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Discuss Thermostatic mixer shower in the Valves & Taps area at

  1. Ric2013

    Ric2013 Plumber

    Hi there,

    Looking to install a thermostatic mixer shower in a flat with night rate unvented cylinder. It's an almost like-for like replacement for a thermostatic bath fill/shower diverter which has a broken thermostatic cartridge.

    During the day, hot water temperatures can fall to 45°C, or perhaps the thermostat is set too low, but the customer seems to like it like that and seems to have paid plumbers to set the temperature so I don't want to interfere, not that I want to touch anything unvented anyway. In any case, my grandmother used to keep her water at this temperature and she never caught anything from it :) so I'm not especially worried.

    It concerns me that most thermostatic shower manufacturers seem to insist on water temperature being 10°C higher than the blended water temperature. A non-thermostatic mixer would be ideal, but I suspect the customer has a touch of dementia so I'm not giving him that option.

    I find manufacturer's instructions can sometimes be taken with a pinch of salt and I'm hoping the unit would still work, even if it doesn't work perfectly outside its comfort zone.

    So what will happen? If the water temperature drops to 45°C, can he still get a 42°C shower, or is that not an option?

    Cheers for any experience on this.
  2. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Have you tested the water temp coming out the tap as I find there can be as much as 10 dc difference
    • Like Like x 1
  3. Ric2013

    Ric2013 Plumber

    Yes. His hot water was at 43°C, but the blended water was, as you say, about 10°C lower (not measured, but it was lukewarm). The thermostatic knob was jammed and he'd tried to turn 'the cold tap' (the temperature control) down to minimum, so obviously it wasn't working as it should anyway.
  4. Harvest Fields

    Harvest Fields Trusted Plumber GSR Top Contributor!!

    Why not just change the cartridge on the old one?
    • Agree Agree x 1
  5. Ric2013

    Ric2013 Plumber

    The old one is a real cheapie and a cartridge costs more than a new mixer. Also he has a bath fill he can't use because the door is broken on the walk-in bath so it can only really be used as a shower. And he doesn't like the bath/shower diverter. Also, he can't grip the controls easily, so may as well change it. It has the 90° elbow things coming up through the bath already so should be reasonably straightforward.
    • Like Like x 1
  6. Ric2013

    Ric2013 Plumber

    So, in your experience, a thermostatic mixer will always deliver water 10°C lower than the hot feed to it?
  7. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Yea can be less also same with the cylinder stat there not 100% accurate that's why you need to check the hot tap
  8. Ric2013

    Ric2013 Plumber

    I have checked the hot tap in the bathroom and it is hot enough for a shower (43°C measured reading), as is the hot tail of the mixer. What the customer wants is to be able to have a shower at that temperature, but his mixer is blending the water down to (at a guess) 35°C.

    I am hoping a thermostatic mixer, with the temperature control set to 46°C would, under these conditions, deliver a shower within a couple of degrees of the measured hot tap water temperature, instead of mixing in cold for no purpose.

    I'm not quite sure what you mean, sorry Shaun.

    Do you mean that even with the hot water at 43°C, and the mixer set to 46°C, it is unlikely to be possible to take a shower with the shower water coming out at 'around' 43°C? Because, if so, he needs a manual, not a thermostatic shower.
  9. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    I would say it would struggle best option is to boost the store hot water upto 60

    And have a tmv on the taps down to 45

    And have the shower before the tmv
  10. Ric2013

    Ric2013 Plumber

    Of course. If I fit TMVs, they would be at point of use 'thus keeping the pipe runs at a sanitary temperature' lol.

    Trouble is we've just turned a £120-£190 job into a £300 job by doing that, and that's probably undercharging. And I don't think he has £300.

    Plus I can't adjust the immersion thermostats on an unvented cylinder.

    Guess I'll have to decline the job then. Bugger!

    Thanks anyway, Shaun - at least I didn't turn up and fit a new mixer only for it not to work. Could have been embarrassing.
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