An expansion vessel absorbs the expansion of water as it heats. When its air charge is lost, expansion has nowhere to go and pressure rises until a relief valve opens. It presents as discharge outside, or as a boiler pressure that swings with temperature.
What an expansion vessel does
Water expands when heated. In an open system that expansion pushes water up an open vent and back into a tank. In a sealed system – an unvented cylinder, or a sealed heating circuit – there is nowhere for it to go, so something has to absorb it.
An expansion vessel is a steel container divided into two by a flexible rubber diaphragm. One side connects to the water system; the other side is sealed and pre-charged with air or nitrogen to a specified pressure.
As water heats and expands, it pushes against the diaphragm and compresses the air on the other side. Air compresses; water does not. When the water cools and contracts, the compressed air pushes back. The vessel is a spring, and it keeps system pressure within a narrow band through every heating cycle.
How they fail
- Loss of air charge. Air permeates slowly through the rubber diaphragm and past the valve over years. The charge falls, the vessel’s capacity to absorb expansion falls with it, and eventually there is effectively no cushion at all. This is the most common failure and it is gradual.
- Diaphragm rupture. The rubber perishes or splits, and water fills the whole vessel. There is then no air at all and no absorption. Tapping the vessel sounds solid and dull rather than hollow at the top.
- Corrosion, particularly on heating-side vessels that have had system water in them for years.
- Schrader valve leakage, which lets the charge out slowly – sometimes fixable by replacing the valve core.
- Wrong size or wrong charge pressure from the start, which means it was never going to work properly.
An expansion vessel is not a permanent component. The charge falls over years in normal use, which is why checking and recharging it is part of every annual service. A vessel that has never been checked in ten years has almost certainly lost its charge.
The symptoms
| Symptom | System | Why |
|---|---|---|
| Water discharging from a pipe outside | Unvented cylinder | Expansion relief valve opening as pressure rises |
| Water running through the tundish | Unvented cylinder | Same – and the tundish is there so you can see it |
| Boiler pressure low when cold, high when hot | Sealed heating | Classic. No cushion, so pressure tracks temperature directly |
| Pressure relief valve discharging outside | Sealed heating | Pressure exceeding the valve’s setting when hot |
| Boiler pressure gradually dropping over weeks | Sealed heating | Water lost through the relief valve each cycle, mistaken for a leak |
| Pressure gauge jumping visibly during a cycle | Either | No absorption at all |
| Banging or knocking as the system heats | Either | Pressure shock without a cushion |
The one worth emphasising is boiler pressure gradually dropping. People assume a leak, look for water, find none, and top it up repeatedly. What is actually happening is that each heating cycle pushes pressure past the relief valve setting, a small amount discharges outside, and the system is a little lower next time it cools. Every top-up also brings fresh oxygenated water into the system, which is separately damaging.
The test
- Look at the discharge pipe outsideOn an unvented cylinder, and on a sealed heating system. A pipe that is wet, dripping, or has a stain or green algae below it is discharging. That single observation identifies the fault most of the time.
- Watch the pressure gauge through a heating cycleNote it cold, then watch as the system heats. A well-charged vessel keeps the rise modest. A failed one lets pressure climb sharply.
- Press the Schrader valveOn the vessel’s air side, with the system depressurised. Air should come out. If water comes out, the diaphragm has ruptured and the vessel needs replacing.
- Tap the vesselA healthy vessel sounds hollow at the top and solid at the bottom. One that sounds solid all the way up is full of water.
- Check the charge pressure properlyWith the system drained of pressure, a tyre gauge on the Schrader valve reads the charge. It should match the specified figure for that system.
The first step is free and it resolves the question in most cases. Go and look at the pipe.
Recharging or replacing
If the diaphragm is intact, a vessel can be recharged.
- Depressurise the water side first. Recharging against system pressure gives a false reading and does not work.
- Use a pump and an accurate gauge, set to the specified charge for that system.
- Repressurise and check through a full heating cycle.
- If it will not hold the charge, the diaphragm or the valve has failed and the vessel needs replacing.
- If water comes out of the Schrader valve, the diaphragm has gone. Replace it.
Replacement vessels must be correctly sized for the system volume and correctly charged. An undersized vessel will not absorb the expansion; a wrongly charged one behaves as though it were failed. Both are found on installations where somebody fitted whatever was available.
On some unvented cylinders the expansion is accommodated by an internal air gap – a bubble of air trapped at the top of the cylinder – rather than an external vessel. Those gaps are gradually absorbed into the water and need recharging periodically by draining part of the cylinder to let air back in. Same problem, different procedure, and it is part of the annual service.
Why it matters beyond the drip
- Repeated relief valve operation wears the valve. Eventually it does not reseat cleanly and you have a continuous leak rather than an occasional discharge.
- Water loss and repeated topping up brings fresh oxygenated water into a sealed system, which drives corrosion and magnetite formation – see radiators cold at the bottom.
- Pressure cycling stresses components, including the boiler heat exchanger and every joint in the system.
- On an unvented cylinder it means a safety device is operating routinely, which is not what it is there for.
- Money down a drain, literally – heated water discharging outside.
It is also one of the cheapest faults to put right, which is the frustrating part: a great deal of unnecessary investigation happens because nobody looked at the outside pipe first.
Boiler pressure dropping? Go and look at the discharge pipe outside before anybody starts looking for a leak.
Questions we get asked about this
Why is water dripping from a pipe outside my house?
A relief valve is discharging, and the usual cause is an expansion vessel that has lost its charge. It is a common, cheap fault and it should not be ignored.
My boiler pressure keeps dropping. Is it a leak?
Check the discharge pipe outside first. A failed expansion 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.
Can an expansion vessel be recharged?
If the diaphragm is intact, yes – depressurise the water side and recharge to the specified pressure with a pump and gauge. If it will not hold the charge, or water comes out of the Schrader valve, it needs replacing.
How often should it be checked?
At every annual service. The charge falls gradually in normal use, so it is a maintenance item rather than a fit-and-forget one.
What is an internal air gap?
Some unvented cylinders accommodate expansion with a bubble of air at the top of the cylinder rather than an external vessel. The air is gradually absorbed and needs recharging periodically by draining part of the cylinder. Same problem, different procedure.
Is it dangerous?
The relief 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.
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.