Expansion Vessel Failure: Signs, Causes, and Replacement Cost

What does the expansion vessel do?

When water heats up, it expands. The expansion vessel absorbs this extra volume so the system pressure stays stable. It is a metal container inside (or sometimes beside) the boiler with a rubber diaphragm separating water from a pocket of pressurised air. When the water expands, it pushes into the vessel rather than building up pressure in the system.

Signs of a failed expansion vessel

• Pressure keeps rising when the heating is on - the gauge goes above 3 bar
• Pressure drops when the heating cools down - you need to top up regularly
• Water dripping from the pressure relief valve (the copper pipe that exits through the wall outside)
• The boiler keeps cutting out on a high-pressure fault

If the pressure relief valve is dripping, it is doing its job - releasing excess pressure to prevent damage. But the underlying cause needs fixing.

What causes it to fail?

The rubber diaphragm inside the vessel perishes over time, usually after 8-12 years. When it ruptures, the air pocket is lost and the vessel can no longer absorb expansion. The air charge can also drop naturally over time, even if the diaphragm is intact.

Can it be repaired?

Sometimes the vessel just needs repressurising - the engineer uses a Schrader valve (like a bicycle tyre valve) to pump the air charge back up. If the diaphragm has ruptured, the vessel needs replacing. During your annual service, the engineer should check the vessel charge as standard.

How much does replacement cost?

• Repressurising only: included in a standard boiler service
• Expansion vessel replacement: £150-£300 depending on the boiler model and whether the vessel is internal or external

Compared to the cost of a new boiler, this is a relatively inexpensive repair that can extend the boiler’s life by years.