Assumes ~4% water expansion. Vessel pre-charge should match the cold fill pressure.
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This free expansion vessel calculator gives you the minimum vessel size for a sealed heating system, so the system can absorb thermal expansion without lifting the pressure relief valve. It is built for UK heating engineers sizing a replacement vessel, adding an external vessel to a system that has outgrown the boiler's internal one, or designing a new sealed system.
Enter the total system water volume in litres, the cold fill pressure and the maximum working pressure in bar. The calculator assumes the water expands by about 4% when heated from 10°C to 85°C, then applies the standard acceptance-factor formula using absolute pressures to find the vessel size that can accept that expansion between fill and maximum pressure.
Minimum vessel size (L) = (0.04 × system volume) ÷ (1 − Pi ÷ Pf), where Pi = fill + 1 and Pf = max + 1
0.04 × system volume is the expansion volume — the roughly 4% growth in water volume from 10°C to 85°C. Pi and Pf are the cold fill and maximum working pressures converted to absolute bar by adding 1 to the gauge readings. The term (1 − Pi ÷ Pf) is the vessel's acceptance factor: the usable fraction of its total volume between those two pressures. Dividing the expansion volume by the acceptance factor gives the minimum vessel size in litres.
Enter the total water content of the system in litres. If you don't know it, estimate it from the radiator count with our system volume calculator, or use around 10 litres per kW of boiler output as a rough starting point.
Enter the cold fill pressure in bar — typically 1 bar on a domestic sealed system. The vessel's pre-charge should match this figure, so check and adjust the vessel charge with the system side depressurised.
Enter the maximum working pressure in bar, which must be above the fill pressure and below the pressure relief valve setting. The default of 2.5 bar suits the common 3 bar PRV with a sensible margin.
The calculator shows the expansion volume in litres and the minimum vessel size. Choose the next standard vessel size up — a slightly larger vessel is never a problem, but an undersized one causes pressure swings and PRV discharge.
The calculation assumes water expands by roughly 4% between 10°C and 85°C, which covers a conventional sealed heating system heated to normal flow temperatures. The acceptance-factor method then accounts for the fact that a vessel can only usefully absorb expansion between its pre-charge pressure and the system's maximum working pressure — which is why vessel size rises sharply as the gap between fill and maximum pressure narrows.
The vessel pre-charge should match the cold fill pressure. A vessel charged below fill pressure is already partly compressed before the system heats up, cutting its usable capacity; one charged well above fill pressure won't accept expansion until the system pressure rises to meet it. Always check the charge with the vessel isolated or the system drained to zero gauge pressure.
Symptoms of an undersized or failed vessel include the pressure gauge climbing steeply as the system heats and the pressure relief valve discharging. If a combi's internal vessel is too small for a system's actual volume — common after adding radiators — fit an additional external vessel sized with this calculation.
Assumes ~4% water expansion. Vessel pre-charge should match the cold fill pressure.
Size it from the system water volume and pressures: minimum size = (0.04 × volume) ÷ (1 − Pi/Pf), using absolute pressures (gauge + 1). For a 100-litre system filled to 1 bar with a 2.5 bar maximum, that gives 4 L of expansion and a minimum vessel of about 9.3 litres — so you would fit the next standard size up.
The pre-charge should match the system's cold fill pressure — typically 1 bar on a domestic sealed system. Check it with the system side at zero gauge pressure (vessel isolated or system drained), because checking against a pressurised system gives a false reading. Recheck the charge at service, as vessels lose charge over time.
The system pressure climbs sharply as the water heats, and once it reaches the pressure relief valve setting the PRV discharges water. The system then reads low when cold, gets topped up, and the cycle repeats — repeated fresh-water top-ups drag in oxygen and accelerate corrosion. Fitting a correctly sized (or additional) vessel breaks the cycle.
This calculator assumes about 4% expansion as the water heats from 10°C to 85°C — so a 100-litre system generates roughly 4 litres of expansion. The vessel has to be bigger than the expansion volume itself because only a fraction of its capacity (the acceptance factor) is usable between fill and maximum pressure.
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