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.
Saunier Duval 0020057805 7L Expansion Vessel

Part: 0020057805

Price: £126.37
Saunier Duval Expansion vessel 0020020744

Part: 0020020744

Price: £90.51
Saunier Duval Expansion vessel 5L 2000801894

Part: 2000801894

Price: £163.09
Saunier Duval Expansion vessel 2000800694

Part: 2000800694

Price: £118.35