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Recently our thermostatic shower has just been putting out luke-warm water, even when turned on to max. I've increased the combi-boiler water temperature but that doesn't make a different.

I took off the shower tap to expose the thermostatic cartridge and turned it counter-clockwise as far as it'll go, (it was already quite far around,) and now once re-assembled I get cold water when turned to cold and once I rotate the temperature around to hot I get very hot water for about 1 second before it returns to luke-warm again.

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Hot taps elsewhere in the house are hot as expected.

Is this a faulty thermostatic cartridge?

Everything in this house is less than 1 year old, (including the boiler which also new,) so I don't really want to be paying £50 for a new cartridge that I would not expect to go faulty so soon in its life span, so I want to confirm if it's not something else in the house.

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  • May need internal adjustment. Make and model would help. Do you have documents?
    – jay613
    Nov 8, 2022 at 12:15
  • It's this one. Documentation is minimal, sadly. It doesn't even reference that a thermostatic cartridge even exits, so who knows what else is in there. crosswater.co.uk/product/…
    – JonW
    Nov 8, 2022 at 13:04
  • That link doesn't work and none of the showers on that site look like yours.
    – jay613
    Nov 8, 2022 at 13:13
  • @jay613 perhaps it's because it's a UK site and you're in a different location, but the link is OK. Direct link to the PDF spec if that's any better: crosswateruksouth.blob.core.windows.net/web/1/product-files/…
    – JonW
    Nov 8, 2022 at 13:17
  • That works. Assuming you've pushed the button to bypass the safety stop, and it still behaves as you describe ... I think it's failed.
    – jay613
    Nov 8, 2022 at 14:13

2 Answers 2

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It may be faulty.

More subtly a different thermostatic/anti-scald valve may be faulty and feeding cold into the hot line (even while "off")

If you can access the pipes at other valve locations, you may be able to diagnose which other valve is misbehaving by feeling the hot pipe turn colder when you run the shower. That may be easier to notice if you first run hot to the fixture in question.

Or, try closing the shut-off valves to other fixtures and see if the problem is affected by a particular fixture being shut off.

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  • Hmm, I translate this as 'get a plumber in to check it over' which isn't ideal. But you may be right.
    – JonW
    Nov 8, 2022 at 13:17
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    @JonW if it's only 1 year old, look into the warranty - you may get a free replacement. You're correct that something like this shouldn't fail after only 1 year.
    – FreeMan
    Nov 8, 2022 at 13:42
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    Come on Jon W -- you've come to a DIY forum, don't be so quick to call a plumber. You can do this! The advice here is to "feel the pipes" ... you can definitely do that. Changing the cartridge is pretty easy too. Is cold water feeding into the hot supply to this valve, or is this valve always mixing cold regardless of its setting? You can figure that out. Do you even HAVE another thermostatic valve anywherE?
    – jay613
    Nov 8, 2022 at 15:32
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According to the document you shared, your thermostatic valve has an adjustable safety stop. That means the temperature dial cannot be turned higher than a certain temperature, and that temperature is adjustable.

You MAY have a problem with that adjustment. Or your valve may be faulty, or a different one may be faulty ... see @Ecnerwal's answer. But I'll address the adjustment.

Your document says it has an adjustable limit but it doesn't say anything about how to adjust it. If it's adjusted too low that might explain things. It might be (just guessing) something to do with that yellow gear. Maybe you pull it off and replace it in a different position, or maybe it has adjustable bits that I can't see.

On some showers there is a button on the OUTSIDE knob, operated buy the end user, that allows you to override the safety stop and turn the knob to 100% hot. If your shower has that button you can use it to test whether the safety stop is in fact the problem.

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