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Thermostatic shower mixer valve won't run hot

Joined
28 Apr 2023
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United Kingdom
Hi all. We live in a hard water area and every few years I take the thermostatic valve out of the en suite shower and use a limescale cleaner on it. This year, when I put the cartridge back, it wouldn't mix in the hot water at all. Presuming I'd damaged the cartridge, I bought a replacement from DiVapor but it's no different and I can't get hot water out of the shower any more.

If I isolate the cold feed and don't insert the cartridge, hot water pours out of the mixer. If I then insert the cartridge, and leave the cold water isolated, no hot water comes out of the shower heads.

The mixer is a Bathstore triple control vertical thermo shower valve, similar to this dual one:

I picked up a spare on ebay a few years ago, so if its the mixer itself (how?!) rather than my cackhandedness, I could probably do a like for like swap.

The hot water is fed from the cylinder on the floor below, heated by a Grandee oil fired boiler.
 

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Possibly Misaligning marks on cartridge when fitting
 

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