Shower mixer failure

7 May 2021
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United Kingdom
After replacing the boiler to a combi from a system the shower mixer no longer works correctly. I assume this is due to now having mains pressure from the hot side but essentially the mixer is always ON. Occasionally the internals seem to move enough to provide an OFF setting but it does not last long.

The shower is OK as we have a shower head with a switch, however, the cold water seems to be crossing through the mixer to affect all the hot taps around the house and has reduced the temp from these taps.

My questions are:

Am I correct in my assumptions of the cold water crossing the mixer?
Is there a quick and cheap solution?
If I replace the mixer, which one will fit?

We are going to get the shower updated ASAP as we have recently moved in but lead times for bathrooms are in the months at the moment so really need a solution before.

Pics attached to help.

Any help would be great :)
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yes, the cause is very likely a worn out shower mixer with internal leakage.

measure the distance between the centres of the pipes coming out of the wall. From memory, the standard distance is xxxx mm (no doubt someone will correct me if I am wrong) edit - corrected - 150mm

as yours is a surface mixer, it is very easy to change if a standard distance.

Modern fitting practice includes a sort of concealed bracket for the pipe connections to hold them tight on the wall. Old showers may not have that.

Of the surface mixers, I think the best is the Aqualisa round-bodied. It is the design they built their reputation on. They are rather expensive. Repair kits are available but should not be needed for 10-20 years.

Bar mixers are cheaper, but IMO not as good, and don't last as long.
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Thanks for the reply and detail, I have zero knowledge of shower mixers so good to know I was on the right lines.

As with everything on an old house a simple replacement is a bigger job. If you have a 180mm centre replacement part what sort of price are you looking for?

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