What Is An Expansion Vessel?

James Elston
Written by James Elston
Updated on 1st February 2024
Posted on 2nd January 2024
Topic: Boiler Advice
What Is An Expansion Vessel

An expansion vessel, also known as an expansion tank or pressure tank, is a device used in gas boilers and central heating systems to help regulate the pressure and volume of the fluid or gas inside the system.

expansion vessel

In this guide, we’re looking at what an expansion vessel is, how it works, where it’s located, and issues to look out for.

If you have a combi boiler, system boiler or conventional boiler in your home then you may have heard about this piece of safety kit but never knew exactly why it’s so important, so on that note, let’s get into it. 

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Expansion Vessels in Boilers Explained

An expansion vessel, commonly found in domestic heating systems like a combi boiler, is designed to regulate the pressure and volume of the water and air in the central heating system.

Its primary function is to prevent the pressure from building up in the heating system as the temperature increases due to heat generation by the combi boiler.

Without an expansion vessel, the central heating system may experience leaks and damage due to excess pressure.

It is important to understand the role of the expansion vessel and to monitor it for any potential issues to ensure safe and efficient operation of the central heating system.

How Does An Expansion Vessel Work?

The expansion vessel is a closed system that connects to the boiler’s water circuit. The inside contains a flexible bladder (diaphragm) that divides into two compartments. 

On one side of the expansion vessel, water from the central heating fills the compartment. On the other side, gas accumulates. This is usually nitrogen in combi boilers. 

Now, as your boiler heats up, the water expands. This is what creates pressure. This pressure pushes against the bladder, compressing the air in the expansion vessel. 

As this happens the bladder absorbs the pressure, preventing it from becoming too high.

When the system begins cooling down, the boiler pressure decreases. This expands the compressed air, pushing the water back into the system. So, even as the pressure and temperatures change, there is always enough water for the vessel to run smoothly. 

Basically, the expansion vessel acts as an overflow tank to relieve pressure, bringing it down to a normal level faster. 

You may be thinking this sounds similar to a pressure relief valve, but the two are different. The expansion vessel works with the boiler, pushing the water back into the system. 

A pressure relief valve is a safety mechanism that gets to work if the pressure build-up in your system is too high, and the water releases into a drain, so you can’t use it again. 

Modern homes have both, with the expansion vessel working constantly as your water heats up. The pressure release valve only acts when the pressure is too high. 

Where Is The Expansion Vessel Located?

The location depends on the type of boiler you have. In a modern combi boiler, the expansion vessel usually fits within the combi boiler itself. It’s stock standard and you probably won’t even realise it’s there. 

Some boilers have separate expansion tanks located somewhere near the water circuit. It’s best to look at your boiler’s user manual to be sure where your expansion vessel is located. 

Expansion Vessel Sizing—Selecting The Right Size

Selecting the right size expansion vessel is critical. Once the water heats it can expand between 4 and 9%. So, if you’ve got the wrong size vessel it might not have the capacity to handle this expansion. 

Generally, you don’t have to worry about modern boilers as they come set up with all the right equipment. However, if you need to replace the expansion vessel it’s important to know what size to get. 

Your user manual should have information on sizing for your specific product. Still, it’s usually best to have a Gas Safe heating engineer come look at your boiler. They know what to look out for and the best possible replacement pieces for your specific make and model. 

How To Fit Expansion Tanks

After reading this you may think fitting expansion tanks sounds easy enough. Don’t be fooled, you should never replace the expansion vessel yourself. 

Remember, the boiler has a lot of pressure. If you don’t have the expertise, this is a dangerous situation. Particularly if something in the boiler isn’t working as it should be.

Always be sure to use an expert heating engineer for this task. 

UK law stipulates that a member of the general public is not permitted to remove the casing of a boiler. A Gas Safe registered engineer needs to ensure the area is safe. So, we wouldn’t risk trying to fix something within the boiler yourself. 

Expansion Vessel Issues And What to Look Out For

As you can see, expansion vessels are a vital part of the boiler system that keeps you and your home safe. However, things do go wrong and it’s important to know what to look out for. 

  • Leaks: If you notice leaks around your boiler or from the expansion tanks, it’s time to call in a professional. Be sure to switch off your boiler and remove any excess water. 
  • Noises: If you hear any whistling, banging or gurgling noises, this may indicate that your expansion vessel isn’t functioning as it should. 
  • Pressure loss: If your closed water heating system water pressure drops rapidly there may be a problem with the expansion vessel not taking water in. Usually, this is an issue with the diaphragm which might be corroded or damaged. 
  • Overheating: If your central heating system or boiler is getting too hot it may be because the expansion vessel isn’t removing enough pressure. This can lead to a boiler explosion if not fixed quickly.  

If you notice anything like this you’ll need to call in a plumber or Gas Safe registered engineer to carry out expansion vessel work.


How do I repressurise an expansion vessel on a combi boiler?

Unfortunately, if you aren’t a qualified heating engineer you can’t. It is illegal under UK law to remove the casing of a boiler. This is for your own safety. 

Is fixing an expansion vessel a big job?

Yes, that’s why the work is carried out by a professional. A boiler has many working components and a professional knows what to look for. Most importantly they know how to make the combi boiler safe for use. 

How long does an expansion vessel last? 

If all other components of your central heating system work correctly, your expansion vessel should last between five to ten years or usually the lifespan of your boiler. You should inspect your heating systems regularly to ensure it’s working properly. Even a slight change in pressure or heat could indicate a problem. 

Wrap Up

Expansion vessels are critical to your closed-water heating system. They help to keep your home safe and running as it should be.

Your boiler has many different components, and understanding how they work can save you a lot of hassle in the long run if you know what to look out for. 

That’s why we created our guide on expansion vessels, to give you a good idea of what this component does and what you can do if something goes wrong.

Written by
James Elston
James Elston Director Of Boiler Central
Posted on: 2nd January 2024
Topic: Boiler Advice

James Elston is our resident boiler replacement and heating expert here at Boiler Central. With over 20 years experience in the boiler installation industry, James ensures that he knows everything there is about our Gas Safe boiler installations, energy saving and home heating solutions. This can be from simply procuring the latest best combi boilers, to reviewing and ensuring that Boiler Central maintains the highest standards across our boiler installation company.

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