Glow worm 30sxi reaches 3.5 bar ?

I've changed the PRV and pressurised the system back up it fires up and runs but when the timer stops the over run fan keeps going.
I have to turn off the boiler to stop it and when I do the pressure has dropped to 0.0 but when the boiler is running the pressure reaches 3.5 bar and the PRV is dumping the water as it should what is its problem ???