Persistent problem with thermostatic shower

Question

Hello, 

I have an exposed thermostatic shower mixer which has been cycling hot and cold, I was told to replace the shower but the new one has the same issue.

The shower isgoes cold and hot after several minutes of working fine, and then starts to cycle. The previous shower worked fine for 8 years before the problem started, the combi boiler, also 8 years old and otherwise working fine, has been checked and the shower valves have been checked for debris. 

I am at a loss to what or where the problem lies. It’s a flat and the water pressure is low as is the flow rate and always has been, and I don’t believe this has changed recently but I can’t say for certain. 

Any ideas?!

Thanks
Rita

Arbo And · Accepted Answer

Hi Rita,

From what you’ve described, there are a few possible causes, particularly given that the shower works correctly for several minutes before beginning to cycle between hot and cold.

As the shower mixer has already been replaced and the valves checked for debris, it would be worth checking the hot and cold supplies under operating conditions, including the flow rates and pressures, as well as how the boiler is responding when the shower is running. A restriction, pressure/flow imbalance or issue with the hot-water supply could potentially cause this behaviour.

As the system is around 8 years old and you’ve mentioned that the pressure and flow have always been relatively low, I’d recommend having a plumber assess the installation rather than replacing further components without diagnosing the underlying cause.

Tom · Answer

Hi Rita,

I’ve had this issue before and had to rule out all of the various issues the other tradespeople have pointed out. One of the easiest ways I’ve found to identify if it’s the shower valve itself OR the boiler is to shut the water off, remove the shower mixer bar from the wall and flip it over so the hot / cold water feeds are reversed. Then refit it in effect upside down. Turn the water back on and try the shower. If it still happens then it’s likely to be the heat exchanger / flow rate issue as suggested on the combi. Good luck.

Darryl · Answer

Possibly piped up wrong. Check the hot pipe coming to the shower is on the left and cold on right

Ashar · Answer

Hi

It’s your boiler, have the heat exchange checked.

Darren · Answer

Hi Rita there is a very good chance the hot and cold feeds are the wrong way around which would cause this to happen

Nilan · Answer

Since two different thermostatic valves have shown the exact same fault, you can rule out the shower itself — the problem is upstream, most likely the combi.
Most likely cause: boiler flow rate / heat exchanger
With a low pressure/flow system, an 8-year-old combi's plate heat exchanger is a prime suspect for partial scaling or debris fouling — even on a "working fine" boiler. As it heats up during a shower run, restricted flow through the exchanger lets the water overheat faster than the boiler's modulation can respond to, so it overshoots, the boiler backs off or the flow drops below its minimum firing rate, and you get the classic hot/cold hunting. A genuinely good TMV cartridge can't fully correct for that because the temperature swing is happening at source, not at the mixer.
Or maybe you need a low pressure exposed shower valve

John · Answer

Heat exchanger in combi boiler either needs cleaning or renewing hope this helps

Mark · Answer

Hi Rita ,this really does sound like the hot/cold pipes have been crossed.The shower will actually work for a short time this way,but long term the thermostat flutters.Put two isolations valves on the feeding pipes and isolate the cold,if it then runs constantly hot, there’s your problem.
Regards
Mark

Lee · Answer

Have you checked the diverted valve it might be sticking

Persistent problem with thermostatic shower

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