Vaillant TurboMax Plus Combi DHW Too Hot

Discussion in 'Plumbing and Central Heating' started by ChewChew, 5 Jan 2016.

  1. ChewChew

    ChewChew

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    I would be grateful if anyone could provide ideas/guidance on this issue with my parents boiler.

    The central heating engineer who fitted and maintains it, spent a lot of time with it and on the phone, and ended up replacing the pump. £300 later the problem was not fixed!

    Problem - Hot water too hot at 65C even when the DHW control is set to its lowest setting of 35C.

    Boiler - Vaillant TurboMax Plus 837e, approx 10 years old, serviced annually.

    Stuff I’ve investigated using a multimeter, thermometer and Vaillants vrDIALOG/2 diagnostic software -

    • All three thermistors (flow, return, warmstart) appear to work normally.
    • DHW temperature control operates normally over the range 35-65C as seen both on the boilers LCD display and vrDIALOG software.
    • DHW flow as measured by the boiler is OK - reading approx 4% higher than measured flow over the range of 2-15 lpm.
    • WarmStart works fine maintaining the DHW heat exchanger at approx 40C.
    • The measured gas rate is as per the manufacturer's specification.
    • CH flow temperature as set by the CH control is correctly maintained.
    Thanks in anticipation
     
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  3. Agile

    Agile

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    Who measured the gas rate and how did they do that?

    Tony
     
  4. Agile

    Agile

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    My view is that you should not have to pay for new parts which don't resolve the problem.

    Tony
     
  5. ChewChew

    ChewChew

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    Thanks for your input Tony.

    I did by setting to boiler to full on, noting the gas usage at the meter over 2 mins and multiplied by 30. I guess you're going to tell me this is not ideal, but it returned a figure close to that in the spec.

    Ref paying for the parts - agreed. I will approach him about this soon, but the priority is resolving the issue hence this post so I can have an informed discussion with him.....
     
  6. Agile

    Agile

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    OK in that case turn the hot tap on with JUST enough flow to keep the boiler burning and measure it again.

    Then tell us what both the readings you obtained were.

    Tony
     
  7. ChewChew

    ChewChew

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    Thanks again Tony. I will do this when I next visit them on Wed and post back later in the week.
     
  8. ChewChew

    ChewChew

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    With HW full on (14.35 lpm as measured by boiler) - 3.66m3
    HW on slow (4 lpm as measured by boiler) - 1.23m3

    During the slow test I noted that the meter stopped briefly a few times. If you look at the log below, you will see that the boiler killed the gas when the DHW heat exchanger and return reached approx 74C

     
  9. wannocks

    wannocks

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    Your engineer needs to check the burner pressure and gas rate at minimum. This needs to be done by running it in test mode at minimum (not slowing a tap down).
     
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  11. wannocks

    wannocks

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    Check the flow rate when its getting too hot. The 837 is quite powerful, if a tap is running too slowly it will get hot at the boilers minimum output.
     
  12. Agile

    Agile

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    That's all very interesting!

    Apart from a possible problem caused by hysteresis ( which I cannot fully mention because of limits on what we can say about gas ) then I am only able to suggest there may be a fault on the PCB.

    But being in front of the boiler there are so many other aspects which could be noticed.

    Tony
     
  13. ChewChew

    ChewChew

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    It’s getting too hot at all flow rates accross 3-15 lpm.

    From the previous flow tests I can see that the boiler is modulating the heat output which I guess is a function based on the measured flow rate, but adjusting the DHW temperature control has no effect whatsoever. If it’s set at 35C or 65C the water remains at 65C.
     
  14. wannocks

    wannocks

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    Have you had the gas rate and burner pressure checked?
     
  15. Agile

    Agile

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    So how did you come to this conclusion?

    It does rather sound like a PCB fault to me as, subject to the above, you do seem to have checked all the usual things.

    I must say that I cannot see how anyone could ever think that the pump is the cause with this fault. A faulty pump will cause less heat transfer if its not running correctly and not a higher DHW temperature.

    Tony
     
  16. ChewChew

    ChewChew

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    Hi Tony,

    With HW running from cold start to hot I monitored the graph in the logging on vrDIALOG (see picture in earlier post as an exapmle) and measured the temperatures on the pipes near to the flow, return and warmstart thermistors. All readings were broadly as expected and similar. If anything the temperature readings from the boiler (shown by vrDIALOG) were slightly higher than my thermometer.

    I agree, it does seem to point to PCB fault, but I’m still not 100% convinced because it appears to read all the thermistors OK, measures flow OK, modulates the burner, and reads the DHW control OK which leaves a failure in the control logic. However, the logic is handled by a microcontroller and the chances that the instruction code became corrupt such to just affect DHW temp seems slim to me. But stranger things happen………

    Anyway, the engineer mumbled something about replacing the PCB was his next step. I will agree to this subject to him not charging if this does not fix the problem.

    Thanks for your continued help.
     
  17. ChewChew

    ChewChew

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    I will ask the engineer if he has done this.

    You will see from an earlier post that I checked the gas rate under min and max DHW flow conditions, albeit very rough and ready, the results of which seemed to fit in with the spec in the manual.

    Thanks
     
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