Boiler shows A09, intermittent lockouts, sometimes after long operation

Safety: High flue gas temperatures can damage the boiler and flue. Investigate promptly.

• Check system flow: Ensure all radiator valves are open enough to allow adequate flow. Verify that any zone valves are operating correctly.
• Pump speed: On Hoval boilers with adjustable pump speed, set the pump to a higher speed to improve flow.
• Clean system: If radiators are cold at the bottom or system water is very dirty, sludge may be restricting the heat exchanger. A professional power flush or chemical clean may be required.
• Flue gas sensor: Inspect the flue gas temperature sensor on the flue outlet. Check wiring and replace the sensor if readings are clearly abnormal compared to boiler water temperature.