HVACopediaViessmann · Boiler
F.691 No combustion air supply feedback
What the control is telling you
Burner in a fault state.
Applies to: Vitodens 200-W B2HE 85, 120, 150, 199
Likely causes
- CAI connection
- Proving switch
- Feedback time set too short
Test and repair steps
- Check the connection at the combustion air interface (CAI).
- Check the proving switch operation.
- If the time is too short, raise parameter 2344.0.
Models that show this code
4 models in Vitodens 200-W B2HE
Other codes on the same units
- F.672 Thermostat-function sensor TS3 open circuit
- F.673 Thermostat-function sensor TS3 short circuit
- F.68 Flame signal present at burner start
- F.69 Ionization current outside permitted range (valve/electrode check)
- F.7 DHW storage tank sensor open circuit
- F.70 BCU internal error
- F.71 Fan speed too low
- F.72 Fan idle state not reached
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Source: Viessmann Vitodens 200-W B2HE 85/120/150/199 Service Instructions 6167 813-24 (09/2026), Fault messages, p.62-73. Summarized in our own words; check the original manual for your model.
Homeowner looking at this code? See what it means for you and when to call a technician.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.