The expansion vessel is a tank that is used to protect closed systems from pressure surges. The tank is filled with air that acts as a cushion against surges in pressure within the system.
This part is present in 16 different boilers in the Worcester range
![WORCESTER EXPANSION VESSEL | 87161055450 [8717]](http://allgasltd.com/cdn/shop/products/WORCESTER_EXPANSION_VESSEL_87161055450.jpg?v=1682072120&width=1445)