I have a mixer shower and the water runs intermittently completely cold for approx 10 seconds approx every 2 mins. No other taps in my apartment do this, but this is the only outlet controlled by a thermostatic valve. All my hot water comes from a combi boiler.

I have already tried changing the thermostatic value and this did not resolve the issue. I have done some tests and I have discovered my boiler goes off briefly just before the temperate drop, it doesn't do this for any other water outlet.

Any idea what the problem is and how I would fix it?

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3 Answers 3


You seem to be describing an anti-scalding device that is malfunctioning. This could either be a thermostatic valve (as you mention) or a pressure balance valve (another type of anti-scald device).

They have been required by code for many years and while I cannot specifically tell if your shower valve has an anti-scald device in it, given that it looks newer it likely has one.

You should see some sort of manufacturer name on the shower valve. Just Google every single word you see on your shower valve. Also try "Google Image Search" and flip through the images until you see something that looks like your valve.

Googling will help you find the specific valve and see if you can replace the internal anti-scald device. These are often accessible from the front so if you're lucky, you will not need to break through the wall.

  • 1
    This shower valve is completely external; there is nothing behind the tile wall except piping. The valve cannot leak inside the wall and replacement of the valve does not require working inside the wall. There is a standard center to center spacing between the hot and cold pipes (15 cm I think) and the fittings on the back allow adjustments to line up. A well thought out system. These seem to be much more commonly used in Europe than in the US. Mar 14, 2021 at 17:14
  • The brand is Bristan. My specific model doesn't seem to exist anymore. So do I just need to replace the whole mixer unit?
    – Dan
    Mar 14, 2021 at 17:46
  • Even if that model isn't available anymore spare parts may be. There msy be an adjustment in the thermal mixer than will solve the problem. Mar 14, 2021 at 18:04

Inside the cylindrical mixing valve - where the shower hose exits - are 2 back flow preventers. They can fail intermittently or permanently, or with any pattern in between depending on the pressure both of the cold and warm water net. Those parts are maintainance parts and should still be available.

Normally the warm side back flow preventer fails first due to higher temperature distress to the membrane. That way the cold water - which has a slightly higher pressure - can no more be stopped from invading the warm water net. When the warm water is being heated up, its pressure increases which can temporarily stop the invasion. The rubber membrane could have a tiny hole which only opens at a certain pressure difference.

This is why it depends on other faucets being used at the same time, inclusive washing and dishwasher machines.

Thermostatic valves directly connect the warm with the cold water - only separated by and relying on 2 working back flow preventers.


One of the two heating elements in your water heater may be failing.

  • 2
    That's not consistent with the other hot water taps working just fine... Mar 14, 2021 at 15:56

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