Main Expansion vessels
This part is in the boiler to ensure that the pressure within the boiler operates within safe parameters normally 1 - 2.5 bar. The expansion vessel should be half full of air and half full of water both separated by a rubber diaphragm. The common problem customers encounter is when air in the expansion vessel escapes over time and allows the boiler to run at higher pressures such as 3 bar. At this point a safety valve operates (pressure release valve / prv) ensuring the boiler
can’t go above this pre-set safety pressure.