Shower Mixer Tap Problems

[TicklyT]

If it's supposed to be a thermostatic mixer, and the hot and cold feeds are reversed it will respond in about the way you describe.

What boiler do you have?
It wouldn't be the first time a combi boiler has been installed, but the bathroom cold feed has been left connected to an old gravity system.

 

[Mikefromlondon]

Indeed, in a similar way many places I have been to where its the opposite, where the hot water comes from a cupboard based tank in a bedroom and no more than about 2 to 3 feet above the bath tub mixer taps, and then the cold water is fed directly, later on the owners go for a mixer shower and find that they can't have a decent hot water shower as the cold water tries to force the hot water back into the hot water tank, even to such an extent so as to make it overflow from the external overflow pipe!

In one installation I had to fit a shower pump to allow the occupants to have a decent shower.