Expansion Vessel Problems — Why Your Boiler Keeps Losing Pressure
If your boiler pressure gauge drops repeatedly despite repressurising with no visible leak, the expansion vessel is the most likely culprit. This guide explains how expansion vessels work and what to do when they fail.
What Is the Expansion Vessel?
The expansion vessel is a small pressurised tank inside (or occasionally external to) your boiler, typically holding 8-12 litres. As water in the sealed central heating system heats up, it expands — the expansion vessel accommodates this expansion, preventing excessive pressure buildup in the system.
Inside the vessel, a rubber diaphragm separates nitrogen gas (at pre-charge pressure, typically 0.75-1 bar) from the system water. As water expands, it compresses the diaphragm against the gas charge. When the system cools, the gas pushes the water back out.
Signs of a Failed Expansion Vessel
The most characteristic symptom of a failed expansion vessel is pressure that drops when the system is cold but rises excessively when the boiler fires:
- Pressure drops to zero when cold, but rises to 2.5-3 bar or more when the system heats up
- The pressure relief valve (PRV) discharges water outside — often seen as water dripping from a small copper or plastic pipe on the exterior wall
- Repeated repressurising needed — you are adding water through the filling loop weekly or monthly but can find no visible leak
The mechanism: when the diaphragm fails (develops a tear or hardens), the gas charge dissipates. The vessel fills with water. There is now no expansion capacity, so as the system heats and water expands, pressure rises dramatically — triggering the PRV to discharge excess water, which then causes low pressure when cold.
Diagnosing an Expansion Vessel Problem
A Gas Safe engineer can diagnose expansion vessel failure by:
- Depressurising the system and connecting a pressure gauge to the expansion vessel's Schrader valve (similar to a tyre valve)
- If the vessel is waterlogged (diaphragm failed), water will emerge from the Schrader valve when pressed
- If the pre-charge pressure has simply dropped (the gas has leaked out), the vessel can sometimes be recharged with nitrogen — but this only works if the diaphragm is intact
Expansion Vessel Repair vs Replacement
If the diaphragm has failed, the vessel must be replaced — diaphragms are not serviceable. If only the pre-charge pressure has dropped and the diaphragm is intact, recharging with nitrogen (using a pump and nitrogen cylinder) may extend the vessel's life, though replacement is often recommended for vessels over 10 years old.
Replacement expansion vessel cost in London: £150-300 including parts and labour for most combi boilers. External expansion vessels (fitted to older system boilers) may cost more due to additional pipework involved.
Frequently asked questions
Why does my boiler pressure keep dropping but there is no leak?
Repeated pressure loss without a visible leak is the classic symptom of a failed expansion vessel diaphragm. The vessel loses its gas charge and fills with water, causing the pressure relief valve to discharge when hot — which reduces pressure when cold. A Gas Safe engineer can diagnose and replace the expansion vessel.
Is a dripping pressure relief valve an emergency?
A constantly dripping PRV is not an immediate emergency but should be addressed within a few days. The PRV is discharging because system pressure is too high — usually caused by a failed expansion vessel or (less commonly) the filling loop stuck open. Water dripping continuously from the PRV will corrode the valve seat, eventually requiring PRV replacement too.
How long does an expansion vessel last?
Most expansion vessel diaphragms last 8-15 years. Hard water areas like London tend to shorten vessel life because dissolved minerals affect the diaphragm material over time. Vessels in properties with London hard water (SW, SE, E, N postcodes) should be checked every 5 years.
Can I replace the expansion vessel myself?
The expansion vessel itself does not contain gas and is technically not a gas fitting, so it is not legally restricted to Gas Safe engineers. However, replacing it involves draining the system, removing the old vessel, fitting the new one, and repressurising — this requires the skills and tools of a heating engineer. The work is often done by Gas Safe engineers as part of boiler servicing.