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We are in the UK, have a non-power-shower, and a gas combi boiler. The shower's temperature tap constantly needed to be bumped up over a period of months to get a warm shower until it's now at max. It does, weirdly, seem to be stable now and staying hot enough - but it's obviously not right.

The whole setup is only about 5 years old, so apparently has some temperature sensing cleverness in the tap to "protect" us - without which I'm sure we'd have no issues, thanks a lot health and safety!

Hot water temperature at all the taps in the house is great, scalding hot, and the heating works perfectly. The boiler's tap water temperature is set to 60 Celcius, no warnings or blinking lights.

What might the issue be, and how approachable is the inside of the tap? I'm hoping it might as simple as pop the tap off and refit it twisted round a bit!

The tap in question:

enter image description here

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  • First thought is furring of the hot water pipe. Do you have soft water?
    – Chenmunka
    Sep 22, 2022 at 11:19
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    Several things: What is a "non power-shower"? what is that contraption in the pic ? Last, 60 C is pretty dangerously hot, here in the US about 120F is normal, which is 50C. Sep 22, 2022 at 12:17
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    Thanks Crip, but geeze, an electrically powered shower head and water? What could possibly go wrong? (sarcasm intended). Their problem is likely mixing valves that have worn out. I still maintain that 60C is too hot and is dangerous, you can get scalded pretty quickly at that temp. Thanks again for the resp. Sep 22, 2022 at 16:20
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    @GeorgeAnderson As I have learned here, in the UK they commonly have one or more on-demand water heaters. There's often one dedicated to the shower and it's often called an "electric shower". That term is fairly unfamiliar to us in the US and causes a chill (or shock) down the spine to us, but it's quite common elsewhere. The "contraption in the pic" is the tub/shower faucet - turn the knobs on the ends to get water flowing. Again, unusual design in the US, but common elsewhere.
    – FreeMan
    Sep 23, 2022 at 14:55
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    ...And they commonly use a system that makes much sense, but seems rare in the USA of one knob to set the temperature, and another to adjust the flow volume. The temperature knob has some thermostatic valve wizardry built in (so it should hold temperature as set, regardless of incoming water temperature) rather than the US common arrangement of one control that either has no flow volume setting, or combines the two, and the only thermostatic feature is anti-scald, but the valve will not hold temperature as set if pressure or temperature coming in change.
    – Ecnerwal
    Sep 23, 2022 at 15:40

2 Answers 2

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There are two filters in this kind of shower, one on the hot and one on the cold side. The hot ones often fur up faster with limescale - you can see in the image that limescale build up prefers the hot pipe - and if this is blocked the tap may struggle to maintain heat.

It may be possible on some models to pull the temperature tap off, and with some fiddling of the innards reattach it "unwound" to some degree, to essentially allow it to be twisted further; even where possible this isn't recommended, and results cannot be guaranteed.

In the end, the thermostatic valve in the unit is not working. Sometimes replacements for the valve alone can be found, though that valve makes up the bulk of the cost of the unit and so complete replacement is the most straightforward way if more expensive.

In the good old days the temperature control simply allowed more or less hot or cold water in, simple moving parts and pipes - now that they have the complexity of the thermostatic valve £200-300 for a unit is considered "cheap" (!!!?!) with £500+ more normal. Theoretically, a more expensive unit should last longer. YMMV.

Turning the tap temperature on the combi boiler down will certainly not hurt the unit at all, and who knows - it could aid longevity, and will save some money on your bills.

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  • I don't like to answer my own question, looks like rep-farming, but I was with the plumber while he took bits off and sucked his teeth, and this answer is basically the findings from our conversations. Sep 26, 2022 at 14:08
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    Self answers are perfectly acceptable here and nobody will look down on you for doing so. It's good that you've come back to close this out. Thanks!
    – FreeMan
    Sep 26, 2022 at 16:22
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Not form UK but guessing: Power-Shower is a shower fed from loft "open vent" hot tank not delivered by gravity only but also by a little pump "sucking" from the open vented tank delivering more pressure.

Your mixer seem to be a (worn) thermostatic mixer valve, I suggest you to replace it and to add a polyphosphate dosing device before your combi (on DHW only, not on the heating flow), doing so much less scale will deposit and he new tap will last much longer.

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  • A power shower is an on demand electrically heated shower, sometimes pumped and sometimes gravity fed. That's an interesting tip about adding the dosing device. Is DHW "demand hot water?" Does the boiler have separate inputs for hot water and heating? That never occurred to me! Sep 27, 2022 at 6:50
  • That's an electric shower, DHW is domestic hot water. Yes, combi usually has two distinct circuits (primary, passing trough the main exchanger) and secondary (where DHW is made), DHW is usually made trough a plate heat exchanger.
    – DDS
    Sep 27, 2022 at 7:20
  • @GrimmTheOpiner electric power showers are generally either pumped or mains pressure. To be gravity fed you'd need your header tank very high up to give the minimum required pressure. @ DDS I have a pumped shower fed from an open vent tank as well as a power shower - and mentions of a power shower here seem to confuse non-UK people
    – Chris H
    Sep 27, 2022 at 10:07

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