Viessmann [F.345] fault codewhat it means and how to fix it

F.345 is the gentle relative of F.62. Its system characteristic is unusual enough to quote in full: "Burner locked out, automatic enabling after appliance cooldown. Independent restart — Temperature limiter has responded." So this is a lockout that Viessmann release themselves: the burner stops, the boiler cools, and it starts again without anyone pressing anything. It is the numeric table's version of what the older two-character units call a "blocked" burner, and it exists so that a brief overshoot — the diverter valve moving over as a tap is turned off, a thermostatic valve closing the last radiator — does not strand the household without heat.

The measures start with a line the F.62 row lacks: "Ensure adequate heat transfer." Then the familiar list — fill level, expansion vessel pre-charge, system pressure, pump flow rate, 3-way diverter valve, vent the system — and one more that is specific: "If the fault occurs during DHW heating: Check DHW cylinder or plate heat exchanger for contamination and scaling." A combi whose plate heat exchanger is scaled cannot dump the burner's output into the hot water fast enough, and the limiter catches it; a system boiler with a scaled cylinder coil does the same. Because the boiler recovers by itself, Viessmann treat F.345 as a message to act on rather than an emergency, and the operating instructions offer the householder two relevant steps: check the system pressure, and bleed radiators if there is air in the system. If the code appears often, or every time hot water runs, the cause is mechanical and the contractor should see it.