BoilerFaultCodesTechnical description
Flue non-return flap closed. Flue non-return flap return signal blocks burner operation (only in conjunction with the multi-functional module) or condensate pump defective, heat demand is blocked.
What causes this fault?
This fault occurs when the boiler's safety systems detect that waste gases cannot exit the flue or waste water cannot be pumped away. The most common cause is a failure in the external condensate pump or a physical blockage in the non-return flap, which triggers a shutdown to prevent water damage or a buildup of exhaust fumes inside the unit.
This fault is more common during the winter, as external condensate discharge pipes can freeze and cause water to back up into the pump or boiler.
Engineer required
This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.