What does an expansion vessel do?

An expansion vessel absorbs the increased volume of water as it heats up in a sealed central heating system. Without it, the pressure would rise dangerously and trigger the safety valve. The vessel contains a rubber diaphragm — one side holds air (pre-charged), the other accommodates the expanding water.

What size expansion vessel do I need?

The vessel size depends on your total system water volume, maximum operating temperature, cold fill pressure, and safety valve pressure. A typical UK home with 80-120 litres of system water at 80°C usually needs an 8-12 litre vessel. Larger systems need bigger vessels. If you do not know your system volume, use our Central Heating Volume Calculator first.

What pressure should the expansion vessel be?

The expansion vessel pre-charge pressure should match the cold fill pressure of your system — typically 1.0 bar for a domestic system. This should be checked with a pressure gauge on the Schrader valve (like a tyre valve) on the vessel. If the pre-charge is incorrect, the vessel will not function properly.

How do I know if my expansion vessel has failed?

Common signs include: the system pressure rising when the heating is on and dropping when it cools, the pressure relief valve (PRV) dripping, or frequent need to top up the system. You can check by pressing the Schrader valve on the vessel — if water comes out, the diaphragm has failed and the vessel needs replacing.

Can I fit an external expansion vessel?

Yes. Many boilers have a built-in expansion vessel, but it may be undersized for larger systems. An additional external vessel can be fitted on the return pipe near the boiler. This is common when extra radiators have been added to a system. The external vessel must be properly supported and pre-charged.

Expansion Vessel Size Calculator — What Size Vessel for My Heating System?

Expansion Vessel Sizing Calculator — What Size Vessel Do I Need?

Calculate the correct expansion vessel size for your sealed central heating system. Based on system volume, temperature, and pressure.

Total System Volume

Use our System Volume Calculator if unsure

Max System Temperature

Default 80°C for standard UK boilers

Cold Fill Pressure

Typically 1.0 bar for domestic systems

Safety Valve Pressure

Typically 3.0 bar for domestic systems

How We Calculate This

Our expansion vessel calculator uses the standard engineering formula for sizing sealed system expansion vessels. It accounts for water expansion at temperature, system pressures, and recommends the next standard UK vessel size.

The formula

Vessel size = (Expansion volume ÷ Acceptance factor) × Safety margin

Where:

Expansion volume = System volume × Expansion factor (e.g. 2.8% at 80°C)
Acceptance factor = (P_max - P_fill) ÷ P_max (using absolute pressures)
Safety margin = 10% added to the calculated size

Water expansion rates

60°C: ~1.7% expansion
70°C: ~2.3% expansion
80°C: ~2.8% expansion (standard UK boiler)
90°C: ~3.6% expansion

Standard UK vessel sizes

We recommend the next standard size up from the calculated minimum: 8L, 12L, 18L, 24L, 35L, 50L, or 80L. Most domestic boilers come with a built-in 8L or 12L vessel. If the calculation shows a larger requirement, an additional external vessel should be fitted.

Frequently Asked Questions

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Expansion Vessel Sizing Calculator — What Size Vessel Do I Need?

How We Calculate This

The formula

Water expansion rates

Standard UK vessel sizes

Frequently Asked Questions

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