combi overpressure

Hi, when heating cuts in, pressure rises to 3 bar and PRV operates.Expansion vessel has been topped up to 1 bar, but problem persists. Noticed if hot water is run same time as heating the pressure does not rise?
Boiler is an Alpha 28CB, 4 years old

Failure of expansion vessel. Lots of threads on this forum covering this.

Thanks for the reply.The CORGI plumber who came today said it was the heat exchanger and has gone off to order one and fit it tomorrow.
I can get the heating to work if I bleed off the pressure when the heating comes on. If I keep it in the green by bleeding a rad whilst it is heating up, when everything is hot the pressure stays in the green. It then works fine. Not very satisfactory, but after 4 days off no heating!!!

Let us know if this replacement works!!!!

Took on board chrishutts advice and also checked other threads.
This morning boiler had no pressure and expansion vessel was flat.
Pumped up expansion vessel to 1 bar, this also brought boiler pressure back in to the green. Heating came on and there was a slight rise in pressure, which I drained off through a rad, heating then ran ok.
Just then plumber phoned to say that the "BITS" had not turned up.
I explained how I got the heating up and running and the flat expansion vessel and he said he would HAVE A WORD and call back.
Ten minutes later he called back and said he had sourced
an exterior expansion vessel which he will now fit on Monday.
We shall see????
l

Had exterior expansion vessel fitted yesterday and everything worked fine untill this morning when I found boiler off due to low pressure.Topped up to 1 bar and it worked fine. When I switched boiler off, pressure went down to zero. Boiler works fine under pressure but as soon as it cools pressure disappears. I have checked PRV and all visible rad joints and have not found any leaks.
Any clues?