PlumbReviseEstimate the vessel volume needed to accept thermal expansion in an unvented cylinder or sealed heating system.
Estimate vessel volume from water volume, expansion and pressure range.
Method: Part G / BS EN 12828-style vessel sizing
An expansion vessel works because the gas side compresses as heated water expands. The useful capacity depends on the pressure range, not just the printed litre size on the vessel.
The calculator first estimates expansion volume: water volume x expansion percentage. The unvented-cylinder preset uses a 10C to 65C potable-water allowance (about 2%); the sealed-heating preset uses a 10C to 90C allowance (about 3.6%) and includes a small reserve volume.
It then calculates the acceptance factor from absolute pressures: 1 - initial absolute pressure / final absolute pressure. The final pressure is kept below the relief-valve setting by the margin you enter.
The initial gas pressure is the vessel pre-charge used in this simplified calculation. The defaults of 3 bar pre-charge and a 6 bar relief valve form an unvented-cylinder example. Heating needs its own pressure assumptions: do not carry the cylinder defaults into a boiler calculation.
Minimum vessel size = (expansion volume + reserve volume) / acceptance factor. Select a real vessel above the result, then check the cylinder or boiler manufacturer instructions, vessel data sheet, pre-charge and safety valve arrangement.
For a 300 litre unvented-cylinder example, assume 2% expansion, 3 bar pre-charge, a 6 bar relief valve and a 0.5 bar margin. Expansion is 300 × 0.02 = 6 litres. Initial absolute pressure is 3 + 1 = 4 bar; final absolute pressure is 6 − 0.5 + 1 = 6.5 bar. Acceptance = 1 − 4 ÷ 6.5 ≈ 0.3846. Minimum vessel volume = 6 ÷ 0.3846 ≈ 15.6 litres, so 18 litres is the next common size to check against the manufacturer’s requirements.
Pre-charge is the gas-side pressure with the water side depressurised. Cold-fill pressure is the water-side pressure after filling. A heating-system design may specify a fill pressure above pre-charge so some reserve water enters the vessel. Enter the initial gas pressure required by the sizing method, rather than assuming the boiler gauge reading is the pre-charge.
Use absolute pressures in the acceptance calculation by adding approximately 1 bar to gauge pressures. Using 3 ÷ 5.5 instead of 4 ÷ 6.5 in the example above would exaggerate the vessel’s usable capacity. The pressure margin and expansion allowance are inputs to check, not universal settings for every cylinder.
A heating vessel accepts expansion from the sealed primary circuit. A potable-water vessel serves the stored domestic water. Check the vessel’s stated application, water-contact suitability, temperature and pressure ratings; the litre size alone does not establish suitability.
For heating, include water in the radiators, boiler, pipework and any buffer or other connected component. For an indirect cylinder, the water inside its heating coil belongs to the primary circuit; the domestic water around that coil is a separate volume. Do not add the whole cylinder capacity to the heating circuit just because the boiler heats it.
Trace the domestic-water side on the unvented cylinder diagram before entering its volume. For a sealed heating circuit, use the separate system-design examples in Level 3 heating revision.
Use this checklist before choosing a vessel from manufacturer data. The calculator gives a minimum volume, not a product approval.
| Check | Record | Why it matters |
|---|---|---|
| System type | Potable unvented or sealed heating | Controls expansion preset and reserve allowance |
| Water volume | Cylinder volume or measured system content | Expansion scales with volume |
| Initial gas pressure / pre-charge | Specified gas-side pressure with water side depressurised | Sets initial absolute pressure |
| Relief valve setting | Safety valve rating | Sets the upper pressure limit |
| Margin below relief | Usually kept below lift pressure | Avoids normal operation opening the valve |
| Selected vessel | Manufacturer model and acceptance volume | Real vessel must exceed the calculated need |
Set and verify pre-charge with the vessel isolated and depressurised on the water side. Final selection follows manufacturer instructions and the applicable hot-water/heating standard.
With 2% expansion, 3 bar pre-charge, a 6 bar relief valve and a 0.5 bar margin, the calculation gives roughly 15.6 litres minimum. An 18 litre vessel is the next common size to check against the cylinder manufacturer data; changing the pressure assumptions changes the result.
The vessel has to accept expanded water within a pressure range. Higher cold pressure or a smaller gap to the relief valve reduces acceptance, so the vessel may need to be larger.
Follow the system and vessel manufacturer instructions. Pre-charge and cold-fill pressure are distinct: some heating designs specify a higher fill pressure to provide a water reserve. Pre-charge is checked with the water side isolated and depressurised, not by reading the filled system’s pressure gauge.
Radiator count alone does not give the system water volume. Ten compact panels and ten large cast-iron radiators can contain very different amounts of water. Add the radiator, boiler and pipework volumes, then use the relevant expansion percentage, pre-charge, reserve and relief-valve setting.
It can estimate sealed-heating vessel volume when you know the system water content and pressure range. Boiler-integral vessels and replacements still follow the boiler manufacturer instructions.
Method: Part G / BS EN 12828-style vessel sizing
Something wrong, unclear or out of date? Report an issue with this calculator.
Put the system principles into practice with 25 free Central heating questions, including answers and explanations.
Training & revision aids — live installations follow the full standard, the manufacturer’s instructions and calibrated instruments.