Mira Excel thermostatic shower valve poor pressure and temperature

[pork_scratching]

I have 2 Mira Excel showers installed in separate rooms on a low pressure gravity fed system. One works fine and produces fairly good pressure and good temperature at a low setting (around 2). The other has had poor pressure and requires the controller to be set to near maximum to get a good temperature. They are both T-ed off the same hot and cold supply pipes/shutoff valves and are within a few feet of each other so only have very short sections of separate pipes. After replacing the hose and shower head, I then assumed it was the valve so got a new 903.33 cartridge set to low pressure and replaced it. The symptoms are still exactly the same. The pressure doesn't noticeably change for any adjustment of temperature. The cartridge is installed with hot on the left as was the previous cartridge.

Any ideas on next steps. All I can think is that either of the short section of separate hot/cold pipe is constricted or the new valve is also faulty. However I would expect that the pressure would change for different temperature settings if there was an imbalance in supply pressures/volumes.
Any ideas/next steps to take?

 

[Johntheo5]

Are you saying that the flowrate (with any combination of hot&cold) is very poor even if you have the flow adjustmet to full?.

 

[pork_scratching]

Yes the flow rate is not as good as the other shower on any temperature setting - but the one with good flow gives hot water at a setting of 2 and the one with poor flow doesn't provide hot water until nearly the maximum setting.

 

[Johntheo5]

That might indicate a blockage of the hot supply, are there any strainers at the shower hot&cold inlets, there should also be non return valves, if the other shower has always been better maybe someone has/had removed the NR valves.
Have you removed and checked the two short sections of piping?.

 

[pork_scratching]

I can't see any strainers other than the ones on the cartridge or any non return valves. You can push a bottle brush into both sides of the cartridge housing with no immediate obstructions. I haven't removed the piping, its all soldered copper and partially in a wall, so I may need a plumber for that. I suppose it be to do with the pipe from the cartridge output and the wall mounted hose fitting. Could the temperature setting be a red herring and one has just had the maximum temperature adjusted?

 

[Johntheo5]

Any difference in flow if you remove the hose??, some showers also have a restrictor installed around where the hose connection is but even if installed couldn't imagine that having much effect on a gravity fed shower as these are installed to reduce a mains or pumper shower flowrate to ~ 6LPM.

 

[pork_scratching]

I think I'll have to do some more investigation and measure and compare the flow rates of both showers with the hoses removed. I'll also try changing the maximum temperature setting on the new cartridge.