i have a Trevi Boost built in shower mixer and cannot get the hot water to flow i have replaced 2 cartridges 2gears for clutch and booster cartridge when i stripped it down testing hot water flows hot and cold flows also but assembling it and its just luke warm The boiler is Potterton Suprima 30-80 and mains fed to shower
If you have a Suprima, this would indicate the hot water is fed via the cylinder, if you are trying to couple that with mains pressure cold, it's no suprise it won't get hot!
You will have to tank feed the cold from the same supply as the hot water to balance the supplies.
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