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Vaillant VCW 242 gb Hot Water Temperature

Discussion in 'Plumbing and Central Heating' started by LanceB, 8 Sep 2021.

  1. LanceB

    LanceB

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    Hi
    yes I know this is an old combi boiler. I've had it for 22 years and it was in the house at least 5 years before we move in I believe.
    Boiler fire up fine. CH is fine. DHW activates diaphragm->servo->diverter valve, boiler starts and the heat exchanger gets hot. Water from tap gets hotish.
    After maybe 15 or 20 seconds the flame shuts off. I guess this is because the heat exchanger is up to to temp as it is very hot. If I disconnect the NCT sensor it carries on and gets even hotter (I didnt do this for long). After a short while the flame reignites. I assume this is because it believes the exchanger needs heating up again.

    So nothing there seems out of the ordinary. However, whilst the flame is off, the DHW going through the exchanger and to the tap gets cooler and cooler until the flame re-ignites.

    I don't understand why. The exchanger is hot throughout the process so should keep heating the water to the tap shouldnt it? if the exchanger was faulty or blocked then I'd expect the water temp to always be cool, regardless of whether the flame is on. I replaced the exchanger 10 years ago from the original plastic one to the new metal one. It may have gotten clogged as we do live in a hard water area but as i say above I'd expect that to have an impact regardless of the flame state. Any ideas?

    The diverter etc are all working fine. Ive also replaced the NCT sensor. I have even disconnected the overheat thermostat on the DHW pipe with no difference. PCBs are not suspect either as the boiler fires up and CH works. I have replaced one or more of them in the past.

    I am loathed to get an engineer in until I have exhausted the obvious. I have had them in before and I usually end up knowing more about the inner workings of these beasts than they do! Most seem to want to use trial and error or just replace the whole thing when one small part is at fault.
    Thanks
     
  2. kidgreen61

    kidgreen61

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    Plate Hex will be gummed up either primary or DHW side
     
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  4. DP

    DP

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    What is the green light doing when flame is on. Watch the intensity

    Taking the lead off the thermistor will effectively run the boiler at max

    Not a boiler every dick and Harry will be able to deal with. It will possibly last another 22 years easily. Have a client with two of these working and three in the garage for spares.
     
  5. LanceB

    LanceB

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    green light comes on initially then after 20 seconds or so (once the heat builds up I guess) its fades out leaving just the amber light and then the flame switches off. After 10 seconds or so it starts again.

    Removing the thermistor lead doesnt sound like an ideal/safe option as I guess the the boiler can over heat?

    Is the exchanger servicable? i.e is it possible to flush it out?
     
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  7. ianmcd

    ianmcd

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    No you cant do that

    the plate heat exchanger can be cleaned, I use one shot drain cleaner , others use various methods, but whichever you choose follow the instructions to the letter , it is nasty stuff and the liquid will get very hot, , have to add I have worked on many of these boilers and never seen the plate blocked but with your symtoms and age of the boiler that is most likely, newer boilers block for fun
     
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