Boiler shows A20, burner stops unexpectedly, flue gas safety device activated

Safety: Do not operate the boiler until the flue system has been checked. Risk of carbon monoxide.

• Inspect flue route: Verify that the flue is installed according to Herz guidelines, with correct lengths, bends, and terminations.
• Check for obstructions: Look for blockages at the terminal and accessible sections of the flue.
• Reset thermostat: Once the cause is removed and the boiler has cooled, reset the flue gas thermostat if it has a manual reset button.
• Persistent A20: Have a Herz technician perform a full flue integrity and combustion check and replace the thermostat if faulty.