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Plumbing question: New shower mixer valve is worse - either hot or cold no inbetween.

Just moved into a bungalow where there is a Combi boiler with mains cold feed to the house.
The shower has a thermostatic mixer valve of the exposed type , hot feed on the right, cold on the left. When operating the mixer valve, it was possible to find a position where the water temp was acceptable but just a minute finger press in any direction on the temp knob would make the water too hot or too cold.
So I thought I would change the mixer valve. Today I went to screwfix and found this Ideal Standard Ceratherm Exposed Thermostatic Mixer Shower Valve Fixed Chrome | Shower Valves | Screwfix.com
Easy to install and all went well until I turn back on the mains feed (under kitchen sink).
Now it is worse. Switching on the shower the water was scolding so I rotated the control knob slowly but not much happened until it got to the backstop then all went freezing cold. So I slowly turned the control knob the other way and it went further than where it was originally and further still before going scalding hot. It's impossible to use. Either scalding or freezing and I tried about 15 minutes .
The cold is mains pressure. The hot is from a combi boiler. The hot water is fine through the kitchen and bathroom taps. 
I have read I might need a pressure release valve but no idea where to look for that if indeed it has one.
Maybe I should not use a thermostatic mixer valve?
No idea what the different flow rates L/per min and pressure (bar rates) mean on these mixers or which one to get.
Can anyone offer advice please?

Comments

  • Are you sure it's plumbed correctly - HtoH and CtoC?

    Step 2 - are you absolutely sure?

    A combi should be ideal for a thermostatic mixer, and yes, you'd be nuts not to have a thermo type - and the one you've chosen should work perfectly with a combi.
    So, are you sure sure sure the H&C are correct? How do you know? Can you feel which one is hot when it's on, by touching the inlet pipe? And are you 100% certain that this is the correct side for that mixer?
    If so, the only other possibilities I can think of - one is that one supply pipe is restricted somehow. When you had the old mixer off, did you check that both supply pipes flowed well? Are there filters on the inlet, and could they have been blocked with debris?
    The only other possibility is that the flow isn't high enough, for some reason, for the boiler to operate properly. Ie, the combi is either coming on or going off, and not modulating properly to supply what the mixer actually needs.
    Make, model and age of boiler?

  • ComicGeek
    ComicGeek Posts: 1,616 Forumite
    Eighth Anniversary 1,000 Posts Name Dropper
    Probably not high enough mains water pressure, the mixer is noted as for high pressure systems only. You may not be getting sufficient hot and cold water pressure together to mix - its only working if you're running only hot or cold separately as that would be at a higher water pressure.

    Do you have low pressure taps in the bathroom? If so, try running the shower in cold and then turn the bathroom tap on hot - does the shower flow rate completely drop off?
  • ninjaef
    ninjaef Posts: 176 Forumite
    Sixth Anniversary 100 Posts Name Dropper
    edited 6 September 2021 at 11:41AM
    Corrected wrong way round! Thanks JC !
    Also looks better with the hose coming from the bottom !!
    To be fair, I just installed the same as the last valve. So that had been fitted incorrectly which is why it also had the same problem. I needed have not bought another . Ah well, at least its new for me. The old one looks tired anyway


    Jeepers_Creepers said:
    Are you sure it's plumbed correctly - HtoH and CtoC?

    Step 2 - are you absolutely sure?

    A combi should be ideal for a thermostatic mixer, and yes, you'd be nuts not to have a thermo type - and the one you've chosen should work perfectly with a combi.
    So, are you sure sure sure the H&C are correct? How do you know? Can you feel which one is hot when it's on, by touching the inlet pipe? And are you 100% certain that this is the correct side for that mixer?
    If so, the only other possibilities I can think of - one is that one supply pipe is restricted somehow. When you had the old mixer off, did you check that both supply pipes flowed well? Are there filters on the inlet, and could they have been blocked with debris?
    The only other possibility is that the flow isn't high enough, for some reason, for the boiler to operate properly. Ie, the combi is either coming on or going off, and not modulating properly to supply what the mixer actually needs.
    Make, model and age of boiler?


  • fezster
    fezster Posts: 485 Forumite
    Part of the Furniture 100 Posts Name Dropper
    ninjaef said:
    The shower has a thermostatic mixer valve of the exposed type , hot feed on the right, cold on the left. 
    Hot feed is always on the left, cold on the right. Dont be confused by the (usually red) temperature adjustment, which is often on the right.
  • Result, Ninja!
    And no-one is sniggering, honest. :smile:

    (Sorry...can't type any more...eyes watery...)
  • NSG666
    NSG666 Posts: 981 Forumite
    Part of the Furniture 500 Posts Name Dropper Combo Breaker
    Doh! well done JC
    Sorry I can't think of anything profound, clever or witty to write here.
  • knightstyle
    knightstyle Posts: 7,112 Forumite
    Part of the Furniture 1,000 Posts Name Dropper
    Well done for having a go at DIY and coming back to us.
    Glad it is sorted without calling in an expensive expert.
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