Viessmann [F.59] fault codewhat it means and how to fix it

F.59 reads "Burner locked out, boiler circuit pump off. No central heating, no DHW heating — Power supply, undervoltage — Check mains voltage. If voltage is correct and the fault occurs repeatedly, replace the fan unit." The B1HG table gives the cause as "Undervoltage, HBMU heat management unit" and the 050-W B0HA/B0KA and B1GA as "Undervoltage, power supply", with the same measure. The boiler's electronics need a supply within tolerance to drive the gas valve and fan safely, and when the mains sags — a brown-out, a long extension of the house wiring, a loose neutral, a neighbour's welder — the unit stops rather than run the burner on a supply it cannot trust. It should recover when the voltage does; the row lists no reset.

The fan unit appearing as the remedy surprises people. On these boilers the fan carries its own drive electronics and reports back to the heat management unit, and a fan whose electronics have begun to fail can present as a supply that keeps dipping. Viessmann make the contractor measure the mains first, and only replace the fan if the supply is proven good and the fault persists. There is no householder step in the operating instructions for F.59 itself; the "heating system is off" checks — ON/OFF switch, mains isolator, the MCB — are for a boiler with no power at all, not one complaining about low voltage. If lights in the house dim when large appliances start, the supply is the suspect and the electricity network operator, not the heating contractor, may be who to call. The two-character B1HC/B1KC reports the same condition as 0C, "Insufficient power supply".