Display shows E9; the furnace will not operate.

Safety first: Allow the system to cool before adding water. Hot water can cause scalding.

• Check pressure gauge: Read the system pressure on the front gauge or digital display. If below the minimum (often 0.8–1.0 bar), the furnace will lock out.
• Top up system: Connect the filling loop (if external) and slowly open the valves to raise pressure to the recommended cold value (typically 1.2–1.5 bar). Close the valves securely afterward.
• Inspect for leaks: Walk around the property and check radiators, valves, and visible pipework for signs of water leakage. Repair any leaks found.
• Reset furnace: After pressure is restored, reset the furnace according to the user manual.
• Sensor or vessel issues: If pressure is correct but E9 persists, the water pressure sensor may be faulty, or the expansion vessel may be waterlogged. A technician should test and replace the sensor or recharge/replace the vessel.