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Intermittent hot water fault (n)

Discussion in 'Plumbing and Central Heating' started by number49, 5 Nov 2018.

  1. number49

    number49

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    Hi All,

    We have a Baxi Combi 30HE which ahs run fine for years and was last serviced earlier this year.

    Our shower has a thermostatic mixer valve; When we turn on the shower full and set the temp to hot it runs fine for a few minutes, then the temp will drop to cold, then after a minute or two will heat up again and it continues to cycle like this.

    At first I thought it must be the shower valve but we have noticed that it happens occasionally with the sink taps too.

    I also noticed we have to open the hot kitchen and bathroom taps fully now when we use them for the boiler to come on and heat the water, whereas we only had to turn them on partially in the past. The actual flow from the taps seems good as it splashes everywhere with it having to be on full.

    (n)(n)(n)

    What is the most likely cause? I am thinking it could be something to do with a flow switch that brings the boiler on. Are there any simple and safe tests I can do to narrow it down without having to call somebody in or getting a roasting from you Gas Safe chaps?

    Any help appreciated.

    Thanks

    Mike
     
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  3. terryplumb

    terryplumb

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    If you turn on a hot water tap fully and let it run continually does the temperature alternate hot / cold / hot/ cold repeatedly ? Or does it eventually stabilise hot ?
     
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  4. number49

    number49

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    Just tried with bathroom tap which is on same leg as the shower. Ran hot continuously for about 5 mins with no issues. Heating currently off if that helps with diagnosis (n)
     
  5. terryplumb

    terryplumb

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    As you stated in first post ,that the fault is intermittent ,sometimes the shower ,sometimes taps ,and todays test all was good on the bathroom tap i think you will need an engineer to visit to establish whats going on . maybe consider a fixed price repair from baxi.
     
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  7. ianmcd

    ianmcd

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    possibly the DHW diaphragm on its way out but would have to be in front of the boiler to be certain
     
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  8. bazdaman

    bazdaman

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    Change the hydraulic venturi and diaphragm both parts for under 20 quid. Does the hot water tap light go on and off on shower demand?
     
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  9. number49

    number49

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    Thanks for the responses and sorry for the late reply.

    Strangely it seems to be working fine again now so I'm wondering if the problem was with the mains coming into the house fluctuating. If does start again at least I have your pointers to start with now.

    Cheers,

    Mike
     
  10. number49

    number49

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    For anyone with a similar fault who finds this thread it got steadily worse and turned out to be the hydraulic Venturi as mentioned by bazdaman (y)
     
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  11. DIYnot Local

    DIYnot Local

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