Shower temperature fluctuations - hot cold hot cold

11 Jan 2018
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United Kingdom
I have a shower with a thermostatic valve driven by a combi boiler. Its worked fine for a few years. Recently it has had temperature and flow rate fluctuations. I hope someone can advise me. More detail follows (look away now if you're not interested):
  • When I turn on the shower it has a reasonable flow rate while it warms up, but when it gets warm the flow rate reduces significantly,
  • It then goes cold, and the flow rate improves slowly. It runs cold for about a minute before warming again. Then the whole cycle repeats itself,
  • The hot water burners in the combi boiler cycle (switching off and then on again) during this process,
  • The other hot taps in the house work fine and consistently with good flow rates and do not produce the same cycling effect in the combi boiler,
  • I had the combi boiler serviced last month and there are no faults,
  • Removing the shower head makes no difference to flow rate or temperature fluctuations.
I assume that the hot water flow rate is being restricted by the shower's thermostatic valve as it warms up, causing the combi boiler to shut down until the thermostatic valve cools down and the flow rate returns. But how do I test and fix this? I think I need to replace the thermostatic cartridge, but are there other possible causes? Thank you in advance for any advice.
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what is the temperature of the hot water when you run it from the basin tap? If it does the same then it is most likely a boiler problem. If it is okay at the tap then it is the shower that is faulty
Thanks snb. basin tap is very hot and runs without going cold. So I'm fairly convinced its a problem with the shower thermostatic valve, but I'd love to be able to test this without removing it because the stainless cover/panel that needs to be removed to extract the thermostatic valve is glued to the tiles - so one of them is likely to get damaged in the process.
Try turning the boiler dhw temperature down to the lowest setting. See what happens then. Had the water temperature been increased recently?
Sounds like the boiler is cutting out either due to insufficient flow when the mixer reduces the protein of hot, or overheating due to not enough heat getting to the tap water. Since the other taps are fine that points to the mixer slowing it down more.

The other option is due to you saying the flow reduces when it gets cold. Perhaps the cold is blocked. Is the valve open properly

I'm not a heating engineer
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Thank you John, I will try/check those suggestions and see how I get on.

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