Viessmann [F1] fault codewhat it means and how to fix it

F1 is "Burner in a fault state — Max. flue gas temperature exceeded" on the WB1B, WB1C, B1HA/B1KA and B1HC/B1KC, the 111-W B1LA to B1LD and the 050-W BPJD. It is not on the WB1A table, which has no flue gas temperature sensor rows at all. The sensor sits in the flue outlet of the Inox-Radial heat exchanger; if the gas leaving the boiler is hotter than the limit, the heat exchanger is not giving its heat to the water, and the control unit locks the burner out before the heat exchanger or the plastic flue is damaged. Viessmann's measures are exactly the first three of the F2 list — "Check heating system fill level. Check circulation pump. Vent the system. Press Reset." — because a heat exchanger that has stopped transferring heat has almost always stopped being fed with water.

What the householder may do is what the B1HC operating instructions permit for any burner fault: check the pressure gauge is above the 0.8 bar minimum, bleed radiators if there is air, then reset once on the unit's own control — rotary selector to RESET under two seconds on WB1B/WB1C/B1HA, "R" for about two seconds on B1HC, MODE and OK on BPJD — and call the contractor if it recurs. A repeat F1 with a full, vented system is a pump or a heat exchanger that has scaled or sooted on the flue side, and the second of those is a combustion problem, which is why the code keeps its carbon monoxide flag. The Vitotronic-controlled 200-W, 222-F and 242-F have the same F1 — "Flue gas temperature has exceeded limit" in the 2015 B2HA edition, "Flue gas temperature limiter has responded" in the 2019 B2HB — with one extra instruction worth knowing: press reset button R only after the flue system has cooled down. On the 2020-on units with the numeric table the equivalent is F.63 (flue gas temperature limiter has responded), and the B1HG and B1GA editions add F.766/F.767 (flue gas temperature too high).