BoilerFaultCodes F21 High
Boiler Fault Code F21
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code F21 appears across 2 boiler brands and 7 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning Your heat pump is having trouble communicating with or receiving power from its outdoor unit, which means your system cannot generate heating or hot water until this connection is restored. (As reported on Ideal Logic Air)
Pick your boiler brand for the F21 fix
Source
Information for the F21 fault code is sourced from the official service / installation manual for each model that displays it. Read more about how we source fault codes →
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026