How do I calculate the correct fill pressure for a sealed system?

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.

What should the expansion vessel pre-charge be set to?

The expansion vessel pre-charge (nitrogen side) should be set equal to the cold fill pressure of the system. Check the pre-charge with a tyre pressure gauge on the Schrader valve when the system is drained. Adjust if necessary using a foot pump.

How do I size an expansion vessel?

The vessel must accommodate the expansion of water when heated. Water expands by approximately 3.5% when heated from 10°C to 75°C. For a 120L system, expansion is about 4.2L. The vessel needs to be larger than this to maintain positive pressure — typically 8-12L for domestic systems.

Why does my boiler keep losing pressure?

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.

What is the safety valve setting for a sealed system?

Most domestic sealed heating systems use a 3 bar safety (pressure relief) valve. The maximum operating pressure when hot should remain below 2.5 bar to provide a safety margin. If pressure reaches 3 bar, the safety valve opens to protect the system.

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System Pressurisation Calculator — Fill Pressure & Expansion

Calculate correct fill pressure from static head and size the expansion vessel. Ensures safe system pressurisation for sealed heating systems.

Static Head

Height from fill point to highest radiator

System Volume

Total water volume in system

Flow Temperature

Maximum flow temperature

PRV Setting

Pressure relief valve setting (usually 3 bar)

How We Calculate This

This calculator determines the correct fill pressure, operating pressure range, and expansion vessel size for a sealed (pressurised) central heating system.

Key formulas

Cold fill pressure: (Static head ÷ 10) + 0.5 bar margin
Expansion volume: System volume × expansion factor (0.035 for 10-75°C)
Pre-charge pressure: Equal to cold fill pressure
Operating range: Cold fill to hot pressure (must stay below PRV)

The expansion factor of 0.035 (3.5%) applies for heating from approximately 10°C to 75°C. For lower flow temperatures (e.g. heat pump at 45°C), the expansion is less. Always ensure the hot operating pressure stays at least 0.5 bar below the safety valve setting.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.