Expansion Vessels

An expansion vessel is normally found on the central heating side of most domestic boilers. It comprises of a vessel in which there is a diaphragm and on one side of the diaphragm is compressed air that is normally pressurised to a set pressure using the Schrader valve on the expansion vessel, on the other side of the diaphragm is the central heating water. The expansion vessel is normally connected to the return side of the central heating and not the flow side. It is important that the pressure on the central heating side of the expansion vessel is reduced before an attempt is made to pressurise the vessel because the water on the central heating side is incompressible, where as the air on the opposite side can be compressed.

Today's modern boilers often have a built in expansion vessel, although some boilers still have external ones. If an internal expansion vessel is not working correctly due to a failed diaphragm it is often possible to leave it alone and just fit an external expansion vessel to the return side of the central heating pipe work, this is often a much cheaper alternative than having the internal vessel replaced as with many boilers a large amount of parts need removing in order to get to the expansion vessel. External expansion vessels are normally available in sizes of between 8 and 24 litres for domestic purposes, larger vessels are available for non domestic applications.