Boiler displays AF or Air Flow Fault; unit will not fire or shuts down shortly after starting the fan.

Safety first: Shut off power before inspecting venting. Never operate the boiler with vent pipes disconnected or modified from the approved Laars installation.

• Check vent terminations: Outside the building, inspect the Laars boiler intake and exhaust terminations. Remove snow, ice, leaves, or other obstructions. Ensure screens are clean.
• Inspect vent slope: For condensing Laars models, verify that horizontal vent runs slope back toward the boiler as specified. Sagging pipes can trap condensate and restrict airflow.
• Check condensate trap and drain: A blocked condensate trap can cause water to back up into the combustion chamber or vent. Clean the trap and ensure the drain line is clear and properly routed.
• Reset and test: After clearing any obstructions, restore power and reset the boiler. If AF returns, the airflow sensor or pressure switch may be faulty.

When to call a professional: Persistent AF faults after vent and condensate checks require a technician to verify vent sizing, combustion setup, and sensor operation.