This is a challenge for you plumbers. I've spent hours reading forums and looking at advice and have tried all of the proposed fixes, none of which have worked.
Up until three weeks ago, my concealed shower mixer valve worked flawlessly, no temperature or flow issues at all.
Assuming it was the cartridge, I replaced it with a new one of the exact same dimensions: Crosswater GP0000250. This did not solve the problem.
I replaced the non-return valves in the brass valve body with like-for-like new ones, no effect. While I had these out, I checked the access point for the water from the hot feed chamber where the hot NRV is, where it enters the thermostatic mixer chamber, no debris, limescale or obstructions.
On the back of the screwcap (access for NRV) there is a mesh filter that covers the channel for the hot water into the mixer chamber. I descaled what little limescale there was and refitted. No effect. I tried it with the mesh removed, no effect.
The pressure on the Worcester Bosch boiler is constant at 1.5 bars, as it has always been.
The hot water to the bath and sink taps is blisteringly hot.
The hot feed to the shower is plumbed correctly to the hot side of the valve. When the shower is running, it is lukewarm despite the entire hot feed pipe, elbow and NRV chamber of the valve getting blisteringly hot to touch. Hot water is getting past the NRV as the valve body is hot about 3 inches past where the NRV is. The hot water just doesn't seem to be getting into the mixer chamber.
NRV's are working as when I had them removed and ran the bath and sink taps, I couldn't get them hot. When I fitted the new NRV's, the hot to the bath and sink worked perfectly.
There has been no work carried out recently on the mains supply, so no change in pressure.
Nothing else has changed with the physical installation of the shower and plumbing. It worked perfectly for 10+ years and only started to get cooler a few weeks ago. We had to keep turning the temperature nob anticlockwise to maintain a bearably warm shower temperature. Eventually it was hard over to the hot side and still lukewarm.
No leaks.
Is there something else I'm missing here? Would new valve sort it? I don't understand how when I've replaced all of the moving parts associated with the temperature and there are no issues with the valve body itself.