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- 8. How to use this expansion vessel sizing calculator
- 9. How to size an expansion vessel
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- 11. Make your job easier withGas Engineer Software
- 1. Create certificates and manage your jobs with Gas Engineer Software
- 2. Other useful heating & plumbing calculators
- 3. Heating System Volume Calculator
- 4. Gas Rate Calculator
- 5. Gas Pipe Sizing Calculator
- 6. Heat Loss Calculator
- 7. Installation Volume Calculator
- 8. How to use this expansion vessel sizing calculator
- 9. How to size an expansion vessel
- 10. Frequently asked questions
- 11. Make your job easier withGas Engineer Software
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Expansion Vessel Sizing Calculator
Work out the correct expansion vessel size for a domestic sealed heating system or an unvented hot water cylinder. Built to the BS 7074 acceptance-factor method. Get the exact system volume from our Heating System Volume Calculator .
Don't know the system volume? Estimate it
Show workings & guidance
1 − (Pi+1) ÷ (Pf+1). Guidance only. Always cross-check against the vessel manufacturer's data and the requirements of BS 7074 / Building Regs Part G (G3) for unvented work, and round up to an available vessel size.
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How to use this expansion vessel sizing calculator
The calculator covers the two jobs you’ll actually meet on domestic work: sizing a potable vessel for an unvented hot water cylinder, and sizing a vessel for a sealed central heating system. Pick the right mode at the top and enter what you know.
- Choose your system type. Use Unvented hot water for a stored hot water cylinder, or Sealed heating for a central heating (radiator) circuit.
- Enter your figures. For unvented, you’ll need the cylinder capacity, the stored water temperature, the inlet (pressure-reducing valve) pressure and the expansion relief valve setting. For heating, enter the system water volume (or estimate it from the boiler output in kW or the number of radiators) plus the max flow temperature, system height, cold fill pressure and PRV setting.
- Add the built-in vessel (heating, optional). Most combi and system boilers already contain a small vessel. Enter its size and the calculator works out the supplementary vessel to fit on top, or confirms the built-in one is enough on its own.
- Read the result. You get a recommended vessel size, rounded up to a size you can actually buy, alongside the raw calculated figure and a few notes on pre-charge, working pressure and anything worth double-checking.
How to size an expansion vessel
Sizing a sealed-heating vessel and an unvented hot water vessel use the same method: work out how much the water expands, then size the vessel from the pressures it works between. Only four things actually differ — the rest is identical.
Some engineers may opt to use a nomogram to quickly size their expansion vessels, such as the one found in the CIPHE Low Temperature Heating and Hot Water Qualification Guide.
Where the two differ
| Sealed heating | Unvented hot water | |
|---|---|---|
| Water volume | Total system content (estimate as boiler kW × 12) | Cylinder capacity (off the data badge) |
| Expansion factor (e) | By temperature — 3.07% at 82 °C (BS 7074-1 uses 5% at the 110 °C fault condition) | Flat 4% |
| Safety reserve | +10% | None (the 4% already includes it) |
| Pressures (Pi / Pf) | Cold fill / (safety valve − 0.3) | PRV inlet ~3 bar / (relief valve − 0.5) |
Step 1 — Water volume (V)
Step 2 — Expansion volume (Va)
Multiply the volume by the expansion factor (e):
Heating takes e from the maximum temperature (BS 7074-1 recommends the 110 °C fault figure for conventional gas/oil systems; heat pumps use their actual flow temp):
| Max flow temperature | Expansion percentage (e) |
|---|---|
| 60 °C | 1.71% |
| 82 °C | 3.07% |
| 90 °C | 3.60% |
| 100 °C | 4.35% |
| 110 °C (fault basis) | 5.00% |
Unvented uses a flat e = 4% — the recognised potable figure that reproduces the standard cylinder charts.
Step 3 — Set the pressures (Pi, Pf)
Step 4 — Acceptance factor (a) — same for both
The fraction of the vessel that's usable capacity, with pressures converted to absolute (add 1 bar):
Step 5 — Vessel volume (Vt) — same for both
Heating: add a 10% reserve (Vt × 1.10). Unvented: no extra reserve. Either way, round up to the next available vessel size.
- Expansion volume: Va = 100 × 0.0307 = 3.07 L
- Final pressure: Pf = 3.0 − 0.3 = 2.7 bar
- Acceptance factor: a = [(2.7 + 1) − (1.0 + 1)] ÷ (2.7 + 1) = 1.7 ÷ 3.7 = 0.46
- Minimum volume: Vt = 3.07 ÷ 0.46 = 6.7 L
- Plus 10% reserve: 6.7 × 1.10 = 7.3 L → round up → 8 L vessel
- Expansion volume: Va = 210 × 0.04 = 8.4 L
- Final pressure: Pf = 6.0 − 0.5 = 5.5 bar
- Acceptance factor: a = [(5.5 + 1) − (3.0 + 1)] ÷ (5.5 + 1) = 2.5 ÷ 6.5 = 0.385
- Vessel volume: Vt = 8.4 ÷ 0.385 = 21.8 L → round up → 24 L vessel
Matches the recognised cylinder chart (210–250 L → 24 L).
Two extras worth knowing. On heating, most combi/system boilers have a built-in vessel — only fit a supplementary one if the system is bigger than it can handle (subtract the built-in size from the total). On unvented, this is notifiable G3 work, and the vessel goes on the cold feed downstream of the pressure-reducing valve.
Frequently asked questions
Does a combi boiler need an expansion vessel?
It already has one — built in for the central heating circuit — so on a standard combi you don't fit a separate vessel. You'd only add a supplementary one if the system holds more water than the built-in vessel can cope with, which tends to happen on larger properties with lots of radiators or long pipe runs. A combi makes hot water on demand, so there's no cylinder and no potable vessel to size.
What size expansion vessel do I need?
It depends on the water volume and the pressures, which is exactly what the calculator works out. As a guide, a heating vessel is often around 10% of system volume, and a potable vessel roughly follows the cylinder chart — about 24 litres for a 210-litre cylinder. Size it properly rather than guessing, and round up to an available size.
What's the difference between the cylinder and the expansion vessel?
The unvented cylinder is the large tank that stores and supplies your hot water. The expansion vessel is a much smaller, separate tank that holds no usable water — it just absorbs the expansion as the cylinder heats up, keeping the pressure in check. They work as a pair.
Can an expansion vessel be too big?
An oversized vessel isn't dangerous, but it costs more, takes up space and isn't necessary. An undersized one is the real problem — it can't absorb the full expansion, so system pressure climbs and the relief valve discharges. Size it correctly with a sensible reserve rather than going as big as possible.
What pre-charge pressure should the vessel be set to?
Match it to the system's cold fill pressure (heating) or the inlet / pressure-reducing valve setting (unvented hot water). If you're fitting a supplementary vessel alongside a boiler's built-in one, set both to the same pre-charge. Always check it before commissioning — vessels can lose charge sitting in storage.
Do I need to be G3 qualified?
For unvented hot water work, yes. Installing or working on an unvented cylinder is notifiable under Building Regulations Part G (G3) and requires the unvented hot water qualification — and sizing and fitting the expansion vessel is part of that job. Sealed heating work doesn't require G3.
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