Shower heat problems

8 Dec 2023
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United Kingdom
Customer has a gas boiler. Shower only gives out lukewarm water. All taps give out hot water. Have replaced the thermostatic mixer valve previously & the shower then worked perfectly. however over a period of weeks to months it gradually dropped in temperature back to where it is now with it only giving out lukewarm water even with the valve on fully hot. Any ideas?
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What type of gas boiler? Make and model?
Remove the head it maybe clogged also check the flex if it has one they can block internally, you should have a large bore flex if it has one not the ones for electric showers
how about the logical route to determine the issue?
Water is hot at other outlets therefore not a boiler issue.
Water temp has reduced at the pipe run from the boiler through uninsulated pipes over a distance of 1 mile? No! then it must be that the water is being reduced in temperature by another event...
what will reduce water temp? well if there are no external events occurring then it can only be colder water is being mixed...
where can this cold water be being mixed? the mixer...
so either cold water pressure is preventing the ingress of hot water or there is a restriction in the hot water feed pipe and creating this differential once again.
If these factors are dismissed then it remains as a logical conclusion that the mixer valve to be the only possible explanation

There is one other possibility that hypothetically can contribute...quantum entanglement...:)
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Scale in head increases back pressure reducing hot flow , try head descale.
If your boiler is a combi then reduced flow from a blocked head or flex will cause overheating in the boiler to cause it to modulate down or even shut off and cycle off/on = reduced shower temp