Thermostatic mixer valve faulty - cold supply going to hot

[fezster]

A few days ago I needed to turn the hot supply off to change a tap valve and noticed that even with the hot supply from unvented switched off, water was still coming out of the hot tap (albeit at a reduced rate). I've done this previously and the hot would always trickle to a few drops after a few seconds.

I've now narrowed this down to one of the thermostatic mixer showers we have - if I isolate it, the hot shuts off as expected to all taps. I've read threads previously where something similar happened and the solution was to replace the thermostatic cartridge. Given it's concealed behind a wall, I'd like to understand if this is the solution before I call someone in to replace it. The cartridge alone is £55 and I can imagine it'll be a bit of a job as I remember the installer plastering behind the metal plate hiding the valve (!).

Is the cartridge likely to be the culprit? Are there not non-return valves fitted precisely to stop this? The shower valve is a Pura Bloque listed here:

https://www.ukbathroomstore.co.uk/p...cealed-valve?gclid=CP_E1sXbptACFWYq0wodfQIN7A

 

[ianmcd]

This is who you need , I know you are looking for a cheap fix but it will just cost you more money in the end

https://www.showerdoc.com/

 

[fezster]

I read that site. They can supply a new theromostatic cartridge. I'm not averse to spending money to fix it - just want to understand I've identified the right component. As well as the cold entering the hot supply, the shower itself is only operating at a lukewarm temperature. Previously it has operated just fine. I've tried adjusting the temperature control to full hot and cold several times to see if that would free it up. I'm wondering now whether there's 2 problems - firstly the thermostatic cartride, and secondly the the non return valve on the hot has failed.

Luckily it's not our main shower so no immediate need to be fixed.

 

[fezster]

Well quick update - as expected, the shower valve had built in check valves. The hot was obviously had some debris in it (looked like solder). Took the thermostatic cartridge out and flushed it through. Problem solved