Checks the pressure chain for a sealed heating or unvented hot-water system: cold-fill pressure, vessel pre-charge, and the final pressure a selected
expansion vessel reaches at max temperature, against the safety-valve margin.
Use the Expansion Vessel Sizing calculator first for the expanded volume (Ve).
For design guidance only. Always verify with a qualified engineer.
About this pressurisation unit calculator
This free pressurisation unit calculator works out the pressure chain for a sealed heating or unvented hot-water system, the cold-fill pressure, the expansion vessel pre-charge, and the final pressure the vessel reaches at maximum operating temperature. And checks that final pressure against the safety-valve margin. It is aimed at building-services designers and heating engineers selecting a pressurisation unit or expansion vessel and confirming there is enough headroom below the safety-valve lift. Run the Expansion Vessel Sizing calculator first for the expanded volume (Ve). Everything runs in your browser. Nothing is uploaded.
How the pressurisation check is calculated
The static pressure is the system height converted at an exact 10.197 m/bar (P ÷ ρg, with ρ = 1000 kg/m³ and g = 9.80665 m/s²), so the cold-fill pressure = static pressure + a 0.3 bar venting margin and the vessel pre-charge equals the cold-fill pressure (Flamco / Grundfos pressurisation guidance, conforming to BS 7074). The final pressure at maximum temperature uses the Boyle’s-law acceptance-factor relation Pfin,abs = P₀abs ÷ (1 − Ve ÷ Vv) for the selected vessel volume (Vv). The safety-valve margin is a fixed 0.5 bar for a setting ≤ 5 bar, otherwise 10% of the setting (Reflex, citing EN ISO 4126), and the check passes when the final pressure stays within setting − margin.
Frequently asked questions
Where do I get the expanded volume (Ve)?
Ve is the volume the system water expands by between cold fill and maximum temperature, and it has no default here because it depends on your system water content and temperature range. Calculate it with the Expansion Vessel Sizing calculator first, then enter it. The pressurisation check needs it to work out the final pressure the vessel reaches.
Why does an undersized vessel give no final pressure?
The Boyle’s-law relation divides by (1 − Ve ÷ Vv), so if the vessel volume is not greater than the expanded volume the term collapses and there is no valid pressure. The calculator flags the vessel as undersized instead, the expanded water reaches the vessel’s capacity before maximum temperature, so you need a larger vessel, often the next standard size up.
How is the safety-valve margin decided?
The final pressure must sit safely below the safety-valve setting so the valve does not lift in normal operation. The margin applied is a fixed 0.5 bar for settings of 5 bar or less, or 10% of the setting above that (Reflex calculation sheet, citing EN ISO 4126). This is design guidance only. Always verify the final selection with a qualified engineer.