potterton promax combi 28 he plus tap hot water changing

[stephan1212]

hello, I have a problem with the hot water to taps and shower, the hot water is running well for a few seconds, than a cold water is coming and after that hot water again. That is all the time. I am not a plumber but i think the issue is caused because the boiler stop working for a second

 

[AGAS]

you need an engineer to probably change the plate hex but could be other issues hence the need of an engineer