Boiler pressure rising

[aggy]

We had an engineer out looking at our boiler two days ago. I phoned before hand and said I think theres a problem with the heat exchanger, he came over and saw that the hot water wasn't heating up enough etc and agreed. Anyway, next time the boiler fired up for heating the pressure rose up to 3 until the prv opened. When the heating is off the pressure drops so its not the filling loop.
I've read that it could be the pressure vessel.
Are there any other possible causes before I take the boiler apart to get at the thing?
Is it just conicidence that this problem occurs for the first time since we bought it four years ago. Straight after his visit

 

[Tom1981]

I'd recommend you call a engineer to fix the appliance!

It could be a simple recharge of the expansion vessel. A recharge will pump up the air bladder inside the vessel. A flat vessel will give no room for water expansion as water can not compress into itself thus causing a rise in water pressure to 3 bar.

Depending on which model of boiler you have and how many litres the system holds the pre charge pressure can vary.

 

[aggy]

Thanks but I intend to try and fix it myself.
I know how the pressure vessel should work, I just want to know if there could be anything else causing it?