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.
Match the pressure fields to the system type. The defaults suit a typical unvented cylinder (about 3 bar cold pressure, 6 bar relief valve). A domestic sealed heating system is usually closer to 1 bar cold fill with a 3 bar relief valve, so change both fields when sizing heating.
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.
A 300 litre unvented cylinder, 3 bar cold pressure, 6 bar relief valve and 0.5 bar margin gives about 6 litres of water expansion. With an acceptance factor around 38.5%, minimum vessel volume is about 15.6 litres, so the next common vessel size is 18 litres or larger.
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 |
| Cold pressure / pre-charge | PRV setting or cold fill pressure | 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 common 3 bar cold pressure and 6 bar relief-valve assumptions, the calculation gives roughly 15.6 litres minimum, so an 18 litre vessel or larger is a typical next common selection. Check the cylinder manufacturer data.
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.
For the usual revision method, pre-charge is set around the cold fill or pressure reducing valve setting. It must be checked with the water side isolated and depressurised.
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
Training & revision aids — live installations follow the full standard, the manufacturer’s instructions and calibrated instruments.