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Boiler Expansion vessel: What It Does and Signs It's Failing

The expansion vessel is a round steel tank mounted inside the boiler or on the pipework beside it. Heating water grows in volume as it warms, and the vessel gives that extra water somewhere to go. Without it, pressure would shoot up every time the heating came on.

How it works

Inside is a rubber diaphragm with air on one side and system water on the other. As the water expands it pushes into the vessel and squeezes the air cushion, then flows back out as it cools. Worcester, Ideal and Vaillant charge the air side to about 0.75 bar, and that charge slowly leaks away over the years.

Signs it is failing

• Pressure climbing well past 2.5 bar when the heating is hot, then dropping right back when cold
• Water dripping from the small copper pipe outside after the heating runs
• Needing to top up the pressure every few weeks with no visible leak

Low-pressure faults that keep coming back and pressure-rise codes often trace to a flat vessel. Vaillant lists the internal vessel among the causes of its low-pressure code.

Can you replace it yourself?

Recharging or replacing the vessel is engineer work; Baxi and Vaillant both say so. On most combis it sits inside the casing, and the charge has to be checked with the system drained of pressure. Watch your gauge cold and hot for a few days and give the engineer those numbers.

Buying the right part

Vessels vary in size (Ideal's Logic Combi2 C uses 8 liters, about 2.1 US gallons, and Vaillant's ecoTEC pro 10 liters, about 2.6 gallons) and in connection type, so the part must match the boiler model or be sized for the system by the engineer.

Error codes linked to the expansion vessel

• E118Low water pressure in the heating circuitBaxi
• E119Low primary water pressure (Platinum Combi)Baxi
• F.22Water pressure in the heating system is too lowVaillant
• F.23Too big a gap between flow and return temperatureVaillant
• F.75No pressure rise when the pump startsVaillant
• CE 207 / H07Heating system water pressure too lowWorcester Bosch

Last reviewed 2026-10-21.