Vaillant [F.1120] fault codewhat it means and how to fix it

F.1120 is the one heater fault Vaillant explain to owners directly rather than only in the installer appendix. The indoor unit, they write, has an internal circuit breaker which switches off the heat pump in the event of short circuits or the failure of one current-carrying phase on a 230 V unit, or several on a 400 V unit. When the breaker has opened on the electric back-up heater, the display shows F.1120, and Vaillant’s instruction to the owner is to inform the competent person so that they can eliminate the fault and reset the internal circuit breaker.

They add a consequence worth taking seriously: if the electric back-up heater is defective, the anti-legionella function is not always guaranteed, because heat pump systems rely on that heater to take the cylinder above 60 °C. The installer appendix gives the causes as a defective back-up heater, poorly tightened electrical connections and mains voltage that is too low, with remedies of checking the heater and its power supply, checking the connections, and measuring the voltage at the heater’s connection.

Vaillant UK’s owner page widens the supply-side list — incoming mains outside tolerance, incorrect phases, reverse polarity, poor earthing, an unstable supply — and says this is a case for the installer in the first instance because it can relate to the power supply, with Vaillant engineers for the heater itself. Two everyday versions are worth knowing. On many installations the back-up heater has its own supply and its own breaker in the consumer unit, and that breaker being off produces a phase failure with nothing defective at all. And a loose connection at a heater terminal heats up under the heater’s current, which is why Vaillant list tightness of connections before the heater itself.