Boiler shows F5 and shuts down, fan may run briefly then stop

Safety: Isolate electrical power before removing any covers. Flue integrity must only be checked and repaired by a qualified engineer.

• Check flue terminal outside: Ensure the flue outlet is not blocked by debris, bird nests, or snow. Clear any visible obstruction without dismantling the flue.
• Check condensate drain: A blocked condensate trap or pipe can cause flue gas recirculation. Inspect the visible condensate pipe for kinks or freezing and clear gently if blocked.
• Reset the boiler: After clearing any obvious blockage, reset the boiler and observe if F5 returns.
• Engineer tasks:
  • Test the air pressure switch for correct switching at specified pressures.
  • Inspect and replace any silicone pressure tubes that are split, kinked, or blocked with condensate.
  • Verify fan speed and operation and check for partial flue blockages within the casing.

Do not attempt to dismantle the flue or sealed combustion chamber yourself; incorrect reassembly can be dangerous.