why dose my worcester 24i rsf combi pressure gauge goes up to 3.5 to 3,8 b?

i have a caravan with a worcester 24i rsf combi.pressure was on 1.5.just put water on for the new season and now it reads between 3.5 to 3.8 bar i have not touched central heating sytem as it is a sealed unit. water is coming out of pipe.like a overflow pipe. has anybody got any thoughts thanks