Viessmann [F.370] fault codewhat it means and how to fix it

F.370 — "Burner in a fault state — Gas valve or modulation valve will not close." — is the numeric table's version of the two-character F8, and the same safety logic sits behind it: when the burner is commanded off, the control unit watches the flame signal disappear, and a flame that lingers past the permitted time can only mean gas still flowing through a valve that should be shut. Viessmann's measures are short: "Reset the appliance. If fault occurs repeatedly, replace fan unit." The fan unit is named because on these boilers the modulation valve that meters gas into the venturi is part of the gas/air unit rather than a separate gas train, so a sticking modulation valve is a fan unit replacement. F.365, "Feedback from gas valve relay contact implausible (relay contact is welded up)", and F.366/F.367, "Gas valve electricity supply does not turn off", are the same fault seen from the control side — the HBMU heat management unit's own relay has stuck or its drive will not release — and for those the only measure is to replace the heat management unit.

This is a row to take literally. A valve that may be passing gas is the reason Viessmann print their warning about resets beside every lockout, and a single reset — the arrow keys, after the burner has cooled — is the most the operating instructions permit before the contractor is called. If F.370 returns, or if there is any smell of gas, the boiler should be switched off at its ON/OFF switch and the gas shut-off valve closed, and the fault treated as urgent. There is no householder repair. The Vitotronic 200-W words the same event "Fuel valve or modulation valve will not close" under F8; the two-character 100-W rows say "Fuel valve closes too late".