Flow problem
The gears and clutch arrangement could be slipping (parts 15 16) remove the gearbox cover (part 11) and make sure that the parts are all moving properly if not the fault could be the gear and clutch arrangement need to be replaced or the cartridges (parts 18) are sticking if all of these are OK then you probably need a new thermostatic cartridge.
www.showerdoc.com/trevi-therm-built-in-post-1997-shower-spares
www.showerdoc.com/ideal-standard-wittlich-3-4-thermostatic-cartridge-d-08-s960134nu
Answered 5 years ago by George Thomson
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Trevi Therm Built In - Post 1997 Shower Spares
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