crosswater concealed shower mixer not adjusting

[stubru1]

Hi all, got a problem with a cross water concealed shower mixer. Unit is 18 months old and has never adjusted properly.
installation is in a modern flat with an electric mains pressure hot water cylinder. Pressure to cylinder is assumed to be balanced because hot water is produced via flat plate heat exchanger ( comments welcome)
shower valve does not adjust for temperature. Cartridge has been replaced but no change. The unit is of the type where if hot and cold are incorrectly plumbed the cartridge can be rotated 180 deg to correct. This doesn't solve the problem (when reversed unit allows cold water only through)
With cartridge removed flow of both hot and cold water into body is ok.

 

[picasso]

The pressure to the shower sounds like its not balanced, maybe worth getting a plumber to put a pressure reducing valve on the cold supply to the shower (assuming that this is the problem)

 

[stubru1]

My initial thoughts too but the cylinder uses a flat plate heat exchanger so any drop in pressure on the cold side as a result of taking off water to heat for hot should be matched by the drop on the hot side. Plus the hot water is getting through rather than the col. I would have expected the opposite.
prior to refit the shower had a grohe thermostatically controlled mixer unit and worked OK.