Boiler displays E09, pressure reading is very high or fluctuates rapidly.

Safety: High pressure can cause hot water discharge. Allow the system to cool before working on it.

• Check pressure cold vs hot: Note the pressure when the system is cold and again when hot. If it rises above 2.5–3.0 bar when hot, the expansion vessel may be faulty.
• Reduce pressure: Bleed water from a radiator or use the drain valve to bring pressure back to about 1.2 bar when cold.
• Check expansion vessel charge: Isolate the boiler from the system and drain pressure to 0 bar. Measure the air charge at the expansion vessel Schrader valve (usually 0.8–1.0 bar). Re-pressurize with a pump if low, or replace the vessel if water comes out of the valve.
• Inspect PRV outlet: Check if the pressure relief valve discharge pipe is dripping. If it leaks continuously, the PRV may be damaged and should be replaced.
• Restart and monitor: Refill the system to correct pressure and observe operation. If E09 recurs, have a technician evaluate the expansion system.