Viessmann [F.63] fault codewhat it means and how to fix it

F.63 reads "Burner in a fault state — Flue gas temperature limiter has responded." on every numeric-table Vitodens. Where F.62 watches the water leaving the heat exchanger, F.63 watches the flue gas leaving it: a sensor in the flue connection that trips when the products of combustion run hotter than the plastic flue is rated for. Hot flue gas is a symptom of the heat exchanger not giving up its heat, and Viessmann's measures are therefore the same as for the high limit: "Check heating system fill level. Check pre-charge pressure in diaphragm expansion vessel. Adjust to required system pressure. Check whether flow rate is sufficient (circulation pump). Check function of 3-way diverter valve. Vent the system." The last line is the one that differs: "Reset the appliance once the flue system has cooled down."

That instruction is about the flue, not the boiler. A plastic flue that has been overheated needs time before the sensor will accept a restart, and Viessmann make the same point on the two-character units, whose F3 row carries the identical condition. The householder may reset once with the arrow keys, and if the code returns the operating instructions say to notify the heating contractor. A flue gas limiter that trips repeatedly on a boiler with good circulation points to the heat exchanger itself — scaled or sludged on the water side, or blocked with combustion deposits on the gas side, both of which stop heat crossing the wall between them. The flue gas temperature sensor has its own rows, F.49 and F.50, when it fails electrically rather than trips.