An expansion vessel absorbs the expansion of water as it heats. The air charge falls gradually over years, and when it goes, pressure rises until a relief valve opens. Recharging works if the diaphragm is intact; otherwise it needs replacing.
Check this before anything else
A small pipe coming through the wall near the boiler or the cylinder. If it is wet, dripping, or has a stain or green algae below it, a relief valve has been opening – and in the great majority of cases that means the expansion vessel has lost its charge. It costs nothing, takes thirty seconds, and it explains most of the pressure mysteries we are called about.
If that pipe is dry and you are still losing pressure, then you have a genuine leak to find – which is a different job, see leak detection.
What the vessel does and how it fails
A steel container divided by a flexible rubber diaphragm. One side connects to the water; the other is sealed and pre-charged with air. As water heats and expands it pushes against the diaphragm, compressing the air – air compresses, water does not. When the water cools, the air pushes back. It is a spring that keeps system pressure in a narrow band.
| Failure | What happens | Recharge or replace? |
|---|---|---|
| Air charge lost gradually | Air permeates through the diaphragm and past the valve over years. Capacity falls until there is no cushion | Recharge, if it holds |
| Diaphragm ruptured | Water fills the whole vessel. No air at all. Tapping it sounds solid throughout | Replace |
| Schrader valve leaking | Charge escapes slowly | Sometimes a valve core; otherwise replace |
| Corroded | Particularly on heating-side vessels holding system water for years | Replace |
| Wrong size or charge from the start | Never had enough capacity for the system volume | Replace, correctly sized |
The gradual charge loss is the common one, and it is why this is a maintenance item rather than a fit-and-forget component.
The symptoms
- Water discharging from a pipe outside – on an unvented cylinder or a sealed heating system.
- Water running through the tundish, on an unvented cylinder. The tundish is there precisely so you can see it.
- Boiler pressure low when cold and high when hot. Classic. With no cushion, pressure tracks temperature directly.
- Boiler pressure gradually dropping over weeks, which gets mistaken for a leak. Each cycle pushes past the relief valve setting, a little discharges, and the system is lower next time it cools.
- The gauge visibly jumping during a heating cycle.
- Banging or knocking as the system heats, from pressure shock with no cushion.
- Relief valve that no longer reseats, which is what happens after repeated discharge – and then you have a continuous leak as well.
The gradual pressure drop deserves emphasis because of what it leads to: people top the system up repeatedly, and every top-up brings fresh oxygenated water into a sealed system, which drives corrosion and magnetite formation – see boiler pressure explained.
Recharging versus replacing
- Depressurise the water side firstA charge reading taken against system pressure is meaningless. This is the step most often skipped.
- Test the chargeA tyre gauge on the Schrader valve. It should match the specified figure for that system.
- Check what comes outAir means the diaphragm is intact. Water means it has ruptured and the vessel needs replacing.
- Recharge with a pump and an accurate gaugeTo the specified pressure, not approximately.
- Repressurise and test through a cycleThe system run up to temperature and the gauge watched, to confirm it now behaves.
- Replace if it will not holdCorrectly sized for the system volume, and correctly charged. An undersized or wrongly charged replacement behaves as though it had already failed.
Some unvented cylinders accommodate expansion with a bubble of air trapped at the top of the cylinder rather than an external vessel. That air is gradually absorbed into the water and is recharged by draining part of the cylinder to let air back in. Same problem, different procedure, and it is part of the annual service.
Where they are and what they serve
- On an unvented cylinder – usually a small vessel beside it, serving the domestic hot water side.
- Inside a combi boiler – serving the sealed heating circuit. Failure here is the classic ‘pressure keeps dropping’ complaint.
- On a sealed heating system with a system boiler – sometimes internal to the boiler, sometimes a separate vessel on the pipework.
- On some installations, both – a heating vessel and a hot water vessel, each with its own charge figure.
It matters because the two get confused. A dripping pipe outside from an unvented cylinder is the hot water vessel; boiler pressure swinging is the heating vessel. They are different components with different charge pressures.
Whichever it is, it is checked at the annual service – on unvented cylinders because it is required, and on heating systems because it is the thing most likely to have failed. See the unvented cylinder annual service and expansion vessel failure.
Pressure dropping, or water outside? Check the discharge pipe before anybody starts looking for a leak.
Questions we get asked about this
How do I know if my expansion vessel has failed?
Look at the discharge pipe outside – wet, dripping or stained means a relief valve has been opening. Also watch the pressure gauge through a heating cycle: a large swing between cold and hot means there is no cushion.
Can it be recharged rather than replaced?
If the diaphragm is intact, yes. Depressurise the water side, check the charge with a gauge, recharge to the specified figure. If water comes out of the Schrader valve, the diaphragm has ruptured and it needs replacing.
How often should it be checked?
At every annual service. The charge falls gradually in normal use, which makes it a maintenance item rather than a fit-and-forget component.
My boiler pressure keeps dropping. Is it a leak?
Check the outside discharge pipe first. A failed vessel lets pressure exceed the relief valve setting each time the system heats, so a little water is lost every cycle – which looks exactly like a slow leak.
What is an internal air gap?
Some unvented cylinders use a bubble of air at the top of the cylinder instead of an external vessel. It is recharged by draining part of the cylinder to let air back in – same problem, different procedure.
Is a discharging relief valve dangerous?
The valve doing its job is the system working as designed. What is unwise is leaving it, because repeated operation wears the valve until it will not reseat – and then you have a continuous leak.
What happens when you call
Call and describe it
You speak to an engineer, not a call centre. Most jobs can be narrowed down on the phone.
We look at it properly
A real diagnosis before a price, and a straight answer if it is not worth doing.
A written quote
Itemised, so you can see what you are paying for.
The job, and the paperwork
Certificates where the work needs them, and what will need attention next.
Where we do this work
Across Norwich and the whole of Norfolk. If you are on the edge of the county, call and ask — we will tell you honestly whether we are the right people.
- Norwich
- Wymondham
- Dereham
- Attleborough
- Aylsham
- North Walsham
- Cromer
- Diss
- Loddon
- Long Stratton
- Watton
- King's Lynn
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.