The Ariston expansion vessel is a unit to cope with the expansion of water in a home heating system or hot water system due to rises in temperature. When water increases in temperature, it expands, in particular when it is stored under pressure, such as in a sealed heating system or an unvented hot water cylinder. Expansion vessels typically contain a cylinder or disc which is divided inside into two compartments separated by a rubber sheet known as the diaphragm.

Brand Name	Ariston