Flue gas temperature too high, boiler shuts down with flue overheat fault

Safety: Risk of flue gas spillage. Shut down the boiler and do not operate until the cause is identified.

• Check flue terminal: Inspect the external terminal for obstructions and clear them.
• Verify combustion air: Ensure the air intake is not blocked and that the boiler room has adequate ventilation if using open combustion.
• Reset once: After basic checks, reset the boiler. If F25 returns, do not continue to operate.
• Professional service: A Viessmann technician should clean the heat exchanger, check the flue system, and perform a combustion analysis to verify correct gas/air ratio.