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This article is provided for information purposes only. You should only attempt any procedures explained here if you are competent to perform the task and have the appropriate tools and materials, including any relevant safety equipment.

Readers acting upon this article do so at their own risk and no responsibility is accepted by the author or publisher for any loss or damage incurred by following the article or failing to follow it.

If in doubt always consult a properly qualified tradesperson or other competent person.


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.


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.


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 expansion vessel.

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 vessel increases.

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 system.

When the water cools the pressure reduces and the diaphragm returns to its original shape.

how expansion vessels work


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 relief valve.

*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.
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