fluctuating/pulsating water flow on mixer shower

24 Dec 2008
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United Kingdom
I’ve just had a thermostatic mixer shower fitted, taking hot water from a Combi boiler.

When I turn on the shower the water flow is constant but as it heats up and once it’s warm the flow of water starts to fluctuate (pump), high pressure, low pressure, high pressure, low pressure, etc, it changes every second or even less than second.

When I run hot water in the kitchen or in bathroom bath or sink then the water flow appears constant.

Any ideas what could be causing this / how to fix it?


P.s. may be the boiler is NOT A COMBI boiler ! it is "SERVOWARM ELITE 50". there is hot water tank up in one of the bedrooms, and another cold water one in the loft.

maybe this is happening because the pressure of the hot and cold water is different.( I can stop the flow of hot water just by pressing my finger to the tap). do you think I could sort the problem by fitting pressure regulator at the bath/sower mixer tap.
what that this mean "Hot and cold supply pressure should be balanced"? It is from hansgrohe web site.
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Please state boiler make and model, also shower make and model.

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