What an expansion vessel does
Water expands when it is heated. Between cold mains temperature and the 70°C or 80°C a heating system can reach, it grows by roughly 3 per cent in volume. In an open-vent system with a loft tank, that extra water simply rises up the vent pipe. In a sealed system, used by every combi and system boiler, there is nowhere for it to go, so the pressure would rise sharply every time the heating came on.
The expansion vessel solves this. It is a steel tank divided by a flexible rubber diaphragm or bladder. One side connects to the heating water. The other side holds air or nitrogen, pumped in at the factory to a set pressure. As the water heats and expands, it pushes against the diaphragm and squeezes the air. When the water cools, the air pushes it back out. The result is a pressure that rises a little when the heating is on, rather than a lot.
Where to find it
In most combis and system boilers the vessel is inside the casing, usually a flat, round or rectangular red or grey tank at the back or side. Some installations, particularly larger systems or ones with a separate heat-only boiler and sealed system kit, have an external vessel, often a red cylinder on the pipework near the boiler or in the airing cupboard. Unvented hot water cylinders have their own separate vessel, usually white or blue, for the hot water side.
How do I know if my expansion vessel is faulty?
The clearest sign is a pressure gauge that swings much more than it should. With a healthy vessel, the pressure rises modestly when the heating runs. With a failed one, the pattern looks like this:
- The pressure is normal, around 1 to 1.5 bar, with the system cold.
- When the heating comes on, it climbs quickly towards 3 bar.
- At around 3 bar, the pressure relief valve opens to protect the system, and water drips or runs from the copper pipe that goes through the outside wall.
- When the heating goes off and the water cools, the pressure drops below 1 bar, because water was lost through the relief valve.
- You top it up, and the cycle repeats.
Other signs include a boiler that locks out with a high pressure code when hot, such as Baxi E117, then a low pressure code when cold, such as Ideal F1 or Vaillant F22. If the pressure climbs even with the heating off, the expansion vessel is less likely to be the cause. A filling loop that is letting water through is the more usual suspect, which our guide to boiler pressure too high covers.
| What you see | Most likely cause |
|---|---|
| Pressure normal cold, near 3 bar hot, then low after cooling | Expansion vessel lost its air charge |
| Pressure creeps up even with the heating off | Filling loop letting water through |
| Pressure drops steadily with no dripping pipe outside | A leak somewhere on the system |
| Water from the vessel's valve when pressed | Split diaphragm, vessel needs replacing |
Why expansion vessels fail
The air charge leaks away slowly over the years through the valve and the diaphragm, which is why engineers check it during a full service. Once the charge is low, the diaphragm flexes further each time the system heats, which wears it faster. Eventually the diaphragm can split, letting system water into the air side. At that point recharging will not work, because the vessel fills with water again.
Overfilling the system can also stress the vessel. If the pressure is topped up well above the figure in the manual, the air side is squeezed before the heating even starts, leaving little room for expansion.
What pressure should an expansion vessel be?
It depends on the boiler, so the figure has to come from your model's installation manual. Many domestic boilers use a pre-charge of 0.75 bar. Two examples from manufacturer manuals:
| Boiler | Vessel details from the manual |
|---|---|
| Worcester Bosch R 28 HE System | 10 litre vessel, charge pressure 0.75 bar. Normal system fill 1 bar, maximum 1.5 bar. Relief valve opens at 3 bar. |
| Ideal Logic+ Combi 24 | Servicing instructions say to recharge the vessel to 0.75 bar. |
The pre-charge is always measured with the water side at zero pressure, which means the boiler's water has to be released first. A reading taken with the system full is meaningless, because the water pressure pushes on the diaphragm and the gauge shows the system pressure instead.
The Worcester manual also shows how the vessel's size limits the system. It says that if the pressure rises above 2.5 bar with the boiler at maximum temperature, an extra expansion vessel should be fitted on the return pipe. Systems with lots of radiators, large radiators or underfloor heating hold more water, and the boiler's built-in vessel may not be big enough on its own.
How to recharge an expansion vessel
On a sealed system, recharging is a job for a Gas Safe registered engineer when the vessel sits inside the boiler, and for a competent heating engineer when it is external. In outline, the engineer will:
- Turn off the boiler and isolate it from the electricity.
- Isolate the boiler's flow and return valves, or drain the system, and release the water pressure so the gauge reads zero.
- Remove the cap on the vessel's Schrader valve, which looks like a car tyre valve.
- Check the air pressure with a gauge, then pump it up to the figure in the manual with a foot or hand pump.
- Press the valve core briefly. If water rather than air comes out, the diaphragm has split and the vessel needs replacing.
- Refill the system to the correct cold pressure, bleed the radiators, add inhibitor if water was drained, and check the pressure stays steady through a heating cycle.
Do not remove the boiler casing to reach an internal vessel. Only a Gas Safe registered engineer should work inside a gas boiler. Opening the Schrader valve with the system under pressure can also release hot water.
Recharge or replace?
If the diaphragm is intact and the vessel holds its charge after recharging, a recharge is enough. It is usually the quickest and cheapest fix. Replacement is needed if water comes out of the air valve, the charge leaks away again within weeks, or the vessel is corroded. On some combis the internal vessel is awkward to reach, and fitting an external vessel on the pipework is a common, approved alternative, as long as the manufacturer's instructions allow it.
How much does it cost to replace an expansion vessel?
These are typical UK prices including labour. They vary by region, boiler model and how hard the vessel is to reach.
| Job | Typical UK cost |
|---|---|
| Recharging the expansion vessel | Typically £80 to £150 |
| Replacing the internal vessel | Typically £200 to £400 |
| Replacing the pressure relief valve, if it no longer seals | Typically £120 to £220 |
A relief valve that has opened many times may not reseal properly afterwards, even once the vessel is fixed. If the outside pipe keeps dripping at normal pressure, the valve may need replacing too. For wider repair prices, see our guide to boiler repair costs.
What you can do while you wait for an engineer
- Keep an eye on the gauge and note the readings hot and cold. It helps the engineer confirm the fault quickly.
- Top up to the normal cold pressure if it drops too low, as explained in our guide to boiler pressure too low. Do not overfill it.
- If the pressure goes very high when hot, turning the heating temperature down can reduce how far it rises until the repair.
- Make sure the relief pipe outside is not blocked and is not dripping onto a path where it could freeze.