How can i put pressure in expansion vessel on vokera 20/80 rs flomatic?

I have a problem with pressure on my pressure gauge when my central heating is on the pressure goes to just comes under 3bar, if i raise the heating thermostat up the pressure goes beyond 3 bar and water drips from safety valve. The pressure gauge is normal at 1 to 1.5 bar when i am using d.hw the pressure remains same , it goes up when when my heating on. any suggestion whats wrong & how can i fix it.and also do i need to drain my boiler & heating system completely to put pressure in expansion vessel using foot pump .