An expansion vessel is a small tank in a closed heating or domestic hot water system, which provides protection from excessive pressure caused by thermal expansion.
The vessel contains a diaphragm that divides it into two sections. One section is connected to the hot water supply, whereas the remaining section is dry and contains compressed air. As the water pressure rises, the diaphragm is pushed against the air on the dry side of the vessel, thus dispersing any pressure build up resulting from thermal expansion. Conversely, as the water pressure falls, the diaphragm pushes water out of the wet section.
In this way, the system is constantly maintained at the correct pressure.