Mira Coda bar shower - pressure problems at hot temperatures

[SlightlyMadScientist]

Today I replaced a very tight flow cartridge on a pre-2008 coda shower. Old valve was very seized and only partially open. That’s now working well.
However, there is only flow from the shower head at very cold temperatures or when the shower head is held below the level of the shower bar.
As I increase the temperature pressure drops considerably. And at hot temperatures water is both very low pressure seems to pulse rhythmically. This feels odd.
Pressure is low out of the hose if I remove the head.

All water is reinstated - cold water header tank is full.
Hot water is reinstated and the water pressure from the bath/sink taps is as I would expect it
Hot water is reaching the bar shower as the body is getting hot, although it was slow to do so. I was surprised.

What do I look at next?
- thermostatic flow valve? This is almost the cost of a new shower? Worth replacing? Or descaling? If I can get it out. Though I can probably fit a new valve myself I am not sure about a new shower unless it will go onto existing fittings. That may be beyond my competence.
- filters and non return valves in the inlet? I don’t want to take the bar off if I can help it as the way it was originally fitted it would require breaking otherwise good silicon sealing. But I will if I have to.
- replacing the bar completely? I would like to add a shower pump at some point in the future. I just wasn’t planning to do so right now.

 

[polesapart]

- filters and non return valves in the inlet?

Are the first things to check. And while the mixer's off the wall flush the supply pipes.

 

[SlightlyMadScientist]

Thank you
I do know that’s the cheapest potential fix, but not what I wanted to hear in another sense.
I’ll strip it all down at the weekend.

Thank you for the advice. Watch this space.