Glossary · Boilers and boiler types
What is an expansion vessel?
An expansion vessel is a small pressurised container fitted to a sealed central heating system. Water takes up more space as it heats, and the vessel gives that extra volume somewhere to go. Without it, the system pressure would rise too far and the pressure relief valve would keep opening.
Key points
- It takes up the extra volume of water as the system heats, so the pressure stays in a safe range.
- The vessel is pre-charged. On the Worcester Greenstar boiler we checked, the built-in vessel is charged to 0.75 bar.
- A larger system can need an extra vessel on the heating return. The same manual sets the trigger at over 100 litres of system volume.
- The manufacturer says the vessel must be checked every year, which is done at the annual boiler service.
- The vessel sits inside or beside the boiler. Work on it belongs to a Gas Safe registered engineer.
How an expansion vessel works
Water takes up more space when it is hot than when it is cold. In a sealed central heating system there is no open tank on the loft floor to take that extra water, so something else has to. That job belongs to the expansion vessel.
The vessel is a closed container that is pre-charged on one side. As the heating warms up, the extra water pushes into the vessel and the pre-charge compresses. When the system cools, the vessel gives the water back. The result is that the pressure in the pipework moves within a limited range as the system heats and cools.
Many modern boilers have a vessel built in. The Worcester Greenstar 24/28/30 combi manual lists a pressure relief valve, a pressure gauge and an expansion vessel as part of the boiler, and says the built-in vessel is pre-charged to 0.75 bar with a valve for adjusting the charge.[1]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: Installation, commissioning and servicing instructions Other models use other sizes and charges, so the figure on your own boiler may differ. The manufacturer’s manual for your model is the place to check.
The vessel is one half of a pair. The other half is the pressure relief valve, which is the safety backstop if the pressure rises too far. The same manual gives the maximum operating pressure as 2.5 bar and says the safety valve opens at 3 bar.[1]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: Installation, commissioning and servicing instructions A healthy vessel keeps the system well away from that point.
Where the vessel sits and when a second one is needed
| Vessel | Where it sits | Why it is there |
|---|---|---|
| Built-in heating vessel | Inside the boiler casing | Takes up expansion of the heating water in a sealed system |
| Extra heating vessel | On the heating return pipe, close to the boiler | Added when the system holds more water than the built-in vessel can cope with |
| Mini vessel on the mains inlet | On the cold mains pipe to the boiler | Added where a non-return valve on the mains supply could let the pressure build up |
On the extra heating vessel, the manual says one must be fitted where the system volume is more than 100 litres or the pressure would exceed 2.65 bar at maximum heating temperature, and that it must be pressurised to the same figure as the built-in vessel.[1]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: Installation, commissioning and servicing instructions It also says a non-return or back flow prevention valve on the mains water supply can cause a pressure build up that could damage the boiler, and that a mini expansion vessel should be fitted on the mains inlet in that case.[1]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: Installation, commissioning and servicing instructions
What you can see and what you cannot touch
You can see the effect of the vessel on the pressure gauge. The Worcester user guide shows the best pressure when the system is cold as between 1 and 1.5 bar, and tells the user to check regularly that the pressure is maintained.[2]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: User instructions If the gauge shows a permanent and significant fall or rise, the same guide says to contact your installer or maintenance engineer.[2]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: User instructions A filling loop is how the pressure is topped up when it is low.
The vessel itself is not a householder’s job. The installation manual says it must be checked annually, that the boiler must be depressurised first and that the pre-charge may need adjusting to suit the height of the system.[1]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: Installation, commissioning and servicing instructions Anyone employed to work on gas appliances in a home must be Gas Safe registered and competent in that area of gas work.[3]Health and Safety Executive, Domestic gas: frequently asked questions The annual visit is the natural time to have it checked, which is what a boiler service is for.
Example
Priya in Bingley notices that her boiler gauge reads 1.2 bar when the heating is cold, which sits inside the 1 to 1.5 bar range in the user guide. After a few weeks the gauge reads under 1 bar again after she has topped it up. One top-up is routine. A gauge that keeps falling is not, so she calls an engineer. The cause could be a leak, a faulty vessel or something else, and only an inspection will tell which.
What people get wrong
Many people think a rising gauge on a hot boiler means the boiler is failing. A small rise as the water heats is what the vessel is there to manage. The thing to watch is a gauge that keeps moving outside its normal range or that sends water out of the discharge pipe.
The second mistake is trying to fix it with a bicycle pump or a tyre gauge. The vessel has a valve, but adjusting the charge means depressurising the boiler first and working inside the casing, which is gas appliance work for a registered engineer.
The bottom line
An expansion vessel is a small part that does an important job. It gives heated water room to expand so a sealed heating system stays at a safe pressure. Check the gauge now and then, top up only as your boiler’s user guide describes, and leave the vessel itself to a Gas Safe registered engineer at the annual service.
Common questions
What does an expansion vessel do in a boiler?
It absorbs the extra volume that water takes up as it heats. That keeps the pressure in a sealed heating system inside a safe range instead of letting it climb.
Where is the expansion vessel?
On many modern boilers it is built in behind the casing. Larger systems can have an extra vessel fitted on the heating return pipe close to the boiler.
Can I recharge an expansion vessel myself?
No. The vessel is part of the boiler and the heating circuit. Anyone paid to work on gas appliances in a home must be Gas Safe registered, so leave it to an engineer.
What are the signs of a problem with the expansion vessel?
A pressure gauge that keeps falling or rising well beyond its normal range is the usual sign. The manufacturer says to contact your installer if a significant change in pressure continues.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Worcester, Bosch Group. Greenstar 24/28/30 RSF combination boiler: Installation, commissioning and servicing instructions. Checked 11 November 2026. ↩
- Worcester, Bosch Group. Greenstar 24/28/30 RSF combination boiler: User instructions. Checked 11 November 2026. ↩
- Health and Safety Executive. Domestic gas: frequently asked questions. Checked 11 November 2026. ↩