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

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 = (P_max - P_fill) ÷ P_max (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.