Hot shower going cold for short periods.

[Clint B]

Morning, hope there is someone to help with this issue. I have a client who is experiencing issues with her shower valve. MAnual only not thermostatic. It is admittedly one at the cheaper end of the market. But only manual as I say. Whilst showering she experiences periods of cold/cool water during the shower. She assures me there is no other appliance on or anyone else in the house. She has an unvented hot water cylinder and I’m a bit perplexed what the issue can be. Any suggestions please ?

 

[Ben-gee]

Is the cold feed taken from the balanced supply? (i.e. after/from the combination valve for the unvented)