Viessmann [F.62] fault codewhat it means and how to fix it

F.62 on the numeric-table Vitodens (100-W B1HF/B1KF, B1HG/B1KG, B1GA; 111-W B1LF, B1LG; 050-W B0HA/B0KA) reads "Burner in a fault state — High limit safety cut-out has responded." It is the same event the two-character units report as F2, and Viessmann's measures are the same list of reasons water stops moving through a heat exchanger: "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. Reset the appliance." The high limit safety cut-out on these boilers is not a separate device but a function of the flow temperature sensor — Viessmann's rows F.184 and F.185 call it the "flow temperature sensor/high limit safety cut-out" — so if the sensor sees the flow run past its limit the burner is shut off and locked.

A locked-out burner on these units is unmistakable: the warning triangle flashes and the "CL" glyph appears, and the operating instructions say a reset is only possible after the burner has cooled down. That cooling delay is not a nuisance, it is the safety device doing its job. The householder may reset once by holding the up and down arrows together for about four seconds; if F.62 returns the instructions say to notify the heating contractor rather than reset again. The reason the fault recurs is nearly always circulation: a pump that is not moving enough water, a diverter valve stuck between heating and hot water, air in the heat exchanger after a top-up, or a system so sludged that the return barely flows. F.345 is the softer version of the same limit, where the burner is blocked and released after cooling without a reset; F.63 is the flue gas temperature limiter, a different sensor with the same list of checks.