System Pressurisation Calculator — Fill Pressure & Expansion

Measure the static head — the vertical distance from the filling point to the highest point of the system (usually the highest radiator). Divide by 10 to get bar (1 bar = 10 metres head), then add 0.3-0.5 bar safety margin. For example: 7m static head = 0.7 bar + 0.5 = 1.2 bar cold fill pressure.

Per Grant UK guidance, the air pre-charge (the gas side, behind the diaphragm) should be set to the system static head pressure at the vessel — roughly 0.1 bar per metre of height to the highest point — with typical air charge pressures of 0.5 bar for a bungalow and 1.0 bar for a two-storey house. The cold fill pressure must always sit ABOVE the pre-charge so that water enters the vessel and there is a positive working margin — this general requirement comes from BS EN 12828. The practical figure is Grant UK's: the cold fill should be about 0.2 bar greater than the air charge (Grant UK's example: a 1.0 bar pre-charge gives a 1.2 bar cold fill). This calculator adds your safety margin (0.5 bar by default) to the static head pressure to set the cold fill, which keeps it comfortably above the pre-charge — for the 7 m example, a 0.7 bar pre-charge with a 1.2 bar cold fill (a 0.5 bar gap). Check the pre-charge with a tyre pressure gauge on the Schrader valve when the system is drained and depressurised, and adjust with a foot pump if needed.

The vessel must accommodate the expansion of water when heated. Water expands by approximately 2.6% when heated from 10 °C to 75 °C (per water density tables: 0.9997 to 0.9749 g/cm³). For a 120 L system, expansion is about 3.1 L. The vessel needs to be larger than this to maintain positive pressure — typically 8–12 L for domestic systems. Use our Expansion Vessel Sizing Calculator for a precise calculation, and our Central Heating Volume Calculator if you need to find your system volume.

Common causes include: a failed expansion vessel (check pre-charge), a leaking pressure relief valve (check discharge pipe for dripping), microscopic leaks at joints, or a leak on the system side of the filling loop. A waterlogged expansion vessel is the most common cause.

Most domestic sealed heating systems use a 3 bar safety (pressure relief) valve. The hot operating pressure should sit around 2.5 bar — about 0.5 bar below the 3 bar PRV — to provide a safety margin. If pressure reaches 3 bar, the safety valve opens to protect the system. If your cold fill plus the heating rise would reach the PRV, fit a higher-rated valve or reduce the cold fill pressure.