SIME combi boiler: pressure goes up and down

[rajk]

When the central heating is off and the system is cold, the pressure gauge reads 0.5Bars.
When the central heating is on and all rad's are at full temp then the pressure gauge reads 2Bar.

Why does the pressure gauge go up/down?
Expansion vessel need replacing?
Do I need to pump air into the expansion vessel? If so, whats the best way of doing this?

 

[fanboy]

Drain the boiler, foot pump onto shredder on expansion vessel pump in 1bar or just under and open up prv again does pressure on exp vessel drop if so its waterlogged and usually draining it completly and refilling with air does the trick.