrishutt

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    If you check the status codes you can see what the boiler thinks it is doing, and then compare that with what it actually does.
     
  5. ACOperson

    ACOperson

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    This boiler has a pump over-run facility which stops it working on CH for a period of time after any meaningful amount of hot water is taken. Are you sure it is not this that is happening?

    As chris says, see what codes are being displayed and then get back to us.
     
  6. fathertobe

    fathertobe

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    Yep. it's showing the pump overrun code - as you predicted.

    I've looked up an explanation of that here:
    http://www.diynot.com/forums/viewtopic.php?t=47699

    [b}edit from here onwards[/b]: WHOSE PROBLEM was solved by A REPLACEMENT pressure switch.

    It seems to be related to this problem reported a few years ago... http://www.diynot.com/forums/viewtopic.php?t=7240 but there was no clear resolution there.

    So far, it seems my problem is related to:

    (i) the pump - but seems unlikely possibly given the HW is ok(?)
    (ii) the air pressure switch?

    Any other thoughts?

    Really grateful for help.

    [/url]
     
  7. Agile

    Agile

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    """Given my other issue with the CH turning off when the HW is turned on, any thoughts on to what I should be looking at?"""

    Surely you dont think that the boiler should carry on giving CH at the same time as its providing hot water?

    Tony
     
  8. fathertobe

    fathertobe

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    :LOL: I don't think anything of worth about plumbing tony. That's why I ask so many questions!

    However, intuitively it didn't seem to me that two such features should be entirely divorced - and, in particular, that on the termination of demand of HW, why the CH should then require the boiler to relight.

    Feel free to tell me that my intuition is horribly mistaken. It often is.

    Alternatively, feel free to contribute your two cents on what's wrong in this particular case. From your posts, you seem to know what you're up to!
     
  9. Agile

    Agile

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    Its entirely up to the designers of the boiler to decide how best to achieve the aims.

    Your boiler is relatively sophisticated and has intentional delays built in. Firstly the DHW preheat needs to be satisfied then the burner turns off and the diverter valve moves to the CH position.

    The boiler then runs the pump for about half a minute to allow the heat remaining after providing DHW to dissapate so that a true measurement of the CH flow temperature can be assessed. Only then, perhaps after a few minutes, will the boiler relight to provide CH depending on the flow temperature and the anticycling time constant.

    The boiler has an anticycling delay of about four minutes built in to avoid repeated short cycle firing as thats inefficient. As the radiators have a thermal time constant to 66% of perhaps 10-20 minutes the odd few minutes is not very relevant and increases efficiency by eliminating unnecessary firing.

    Please dont stand in front of your boiler watching it, that makes it feel uncomfortable. Sit in your armchair and watch a digital room thermometer instead!

    Tony
     
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  11. fathertobe

    fathertobe

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    Ok. Point taken... thanks!

    But I've still got a problem ...
     
  12. chrishutt

    chrishutt

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    Tony's point is that you don't have a problem, or at least the boiler doesn't, based on the information that you've given. Your problem is that you haven't read or understood the boiler manual that explains its operation. You should be able to download a copy from the Vaillant site.
     
  13. fathertobe

    fathertobe

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    Hi Chris,

    Ok, I think the conversation with Tony somewhat obscured the fact that I do still have a problem.

    The central heating seems to switch itself off even though the radiators/rooms have not reached the set temperature. It is similar to the problems described in the links in my earlier post in this thread.

    When it is switched off, the code shows Pump overrun - which I realise is a status code rather than a fault. But given the information about how the boiler is behaving, what are your opinions of possible faults?

    Really appreciate help.

    best,

    Nick

    ps sorry for dredging up old thread about the same problem. The thought of confusion did occur to me - I just thought the original poster might have email notification on the thread and be prompted into returning and explaining a resolution...
     
  14. ACOperson

    ACOperson

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    What is the set temperature that you describe? How are you measuring this and what are you comparing it to?
     
  15. Agile

    Agile

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    I am beginning to think that what he means is that he sets the room stat to say 22 *C and thinks the boiler should continue firing constantly until it reaches that temperature.

    In reality the boiler's duty is to present a flow temperature of say 70*C and turn off for a few minutes until the anticycle has timed out and then consider what it has fallen to and decide whether it needs to fire again.

    If the gas valve minimum is set wrongly this will affect the operation as it will reduce the modulation range.

    Perhaps he just needs some larger rads?

    Or to get his boiler serviced by a competent engineer BUT he says he has a "plumber" who he likes even though he is "untimely". Perhaps this "plumber" could check it out?

    Tony
     
  16. Crystal

    Crystal

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    Worrying, very worrying.
     
  17. fathertobe

    fathertobe

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    Agile - you are beginning to think correctly, I fear. But also the pin on the diverter was stuck meaning rads were not getting to temperature before the boiler shut off... or at least that's what I (half) understand. Any fixed now and I'm feeling a lot more confident that there will be heat in the little one's room.

    ps It's great to be spoken about in the third person. Somewhat reminiscent of the wonderfully entitled 'Does He Take Sugar?' on R4. Yes, I may be 'cerebrally-challenged' when it comes to this forum but points for trying, no?
     
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