Viessmann [F.74] fault codewhat it means and how to fix it

F.74 is the low-pressure row of the numeric table used by every Vitodens with the black-and-white arrow-key programming unit: 100-W B1HF/B1KF, B1HG/B1KG and B1GA, 111-W B1LF and B1LG, 050-W B0HA/B0KA. The row reads "Burner locked out. Internal circulation pump off. No central heating and no DHW heating — System pressure too low — Top up with water. Vent the system." It is a lockout of the waiting kind: no "Reset the appliance" appears in the measures, and in our reading the boiler releases itself once the sensor sees enough pressure, because the pressure sensor is what is holding it. Only if the fault occurs repeatedly does Viessmann send the contractor further: check the system pressure sensor against an external gauge, check the pre-charge pressure of the diaphragm expansion vessel, and check the set system pressure and range in the parameters.

This is one of the few rows where the operating instructions give the householder the fix. On switching on, the 100-W/111-W and 050-W operating instructions say to scroll with the arrow key until the pressure-gauge symbol lights, and "If the pressure shown is below 1.0 bar: Please top up with water or notify your heating contractor." The filling method depends on how your installer plumbed the boiler — usually a filling loop with two valves under it — and the instructions assume the contractor showed you. A boiler that needs topping up more than once or twice a year is losing water somewhere, or its expansion vessel has lost its air charge, and that is the repeat-fault branch of the row, which is the contractor's. The two-character units report the same condition differently: the WB1A as E6, the WB1B to B1HC by flashing the pressure reading itself. F.87 is the opposite fault, pressure too high, and F.471 is the pressure sensor failing outright.

1. 1

   Press the arrow key repeatedly until the pressure-gauge symbol lights and read the pressure. Viessmann's operating instructions say that if it is below 1.0 bar you may top up with water or notify your heating contractor.

   Top up only if you were shown how at handover and know where the filling loop and its valves are. Fill slowly, cold, to the pressure your installer set (typically 1.0 to 1.5 bar), and close both valves afterwards.
2. 2

   Bleed any radiators that are cold at the top, then check the pressure again and top up if it has dropped. "Vent the system" is the second half of Viessmann's own measure.

3. 3

   If F.74 has not cleared once pressure is restored, use the burner reset for these units: arrow keys to show the fault number, then hold the up and down arrows together for about four seconds until the rotating bar appears. If the code comes back within days, stop topping up and call a Gas Safe engineer — the water is going somewhere.