You probably need to charge the pressure vessle within the boiler. This absorbs the expansion of the water from cold to hot. Look inside the boiler and you'll see a tank (Usually red) there will be a valve on it the same as a car tyre valve. Release all the pressure from the system and check the pressure at this valve. It should be just under what you run the system at (1 Bar is the norm). If water comes out you need a new pressure vessle. If no water comes out just pump it up as you would a car tyre.
An alternative way to check if you have a pressure vessle problem is to watch the pressure while the boiler starts from cold. If you get a pressure rise of more than half a bar or so you have a problem. and need to carry out the checks above
Rupe
September 2008