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What Is An Expansion Vessel?

What is an expansion vessel?

Expansion vessels, also known as expansion tanks, can be found in most domestic pressurised heating systems. If you have a combi boiler, you will have an expansion vessel. This vessel contains air and water from the central heating system, and its purpose is to maintain the right level of pressure in the system.

There are various expansion tank designs, but the most common among modern boiler designs is the bladder type system. This design consists of a cylinder or disc which is split into two parts by a thin rubber sheet known as a ‘diaphragm’. One side is filled with water from the central heating system, while the other is filled with nitrogen. An expansion vessel also has an air valve that allows for the system to be depressurised and repressurised over time.

How does an expansion vessel work?

When heated, water molecules vibrate and move, causing the water to expand in size. This means that water inside the boiler will expand upwards and, if left unchecked, increase the pressure in the system to dangerous levels. To counteract this, the air in the expansion vessel compresses to absorb the added pressure and keep the boiler system stable.

Expansion vessels should be able to operate for years without any intervention, except for periodic draining and recharging as the unit gets older. Over time, however, hard water can damage the diaphragm and cause it to fail, at which point a replacement is needed.

Where is the expansion vessel?

It is likely that your expansion vessel is located inside the boiler casing. However, make sure to refer to your boiler manual to double check this. If you are having trouble identifying the vessel or need advice on finding a replacement, speak to a qualified heating engineer.

Please note: It is illegal for consumers to maintain, service or permanently adjust a gas appliance in the UK. By law you will need to ask a Gas Safe registered engineer to remove your boiler’s casing.

How to install an expansion vessel

Only engineers registered with the Gas Safe Register (GSR) are legally able to install an expansion vessel. This is because the gas appliance will have to be partially deconstructed, with certain parts needing to be taken out depending on where the tank is located. Always call a heating engineer to install a new expansion vessel.

What causes an expansion vessel to fail?

An expansion vessel can fail due to rust and subsequent water leakage, although this is unlikely in most modern sealed water heating systems. It is more common for the diaphragm to become thinner and perforate over time, causing the whole system to fail. The Schraeder filling valve can also leak air and cause pressure failure if left unchecked.

How long should an expansion vessel last?

A typical tank will last between 5-10 years. Some may last longer, but over time they will wear out and need replacing.


  • Expansion vessels, also known as expansion tanks, keep the pressure in a combi boiler stable while water heats and cools.
  • They are typically located inside the boiler casing.
  • The typical pressure a boiler system should be set to is 1 bar. If you identify significant fluctuations above or below this, you should carry out tests to see if there is a fault.
  • Only boiler engineers registered with the GSR are legally allowed to carry out maintenance on a heating system. Always call a professional if you need to replace or fix a boiler system.
  • An expansion vessel should last between 5-10 years.