Viessmann [F.71] fault codewhat it means and how to fix it

The fan on a numeric-table Vitodens is more than a fan. Viessmann call it the fan unit because the gas/air mixture is formed in it — the gas train feeds a venturi in the intake and the fan speed sets the air — so the control unit watches it closely and has five rows for it. F.71 and F.371 both read "Fan speed too low"; F.71's measures are "Check fan for blockage. Check setting for gas type and flue system. Reset the appliance", F.371's "Check the fan. Check the fan connecting cables. Check the fan power supply. Reset the appliance." F.72 is "Fan idle state not reached" (the B0HA table says "Fan standstill not reached") — the fan was still turning when it should have stopped, and the remedy is a reset then a fan unit if it repeats. F.457 is "Fan sluggish or blocked", with the memorable measure "In the case of severe contamination or grinding noises, replace fan unit." F.349 is "Air mass rate flow not detected correctly in fan unit", whose measures start with "Check for dust contamination in the supply air" and the burner gauze, and end with the fan unit if it recurs. All five are "Burner in a fault state" — no fan, no safe combustion, so the burner locks out rather than run.

The cause behind most of them is mechanical: a bearing that has begun to grind, a rotor that has picked up lint or plaster dust from the air supply, or the drive electronics on the fan failing to reach or report speed. The cable and supply checks in F.371 are there because the fan reports its speed back to the HBMU heat management unit on a signal line, and a poor connection looks like a slow fan. F.71's odd second measure — "Check setting for gas type and flue system" — reflects that the expected speed depends on the flue length and gas type coded into the unit at commissioning; a boiler set for a long flue run demands a speed a short one never needs. The householder can reset once with the arrow keys and should call the contractor if the fault returns or if the boiler has started to make a grinding or whining noise, which is the fan asking to be replaced before it fails.