When the heating system is cool, the pressure should be between 1 and 1.5 bar on the pressure gauge (the indicator needle would usually be in the green section).
If pressure is below 0.5 bar (down in the red section), water has been lost from the system and must be replaced.
Note: The red part of the dial could also indicate the pressure being too high (about 2.75 bar or more).
If the pressure gauge indicates high pressure as a result of over filling, you will need to bleed a radiator until the pressure gauge returns between 1 and 1.5 bar.
If pressure rises by more than 1 bar when the heating is up to temperature then the expansion vessel may require repressurising.
This will require a service engineer.
The pressure in the system will usually require topping up once or twice a year.
If you are having to repressurise your heating system much more frequently, please contact your installer.