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Combi Boiler - One shower seems cold...

Discussion in 'Plumbing and Central Heating' started by garmcqui, 10 Dec 2018.

  1. garmcqui

    garmcqui

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    Hi all,

    Just moved into a house with a combi boiler, (28kW Baxi DuoTec, recently serviced) it's the first time I've ever owned one, always being a system boiler and cylinder guy previously. Seems to work ok, and water pressure is decent.

    However, we have two showers, one is a standalone shower and is fine, stays warm etc.

    The other is above a bath and is fed from the (non-thermostatic) bath mixer tap, seems to fluctuate a lot in temperature, and the water generally seems cooler than other hot water outlets in the house. For example, the boiler water temp is set to 65C, I cannot hold my hand under the hot tap in the bathroom, however I can comfortably stand under the shower in the same bathroom, even when only the hot tap is turned on.

    It was such a difference that I assumed there must be one of those anti-scald devices fitted below the bath (the ones which mix cold and hot water), but there isn't.

    Any advice appreciated?

    Thanks!
     
    Last edited: 10 Dec 2018
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  3. ianmcd

    ianmcd

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    try slowing down the flow rate on the hot tap that is the problem and see if it gets hotter
     
  4. garmcqui

    garmcqui

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    Will try that when I get back home, thanks. :)

    Doesn't the combi boiler govern the flow rate in order to maintain a temperature though, in the same way an electric shower does?
    The other shower has an equal, if not better, flow rate and seems to maintain temperature ok.

    Am thinking of replacing the bath mixer tap with a thermostatic version - would this help at all?
     
  5. ianmcd

    ianmcd

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    some boilers have flow restrictors some have to be set manually, if you have a greater flow than the boiler is designed to produce it will be cool, for some reason only the Duo tec 40 HE has a flow restrictor fitted according to the Manufacturers instructions, come back and let us know if running the problem tap slower works and I will tell you how to set it so that you wont need to change your taps
     
  6. garmcqui

    garmcqui

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    Cool, won't be home until lunchtime, so will give it a go then, cheers.

    Are you thinking that it's likely due to the naturally increased flowrate of a bath tap vs a basin tap, that the boiler simply can't keep up?
     
  7. Gasguru

    Gasguru

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    Yes..if you have a traditional 3/4 valved bath tap opening the hot tap more than an 1/8 turn can result in too much water flow and reduced hot temperature.
     
  8. garmcqui

    garmcqui

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    From below, seems like 15mm pipe supplying the bath, not 22 - does that make a difference?
     
  9. Gasguru

    Gasguru

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    Not really.
    At this time of year as soon as you exceed perhaps 10 or 11 litres/min hot flowrate you won't achieve a sufficient showering temperature.
    Non thermostatic bath mixers are hopeless to set a stable temperature.
     
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  11. garmcqui

    garmcqui

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    Ok, so first thing I did was measure the flow rates from hot and cold taps in bath, and other shower.

    Bath tap flow rate (cold): 18L/min
    Bath tap flow rate (hot): 13.95L/min
    Bath tap flow rate (hot shower): 13.95L/min, same as bath.

    Other shower flow rate (at showering temp):11.7L/min (possibly due to a different shower head)

    So it looks like that’s the issue then. What to do about it?
     
  12. ianmcd

    ianmcd

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    set the flow rate to what it should be 13.95 L/min is too much for your boiler
     
  13. garmcqui

    garmcqui

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    is this just done by adjusting the main stopcock under the kitchen sink?

    My only concern is this will reduce pressure too all outlets, including the shower which is working well. Would it be preferable to fit a valve on the feed to the bath tap, so the flow to this could be reduced alone? They are on flexible pipes, so shouldn't be difficult to replace with one with valve?

    OR, would fitting a thermostatic mixer tap to the bath work, as (in my head, anyway) this should reduce the flow from the boiler as it will be combining it with cold?
     
    Last edited: 10 Dec 2018
  14. ianmcd

    ianmcd

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    please just do as I asked you, reduce the flow and if it works I will tell you how to sort it and no dont touch the main stop cock
     
  15. garmcqui

    garmcqui

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    Sorry, meant to say, opening the tap less seems to result in the water being hotter, and staying hotter. Apologies, did test that but got distracted calculating flow rates...
     
  16. Gasguru

    Gasguru

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    So either keep the flowrate lower, or fit flow restrictors to the feed or preferably fit a mixer with a thermostatic shower.
     
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  17. ianmcd

    ianmcd

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    on the cold water inlet to your boiler there is an isolating valve , open the problem tap up full and then gradually close the isolating valve at the boiler, a bit at a time till you have the temp you want at the problem tap and you will be good to go, there is a much more technical way using a thing called a Weir guage but this should work for you
     
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