Shower thermostat knob seizes up

[alankearn]

I have a Bristan Quadrato shower set up
Click on the link below

http://postimg.cc/rDp9FJyv

On the right side of the water control bar is a square knob that controls the water temprature.
When the shower is in use and warm water starts to flow through the shower head the square thermostat knob begins to get harder to turn until it is almost impossible to turn it.
Anyone Know what causes this problem and a way rectifying it.

Thanks

 

[oldbuffer]

1. If it only happens when the water gets hot, could well be thermal expansion making it bind.
2. Probably worth isolating water, taking off thermostatic control knob, removing cartridge, de-scaling / cleaning it, lubricate with silicone grease and re-assemble.