WHAT IS AN EXPANSION VESSEL?
An expansion vessel is a device to cope with the expansion of water in a heating or
hot water system due to increases in temperature.
When water increases in temperature it expands, especially when it is stored under
pressure such as in a sealed heating system or an unvented
hot water cylinder.
Expansion vessels typically consist of a cylinder or disc which is divided inside
into two compartments separated by a rubber sheet, known as a 'diaphragm'.
Inside the expansion vessel there is water on one side of the diaphragm and air or
an inert gas (such as nitrogen) under pressure* on the other side.
An air valve similar to those found on bicycle and car tyres is typically provided
on the air side to allow the system to be re-pressurised.
HOW DO EXPANSION VESSELS WORK?
When the water in a sealed system of pipes expands it increases in pressure. Eventually
the pressure exceeds that of the air or gas, causing the rubber diaphragm in the expansion
vessel to bend back into the air space, compressing the air and creating more room
in the sealed system to cope with the expanded water.
When the water cools it contracts back to its original volume and the air/gas pressure
makes the diaphragm revert back to its original shape.
EXAMPLE OF A HOW AN EXPANSION VESSEL WORKS
In the diagram below you see an example of an expansion vessel with a diaphragm
in the middle.
The diaphragm separates the water in the heating system from the air charge in the
Initially the water and the air in the expansion vessel are at the same pressure.
As the water gets hotter it expands and the pressure in the water side of the expansion
The increase in pressure on the water side of the diaphragm causes the diaphragm to
distort, compressing the air and absorbing the expansion of water within the sealed
When the water cools the pressure reduces and the diaphragm returns to its original
PROBLEMS WITH EXPANSION VESSELS
The air/gas pressure in an expansion vessel should be tested and recharged every 12 months.
Most vessels are fitted with a bicycle tyre pump type adapter to allow recharging.
If the vessel loses its air/gas pressure then the diaphragm can become permanently distorted by increases in water pressure.
Eventually the diaphragm may rupture, allowing water into the air chamber. The remaining air is then forced out and
the expansion vessel ceases to work, allowing the water in the system to over-expand
and trip one of the other safety devices on the system, such as the pressure
*The pressure of the air or gas depends on the size of the heating or hot water system
and the manufacturer's specification. Typically it is between 0.75bar and 1.5bar.