Combi Boiler - No hot water if heating on

[UpTheIron]

One for the PH plumbers please... GF has a combi system - all appears to be working fine however if the heating is on the hot water is no longer heated, resulting in a cold shower. If the heating is off, the boiler works fine.

Diverter valve perhaps?

I'm familiar with conventional systems but not combi, so any help appreciated.

[Deva Link]

Make and model of the boiler might be useful.

It does sound like the diverter valve though. Fiddly to replace. If you don't have a pet gas fitter then you might find the manufacturer has a fixed price repair service or you could even use British Gas's similar facility.