Boiler pressure gets too high & cuts out with heating but hot water is fine!

[Rose13201]

Hi!
I have a 9 year old Baxi combi boiler. When I turn the central heating on, the boiler pressure is getting so high that it keeps cutting out completely. It's absolutely fine when I run any hot water though. Does anybody know what the problem is likely to be please?
Thank you

 

[Chuck]

Does anybody know what the problem is likely to be please?

There is an expansion vessel that is supposed to prevent this behaviour. It is evidently not doing its job and needs attention. There'll be a pressure relief valve that needs checking at the same time. Should be a straightforward fix for a heating engineer.