Cold water in Basin/toilet hot when turning on Bar Thermostatic Shower

[SafeT]

I have a strange problem that has recently arisen.
There is a separate shower with a bar thermostatic shower control, a basin and bath.
When the thermostatic shower is turned on to warm/hot then the entire bathroom including the toilet cold water has very hot water coming out. There is no cold water in the bathroom, all taps are not hot.

This is a recent event as all was working fine for a few years.

I assume it may be the thermostatic shower that is faulty causing a backflow of hot water.
The flat has a boiler and cold water supply from the buildings main tank.

Any advice or suggestions would be appreciated.

 

[Plumbstar Tom]

check the not return valves on the shower

isolate it and undo the nuts behind the bar and you should be able to see them

I suspect the cold one is stuck