A: If you are on a combi boiler carry out the check below
Regarding the temperature fluctuation or cool running it could be a faulty cartridge however to check this, see if you can get hold of the hot supply pipe before it enters the shower or perhaps as it leaves the boiler.
If when holding this pipe the temperature fluctuates or goes cool then the problem is not with the shower but more likely with the boiler as the shower can only deliver what the boiler sends to it. #
Disregard the temperature at the taps as they have a different flow rate.
If you are not on a combi then the first thing to do is strip down the temperature side of the shower strip the cartridge give it a clean lube and fit a service kit.
Answered 186 days ago | The doc