What is an expansion vessel?
An expansion vessel is a sealed container that absorbs the expansion of water in a heating or water system. When water heats up, it expands and the system pressure rises. Without an expansion vessel, that pressure could rise too high and damage the installation.
How does an expansion vessel work?
An expansion vessel is a metal tank with a rubber membrane inside, dividing it in two:
- Water side — connected to the heating system
- Gas side — filled with nitrogen or air under pressure
When the water expands, it pushes against the membrane and compresses the gas. When the water cools, it retracts and the gas pushes the membrane back. This keeps system pressure stable.
Application
Expansion vessels are used in many installations:
| Type | Application | Pre-charge |
|---|---|---|
| Central heating | Boiler systems | 1.0 bar |
| Hot water (potable) | Cylinder, hot water | 4.0 bar |
| Booster set | Pressurised water supply | 3.5–4.0 bar |
| Cooling/HVAC | Industrial | Variable |
Heating expansion vessel
Most modern homes have an expansion vessel built into the boiler. For underfloor heating or larger installations, an external vessel is added, typically 8 to 25 litres.
Pre-charge and maintenance
Expansion vessels need regular checks:
- Check pre-charge — annually during service
- Low pre-charge — vessel can no longer expand properly
- Failed membrane — vessel needs replacing
- Waterlogged vessel — pressure swings, failed cushion
Sizing the vessel
Vessel size depends on:
- System volume — total water content of the heating system in litres
- Maximum temperature — hotter means more expansion
- Static height — difference between vessel and highest point
- Pre-charge pressure — standard 1.0 bar
Rule of thumb: vessel size ≈ 5% of system volume.
Symptoms of failure
- Pressure drops after cooling — vessel does not hold charge
- Pressure rises sharply when hot — vessel cannot expand
- Pressure relief valve discharges — water lost through overflow
- Sloshing noise — vessel has filled with water
Related terms
- Boiler
- Radiator
- Boiler and radiator capacity
- Convector
- Drain valve
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