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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.
Use our System Volume Calculator if unsure
Default 80°C for standard UK boilers. Expansion is measured from a 10°C cold fill; a warmer fill shrinks the vessel size slightly
Typically 1.0 bar for domestic systems
Typically 3.0 bar for domestic systems
Extra capacity added to calculated size (default 10%)
Indicative rule of thumb: adds ~1% vessel size per 1% glycol. For design, use the antifreeze maker's expansion data (default 0)
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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. 2.9% at 80°C)
- Acceptance factor = (Pmax - Pfill) ÷ Pmax (using absolute pressures)
- Safety margin = 10% added to the calculated size
Water expansion rates
Volumetric expansion of water from a 10°C cold fill, derived from IAPWS-95 water density and matching published UK heating-supplier coefficients (Anglian PHE / CIBSE Guide G):
- 60°C: ~1.7% expansion
- 70°C: ~2.2% expansion
- 80°C: ~2.9% expansion (standard UK boiler)
- 90°C: ~3.6% expansion
- 100°C: ~4.3% 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
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.
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. For sealed system pressure settings, see our System Pressurisation Calculator.
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.
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. Use our System Pressurisation Calculator to check correct fill and operating pressures.
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.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.