Intermittent hot water

20 Aug 2007
Reaction score
United Kingdom
My hot water system has developed a strange fault. We have a pressurised system with no cold water tank i.e all taps fed mains cold water.

When using the hot water taps (all baths sinks and showers) in the upstairs rooms we get a few seconds of hot water and then it runs to a tepid temperature. Meanwhile the taps downstrairs run hot with no problems. The boiler is obviously heating the water and it is getting to the upstairs taps as they do run hit for a few seconds but why are they not staying hot?

Any help greatly appreciated, please do not subject me to another cold shower :)
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The shower or bath mixer (or tap) upstairs is letting cold water go across to the hot, internally.
First thing, wiggle any temperature controls from end to end several times to clear limescale.
Otherwse, new cartridge, fit non return valves, or new mixer.
Thanks Chris,

Your idea makes sense in terms of cold water crossing into the hot supply however what still confuses me is why this has happend to 5 sets of mixer taps and 2 showers in one day. Could your theory be therefore happening further up the chain, before it gets to the taps and if so where should I be looking?


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