Pressured systems, common with most combi boilers have a pressure vessel. This contanes a rubber ballon like item that is filled with air. The system water when it is heated expands the air in the pressure vessel is compressed. The air is more compressable than the water and therefore the pressure in the system does not increase. If the air buble fails then when the water in the system is heated the pressure would increase, resulting in an explosion. To prevent this a pressure release valve is fitted which opens up at 3 bar. This items oftern has dirty/rust (from the system water) in it and will not shut properly. Check to see if this is weeping, normaly it is routed to the out side near the boiler.
Peter, November 2009
