Boiler Fault Code A2:1

Also written as A21 — same fault.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026

Fault code A2:1 appears across 1 boiler brand and 5 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler is temporarily prioritizing the heating of your hot water over your radiators, meaning your heating may feel cool for a short period while the water tank refills. (As reported on Viessmann Vitodens 300-W)