Free tool
Expansion vessel calculator
Minimum vessel size for a sealed system, and the pre-charge to set it to.
Pressures are gauge readings. The calculator converts them to absolute by adding 1 bar.
The formula
Vt = (V × e) ÷ (1 − (Pi ÷ Pf))
V is the system water volume, e the expansion factor, Pi the absolute cold-fill pressure and Pf the absolute safety-valve pressure. Absolute means the gauge reading plus 1 bar, so a 1.0 bar cold fill is 2.0 bar absolute and a 3.0 bar safety valve is 4.0 bar absolute.
Worked example
100 litres, filling to 1.0 bar, 3.0 bar safety valve, expansion factor 0.04.
| Step | Working | Result |
|---|---|---|
| Expansion volume | 100 × 0.04 | 4.0 L |
| Absolute pressures | 1.0 + 1 and 3.0 + 1 | 2.0 and 4.0 bar |
| Minimum vessel | 4.0 ÷ (1 − 2.0/4.0) | 8.0 L |
That is a minimum. Fit the next vessel size up rather than the exact figure, and set the pre-charge to the cold-fill pressure with the water side drained so the gauge is reading the gas charge and not the system.
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Common questions
What size expansion vessel do I need?
Work it out from the system water volume, the cold-fill pressure and the safety-valve pressure. The formula is Vt = (V × e) ÷ (1 − (Pi ÷ Pf)) using absolute pressures, which are the gauge pressures plus 1 bar. A 100 litre system at 1.0 bar cold fill with a 3.0 bar safety valve needs at least an 8 litre vessel.
What should the expansion vessel pre-charge be set to?
Set the pre-charge equal to the system cold-fill pressure. If the system fills to 1.0 bar, charge the vessel to 1.0 bar with the water side drained down so the reading is honest.
What expansion factor should I use?
0.04 is the usual figure for a sealed heating system taken to around 80 °C, and it is what this calculator starts with. Use the figure the system designer or the manufacturer gives where you have one.
Why does my pressure keep climbing when the vessel is new?
A new vessel that is the wrong size, or charged to the wrong pressure, behaves like a failed one. Check the pre-charge against the cold-fill pressure with the system drained, and check the filling loop is fully closed and not passing.
Flint is a reference tool for competent, Gas Safe registered engineers. Everything here informs your judgement and never replaces it. Always work to the manufacturer's instructions and the current standards, and check every figure against the appliance data badge and manual.