Boiler fan runs at incorrect speed or does not respond properly; may cause ignition failure or noisy operation.

Safety: Isolate power and gas. Wait for the boiler to cool. Working on the combustion fan involves the sealed combustion chamber; only competent persons should proceed.

Inspection steps:

• Access the fan: Remove the front cover and inner combustion cover as per Bosch instructions. Locate the fan assembly attached to the gas valve and burner.
• Check for obstructions: Inspect the fan impeller for debris, soot, or signs of water ingress. Gently rotate the impeller by hand; it should spin freely without grinding.
• Inspect wiring: Check the fan connector and harness back to the PCB. Look for heat damage, loose plugs, or broken conductors. Reseat the connector.
• Test fan operation: With covers refitted and safe, restore power and call for heat. Listen for the fan starting and ramping up. If the fan hums but does not spin, or does not start at all, the fan motor is likely faulty.

Professional repair: Fan replacement and any adjustment of combustion settings must be carried out by a Gas Safe / certified Bosch engineer, as it directly affects flue gas safety.