Viessmann [F.347] fault codewhat it means and how to fix it

F.347 — "Burner in a fault state — Flue gas recirculation" — is the combustion controller recognising its own exhaust in the air it is burning. Measures: "Check the flue system for leaks; remedy if necessary. Check the flue system for flue gas back pressure, e.g. caused by an insufficient fall in the flue system, constrictions or blockages. Rectify if necessary. Reset the appliance." F.463, "Contaminated combustion air, flue gas recirculation", is the same conclusion with the wider net: "Check flue system for contamination and flue gas recirculation. Clean flue system if required", plus Viessmann's standing note that deposits on the electrodes indicate foreign bodies in the combustion air — laundry detergents, cleaning agents, toiletries, a chimney used as an air supply — and that a boiler whose gauze assembly and electrode have been replaced should have its fan unit, gas/air channel and venturi extension cleaned as well.

On a concentric balanced flue the exhaust travels up the inner pipe and the air comes down the annulus around it; the two are separated by seals at every joint. A seal that has failed, a joint pulled apart in a loft, or a terminal fitted where the wind pushes exhaust straight back into the intake defeats that separation. A run with too little fall lets condensate pool until the flue gas has to push past it, and the back pressure forces exhaust out of any weak joint. This is not a householder repair at any point; the flue is part of the gas installation and inspecting it is Gas Safe work. The permitted single reset is available, but a recirculation code that returns after a reset should be left, because the next stage is F.352, the CO limit itself. Householders who can see their flue terminal can usefully look at it for obstruction, nests or ice while waiting for the engineer, and mention what they saw.