How do I size an expansion vessel?

Enter system water volume, max/min system temperatures, and pre-charge pressure. The vessel must accept the expanded water volume at max temperature within pressure limits.

What pre-charge pressure should I use?

Typically static head at vessel location plus 0.3–0.5 bar margin. Too low causes early safety valve lift; too high reduces usable acceptance volume.

Do I need a separate vessel for DHW?

Yes — DHW expansion is sized separately from heating/chilled water circuits. Combined systems need independent expansion provision per circuit.

BuildFlo Design Guide · Water · BS 7074

Sealed system expansion vessel calculator

Size expansion vessels for DHW, LTHW or CHW per BS 7074. Free expansion vessel sizing calculator.

What this calculator does

Expansion vessel helps UK building services engineers with bs 7074 sealed expansion vessel sizing for dhw, lthw or chw. The calculation follows BS 7074 methods used in everyday MEP design — enter your project inputs below for a first-pass result.

Whether you searched for "expansion vessel", "expansion vessel calculator", or "how to expansion vessel", this page gives you a working tool plus the context to use it on a real job.

When to use it

Use this for early-stage sizing and sanity checks on UK building services work. It is not a substitute for issued calculations — verify against the project specification, drawings, and relevant standard before sign-off.

How to use it

1. Fill in the inputs for your scenario (defaults are starting points only). 2. Click Calculate — results include the method and assumptions where applicable. 3. Verify against the project specification and relevant standard before issuing.

Typical output unit: litres.

Expansion vessel calculator

BS 7074 sealed expansion vessel sizing for DHW, LTHW or CHW

SystemVolume entryTotal system volume (litres)

Vs including contents margin if already applied

Pipework volume (litres)Calorifier / cylinder (litres)Boiler / heat source (litres)Buffer / thermal store (litres)Emitters (rads / UFH / FCUs) (litres)Other equipment (litres)

Coils, AHUs, heat exchangers, etc.

Contents margin

e.g. 0.1 = 10% (applied when building from parts)

Max system temperature (°C)

For expansion fraction lookup

Fill / cold temperature (°C)FluidGlycol concentration (%)

Approximate uplift vs water — prefer chart override below

Expansion fraction e (override)

Use for glycol mixture charts

Static height (m)Pressure margin (bar)Initial pressure Pi (override) (bar)Final design pressure Pf (bar)

Typically 0.5 bar below lowest equipment WP

Vessel selection margin

e.g. 0.2 = 20%

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Tips

Defaults are indicative — replace with project-specific values from the architect, structural, or MEP spec.
For issued calculations, save results in your BuildFlo project and attach to the calculation folder.
Need the regulation citation? Open BuildFlo chat in the app and ask about expansion vessel under UK standards.

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Frequently asked questions

How do I size an expansion vessel?: Enter system water volume, max/min system temperatures, and pre-charge pressure. The vessel must accept the expanded water volume at max temperature within pressure limits.
What pre-charge pressure should I use?: Typically static head at vessel location plus 0.3–0.5 bar margin. Too low causes early safety valve lift; too high reduces usable acceptance volume.
Do I need a separate vessel for DHW?: Yes — DHW expansion is sized separately from heating/chilled water circuits. Combined systems need independent expansion provision per circuit.