An expansion vessel is a small tank in heating or domestic hot water systems, which provides protection from excessive pressure caused by thermal expansion in a closed system. 

The vessel contains a diaphragm that divides it in two. One section is connected to the hot water system, whereas the other section is dry and contains compressed air. When the water pressure rises, the diaphragm is compressed against the air on the dry side of the vessel, thus relaxing any water pressure build up resulting from thermal expansion. Conversely, as the water pressure falls, the diaphragm regains its shape and returns water back into the hot system. 

In this way, pressure excesses and risk of rupture are avoided.