What is an expansion vessel and what happens when it fails?
The component behind most boiler pressure problems, the cold-low hot-high pattern that identifies a failed one, and why topping the system up does not fix it.
- Difficulty
- intermediate
- Time
- 15 min
- Read
- 5 min
- Safety
- warning
Short answer
An expansion vessel is a sealed container with a rubber diaphragm and an air cushion behind it, giving heating water somewhere to expand into when it heats. When the cushion is lost, pressure rockets as soon as the boiler fires and the relief valve dumps water outside — leaving the system low and cold the next morning. That cycle is the diagnosis.
Water expands when it is heated, by a small but absolutely unstoppable amount. An open-vented system deals with this by letting the water rise into a tank in the loft. A sealed system has nowhere to go, so it is given an expansion vessel: a steel container split by a rubber diaphragm with compressed air behind it. That air cushion is the only thing absorbing the expansion of every litre in your heating system, and when it goes, the symptoms are dramatic and very characteristic.
Safety
Step by step
- Understand what it is doing.As the system heats, the water expands and pushes the diaphragm across, compressing the air behind it. The gauge rises modestly. As the system cools, the air pushes back and the gauge falls again. That gentle rise and fall between cold and hot is a healthy sealed system.
- Find it.In a combi it is inside the boiler casing and you will not see it. On a system boiler installation it is often a red vessel on the pipework nearby, sometimes in the airing cupboard or the loft. An unvented cylinder has its own separate vessel for the hot water side, which is a different vessel doing the same job for a different circuit.
- Read the two gauge figures.Note the pressure with the system stone cold and the boiler off. Then run the heating until everything is hot and note it again. Those two numbers are the diagnosis and they cost nothing to obtain.
- Recognise a healthy pattern.Cold pressure sitting in the manufacturer's normal band, rising modestly when hot, and returning to the same cold figure. Stable across weeks. Nothing discharging outside.
- Recognise the failed pattern.Cold pressure low, and a steep climb to the top of the dial as soon as the boiler fires. The relief valve then lifts and dumps water outside. Everything cools, and because water was lost the cold pressure is now lower still. Repeat. Topping up makes the next cycle worse, not better.
- Distinguish it from a leak.A leak loses pressure gradually and steadily, hot or cold, and does not produce a dramatic rise when heating. A failed vessel produces the high-hot, low-cold swing. If pressure falls slowly and evenly over weeks, look for water; if it swings wildly with temperature, look at the vessel.
- Stop repeatedly topping up.Every top-up puts more water into a system with no expansion space, which pushes more out of the relief valve. It also introduces fresh oxygenated water, which corrodes the system. Two or three top-ups in a season is the point to stop and diagnose.
- Hand it over with the numbers.Tell the engineer the cold reading, the hot reading, whether anything discharges outside and how often you have had to top up. That is a complete diagnosis and often means they arrive with the right part.
Tips
- Write your cold pressure reading on a label by the boiler with the date. Comparing against that in three months is worth more than any single reading.
- A pressure gauge with a red or green marked band is showing the manufacturer's normal cold range. Use it rather than a remembered figure.
- Vessels can often be recharged rather than replaced, which is cheaper. Whether that is possible depends on the diaphragm — if it has split, only replacement works.
- An external vessel can be fitted alongside a combi's internal one where the internal vessel is undersized. That is a common and legitimate remedy.
If it doesn't work
Pressure climbs to the top of the gauge when heating, drops below normal when cold
Cause: The expansion vessel has lost its air charge or the diaphragm has split — Fix: Recharging or replacing the vessel. This requires the system side depressurised and is engineer work — but it is a well-defined, routine job once diagnosed.
Slow steady pressure loss with no dramatic rise
Cause: A leak — a radiator valve, a joint, a weeping filling loop, or the relief valve itself passing — Fix: Look for damp patches at radiator valves and joints, check the loop is closed, and check whether the relief valve discharge pipe outside is damp. A pressure test finds the rest.
Pressure rises slowly over days with the heating off
Cause: The filling loop is passing, or a failed cylinder coil is letting mains water into the heating circuit — Fix: Check the loop valves are fully closed and disconnect one end. If it still rises, the coil is the suspect and needs an engineer.
Relief valve discharges only after topping up
Cause: The system has simply been overfilled — Fix: Bleed a radiator carefully to bring the cold pressure back into the band. Top up to the low end of the band, not the high end.
Vessel replaced but the problem returned within a year
Cause: Undersized vessel for the system volume, or the vessel is fine and the fault is the relief valve passing — Fix: Both are worth raising. Vessel sizing is a calculation based on system volume and it is sometimes got wrong.
Questions people ask
Can I recharge it myself with a bicycle pump?
No. The system side must be depressurised first, or you will simply be reading and pumping against system water pressure and achieve nothing while risking a discharge of hot water. The procedure and the correct charge pressure are engineer work.
How long do they last?
Commonly around ten years, sometimes much less if the charge was never checked. Checking the charge is a normal part of an annual service and it is often skipped.
Does an open-vented system have one?
No. It uses the feed and expansion cistern in the loft instead — the water simply rises up the vent and feed pipe. That is why open-vented systems have no pressure gauge and no filling loop.