Boiler shows H4; burner shuts down, often with flue-related issues

Safety: Risk of carbon monoxide. Turn off the boiler and ventilate the area. Do not use the boiler until the flue system is checked.

• 1. External flue check: Inspect the flue terminal for obstructions and ensure correct clearances from walls, windows, and other structures.
• 2. Condensate system: Check that the condensate trap and pipe are not blocked or frozen, which can cause flue gas backup.
• 3. Flue joints: Have a technician inspect all accessible flue joints for disconnection, sagging, or water accumulation.
• 4. Flue gas sensor: The flue gas temperature sensor should be checked for correct mounting and resistance. Replace if faulty.
• 5. Professional inspection: A certified Buderus technician should perform a full flue and combustion safety check before returning the boiler to service.