An expansion vessel is a small tank that is installed in a closed water heating system. It is also known as an expansion tank. The purpose of an expansion vessel is to maintain the right level of pressure in the system.
Expansion vessels are typically located inside the boiler casing. They keep the pressure in a combi boiler stable while water heats and cools. The typical pressure a boiler system should be set to is 1 bar.
The expansion vessel gives the additional hot water a sort of overflow tank, that can help to bring the pressure in the system back to a normal level. Acting like a spring, the air that is present in the vessel compresses. In this way, it absorbs the extra pressure, helping to keep the system stable.
Expansion vessels are intended to operate in heating installations and enclosed cooling systems. They absorb the volume increase produced by the increase in temperature of the heating circuit. The expansion vessels (type CMF and SMF) are only suitable for use in closed circuits.
The expansion vessel should be installed vertically above the pipe with the water connection at the bottom and the air valve uppermost. Expansion vessels are supplied pre-charge with air, this may be increased or reduced via the air valve to suit site conditions.
Maintenance should only be performed by authorized and qualified personnel. At least once every six months check the air or nitrogen pressure using the air valve at the top of the vessel. The pressure can be measured by a tyre gauge or an inflation device, if it has an integral gauge, and should be returned to the pre-charge or specified pressure as necessary. Never remove the expansion vessel from the system without de-pressurizing the system, isolate the system where possible to minimize water loss and damage.