Mechanical

Expansion Vessel Calculator

Estimate expansion vessel size for sealed heating systems from system volume, water expansion and working pressures.

Use this calculator for preliminary sealed system checks where system water content and operating pressure range are known. The expansion factor can either be entered manually or estimated from cold fill temperature and maximum system temperature.

Inputs

Enter the estimated volumetric expansion of the system water across the operating temperature range.
This should be established from static head, vapour pressure and safety margin in accordance with project design.
Typically based on the allowable working pressure, safety valve setting and selected operating range.

Result

Expansion Vessel Size
-
Enter values and click calculate.

Method

Expansion volume is estimated from the system water content and selected expansion factor:

Vₑ = Vₛ × e

A reserve volume is then added:

Vᵣ = max(0.5% of Vₛ, 3 L)

Total acceptance volume is:

Vₐ = Vₑ + Vᵣ

The vessel acceptance factor is estimated from the pressure range:

a = (Pmax - Pmin) ÷ (Pmax + 1)

Minimum vessel size is then:

Vv = Vₐ ÷ a
  • Vₛ = system water volume (L)
  • e = water expansion factor
  • Vₑ = expansion volume (L)
  • Vᵣ = reserve volume (L)
  • Vₐ = total acceptance volume (L)
  • a = vessel acceptance factor
  • Vv = minimum vessel size (L)
  • Calculated result = -

Reference

  • This calculator estimates nominal expansion vessel volume from system water content, thermal expansion and the selected working pressure range.
  • Relevant guidance may include BS EN 12828 for heating system design and CIBSE AM14 for expansion and pressurisation vessel sizing principles.
  • The reserve allowance shown reflects commonly adopted building services practice for sealed heating systems.
  • Where applicable, final design should also align with project specifications, manufacturer requirements and selected pressurisation equipment arrangements.
Design note: This calculator is an indicative estimating tool only. In auto mode, the expansion factor is estimated from water density change between fill temperature and maximum temperature. Final design should confirm the correct system pressures, operating temperatures, reserve allowance and selected vessel against project requirements and manufacturer data.

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