Visible smoke leakage from furnace doors or joints into the boiler room

Safety: Smoke contains harmful gases. Ventilate the room immediately and avoid prolonged exposure.

• Inspect the door gaskets (burner door, ash door) for cracks, flattening, or missing sections. Replace if not sealing properly.
• Clean the heat exchanger passages and flue outlet according to the Defro maintenance schedule.
• Check the chimney cap and flue for obstructions; have a chimney sweep clean the flue if heavily sooted.
• Verify that combustion fan settings match the chimney characteristics recommended by Defro; avoid reducing fan speed too much.

If smoke leakage persists, stop using the furnace and have the flue system inspected by a professional.