Furnace indicates flue gas recirculation or CO risk; unit shuts down

Safety: Turn off power and gas immediately. Ventilate the area. If a CO alarm has sounded, evacuate and call emergency services.

• Inspect terminations: Outside, verify that the IVT flue outlet and air intake are correctly spaced and not damaged. Repair or replace any cracked or separated concentric flue components.
• Check room ventilation: Ensure required combustion air openings to the room are not blocked or undersized.
• Assess building pressure: Excessive use of exhaust fans can create negative pressure; reduce their use or provide additional makeup air.
• Professional testing: Have a certified technician perform combustion and draft tests to confirm safe operation before returning the furnace to service.

Do not operate the furnace until the cause of flue gas recirculation is fully resolved.