flowmax high pressure problem

[fordson1]

I have a flowmax tank heated by grant oil fired boiler with mira showers .
A prv on main supply to house set at 3 bar . hot and cold water pressure at shower is 3 bar when heating is off .
Problem starts when water is hot and the house hot water taps are turned on and off ,the pressure at the shower hot pipe rises to 15 bar ,the pressure releases slowly ,the cold pipe stays 3 bar .
None of the pipework has been changed or modified in the last 20 years
Have any plumbers ever come across this problem and how did you fix it .
I thought someone may have had the same problem in the past

Thankyou

 

[dcawkwell]

Pressure is trapped in the hot pipe and the pipe is being heated.
Not quite sure why this is the situation. My guess is your piping is wrong somewhere in the house.