Boiler pressure (on sealed systems) rises rapidly when heating and may discharge through safety valve.

Safety first: Hot water may discharge from the safety valve. Keep clear of the discharge pipe when the boiler is hot.

• Check cold pressure: When the system is cold, verify that pressure is around 1.0–1.5 bar. If higher, drain some water from a drain cock to reduce pressure.
• Test expansion vessel: With the system depressurised, check the air charge on the expansion vessel using a tyre gauge. It should match the manufacturer's specification (often around 0.8–1.0 bar). Repressurise with a pump if low.
• Tap expansion connection: Ensure the pipe between the system and expansion vessel is not blocked. A blocked connection will prevent expansion from being absorbed.
• Check safety valve: If the valve has lifted repeatedly, it may not reseat properly and could drip. Replace if necessary.
• Monitor pressure rise: After adjustments, run the boiler and observe pressure. If it still rises excessively, the expansion vessel diaphragm may be ruptured and the vessel must be replaced.