Cold water backflow to hot water pipe

You can fit double check valves that stop backflow

It sounds like the cold water feed is not at equal pressure with the hot water feed. This is normally caused by a poor installation of a shower as the hot and cold to a shower (if tanked hot water) should be equal pressure. If its a shower run from a combination boiler the chances are the thermostatic cartridge in the shower is faulty.

To provide the answer for the fix one would need to know the type of system installed in the customers home. The systems that exist are: Open vented (low pressure system sometimes referred to as gravity comprising hot water cylinder in airing cupboard and cold water tank in roof space). Combination boiler. Unvented hot water cylinder. Thermal store (not that common). The previous answer given sounds like customer has a combination boiler but to provide further advice on fix I would need to know more about the system, the shower make, model etc.

Firstly I'm not a boiler engineer but it strikes me as very odd for backflow from cold to hot being the issue here. I would suggest the pipework to the shower is a fair distance away from the heat source and therefore takes awhile for warm water to reach the shower outlet. Having said that, the previous answer, check valve installation, is a good answer if backflow is the cause.