Expansion vessel size calculator
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Water in the system | 100 L |
| Maximum water temperature | 80 °C |
| Expansion factor e | 0.029 |
| Vessel pre-charge pressure | 1 bar g |
| Maximum working pressure | 2.5 bar g |
Vessel: at least 6.77 L (indicative)
| Result | Value |
|---|---|
| Vessel size needed (indicative) | 6.77 L |
| Expansion volume | 2.9 L |
| Acceptance factor | 0.429 |
| Expansion factor used | 0.029 |
Show the working
- Acceptance factor: 1 − (1 + 1) ÷ (2.5 + 1) = 0.429
- Expansion: 100 L × 0.029 (your figure) = 2.9 L
- Vessel: 2.9 L ÷ 0.429 = 6.77 L
How it works
- Expansion volume = system volume × e (the fraction the water expands by from cold to the maximum temperature)
- Acceptance factor = 1 − (pre-charge + 1) ÷ (maximum pressure + 1), using absolute pressures (gauge + 1 bar)
- Vessel size = expansion volume ÷ acceptance factor, then the next stock size up
For a heating system with no measured volume, ask the installer: radiator and boiler makers give water contents.
Indicative only: the installer sizes the vessel to the maker's instructions and sets the pre-charge.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
This calculator has no built-in defaults: every figure comes from you or your product's data.
Regulations and standards
- BS 7074-1 (expansion vessels for sealed systems; Researcher to cite)
- Building Regulations Part G3 (unvented hot water storage)
Explained: Approved Document G (hot water safety).
The sizing method is the same across the UK; building regulations for unvented systems differ by nation.
| Table | Clause | Nations | Status |
|---|---|---|---|
| Expansion of water from cold fill to the stated temperature TODO: Researcher to add the maker or standard URL | BS 7074-1 / water property tables (Researcher to fill and cite) | UK-wide | 0/9 values entered Not yet checked against the source |
| Expansion vessel stock sizes TODO: Researcher to add the maker or standard URL | Expansion vessel makers' ranges (Researcher to fill from at least two UK makers) | UK-wide | 0/8 values entered Not yet checked against the source |