Reverse flow

[RightShower]

Gravity feed shower with manual control installed for 9 yrs. Was working OK except for poor pressure. Due to only 4' head of water.
Cold water to shower on 15mm independent feed from cold tank in loft. Hot water 15mm feed teed off main supply pipe at top of hot water tank.
Problem - Periodically, when hot taps are turned on in rest of house, cold water seems to flow back from the tank. Past the control valve, entering the hot water system. Causing the hot water tap water to go luke warm.
I was thinking a non return valve in the hot feed to the shower control may be the answer. But do not know if the shower would still work due to low pressure/head.

Any suggestions?

 

[doitall]

Its not the shower you should be looking at, but a mixer tap with a mains cold supply, first place to look is the kitchen taps followed by the washing machine.

You then should have check valves (water regs) to prevent backflow on the hot and cold.