Usually the pressure is set by adding water via the filling loop - up to the spec for the boiler 1-1.5 bar usually. Then as the water heats up the pressure will increase slightly.
Is you boiler pushing water through the overflow? When the boiler pressure gets too much it should vent through this pipe (usually outside) to avoid melt down. If it is venting through this pipe you have a problem - could be the pressure vessel.
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