Thermostatic Shower Mixer Bar - Erratic Performance

29 Mar 2010
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United Kingdom
Recently installed a thermostatic mixer bar and cannot get it to operate properly.

The water will run at a good temperature for 10-15 seconds but will then be interrupted by a colder burst for 3-4 seconds, return to the original temperature then repeat this again and again.

The cold feed is direct from the mains, the hot comes from a recently serviced gas board installed combi.

I've replaced the cartridge inside the mixer, then replaced the entire shower control (with another new cartridge too). With each new cartridge the water still behaves the same (hot then colder) but the difference in temperature of the water is more extreme.

There are four filters around the mixer bar, all are clear.

The property already has a pressure reducing valve fitted to the cold mains (set to 3.5 bar). I got a reading at the shower of 3 bar on both the cold and hot side (does this sound reasonable for a Combi to achieve 3 bar of water pressure?)

Any ideas what I should try next?

Much appreciated.
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Thanks Muggles.

I forgot to say in the original post, confirmed that hot and cold both installed on correct sides.
is the boiler constantly running or cutting out when shower running ?
is the hot pipe to shower staying hot or hot/cold ?
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Thanks Seco,

Never thought to check boiler behaviour, will do tomorrow.

I'll check the hot water pipe too.
either run the shower and hold the hot pipe see if it cycles hot/cold.

then watch the boiler see if it's cutting in/out.

Finally got round to having another look at the shower over the Easter weekend.

Boiler ran fine, never missed a beat, while the shower was running.

Hot pipe seemed to keep a constant temp too.

Really stumped by this one
double check the hot/cold connections are the right way round, read the instructions that came with it.

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