Viessmann [F8] fault codewhat it means and how to fix it

F8 is "Burner in a fault state — Fuel valve closes too late" on the WB1A, WB1B, WB1C, B1HA/B1KA, B1HC/B1KC, the 111-W B1LA to B1LD and the 050-W BPJD. When the burner is told to stop, the control unit watches the flame signal disappear and expects it to do so within a defined time after the gas train is de-energised. If the flame persists past that window the only explanation the control unit will accept is that the fuel valve did not close when it was told to, and it locks out. Viessmann's measures are "Check gas train. Check both control paths." — the two control paths being the two independent ways the control unit drives the valve shut, either of which failing is a safety-chain fault.

Because the fault is about a valve that may be passing gas, this page gives no householder steps and carries the carbon monoxide flag. A single reset on the unit's own control (rotary selector to RESET on WB1B/WB1C/B1HA, "R" for two seconds on B1HC, the Reset key on WB1A, MODE and OK on BPJD) is what Viessmann's operating instructions allow before the contractor is called; an F8 that clears and returns is a gas train on its way out, not a glitch. The Vitotronic-controlled 200-W, 222-F and 242-F carry the same F8 — worded "Fuel valve or modulation valve will not close" in the 2019 B2HB edition, with "Check the gas train and replace if required. Press reset button R." — and the 2020-on numeric-table units report it as F.370 (gas valve or modulation valve will not close), which has its own page. A numeric-display boiler showing a plain 8 with the triangle is a different matter altogether — F.8 there is a cylinder temperature sensor short circuit, covered under 50/58.