F12
Viessmann
Combustion fan / speed feedback fault
Fan speed error, boiler does not ignite, fan either not running or running at incorrect speed
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Defective Viessmann combustion fan, Blocked fan impeller, Faulty fan speed sensor (Hall sensor), Loose fan connector on control board
How to Fix / Troubleshooting
Safety: Disconnect power before accessing the fan. The fan is part of the sealed combustion system.
- Listen for fan operation: On a call for heat, you should hear the fan start. If it is silent, suspect power or fan failure.
- Inspect wiring: Remove the front cover and locate the fan assembly. Check the multi-pin connector for secure fit and no corrosion.
- Check for obstructions: Look into the fan intake for debris or insulation that may be blocking the impeller. Do not insert tools into the fan while powered.
- Reset and test: After reseating connectors, restore power and reset the boiler. If F12 persists, the fan or its speed sensor likely needs replacement.
- Professional replacement: Fan replacement requires resealing the combustion chamber and verifying combustion; this should be done by a Viessmann-trained technician.
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Repair Difficulty
Professional Required
5/5
Required Part
Combustion Fan Assembly
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