Fluctuating boiler pressure

[Bogdan Cosacenco]

i have been having a problem with my Vaillant Ecotec plus combi boiler for a while. The pressure in the system is normal to 0.8 bars, i put the heating on, the temperature in the radiators at 50 C, then, in 30 min, the pressure goes up above 3 bars and the pressure relief valve goes off. once the radiators cool down, the pressure in the system is about 0.2-0.3 bars, and the boiler gives me the (not the finger) F22 fault. i put pressure back up and the circle starts again. I found that if I leave the radiators at 43-44 C and pressure at 0.5 is works, but the house is not heating up above 18 C.
What can I do? would an expansion vessel do the trick?
Thanks

 

[Cgas]

Yes the expansion vessel reqs pumping up/checking pipe not blocked and PRV replacing but at a guess as you've not given exact model the vessel is inside the room sealed case and you should seek the advice of an RGI to sort.

Edit.