F4
Viessmann
Fan / Air Pressure Fault
Boiler shows F4, burner does not start, fan may not run
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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
Faulty fan motor, blocked flue or air intake, defective air pressure switch, wiring fault between fan and PCB
How to Fix / Troubleshooting
Safety: Do not operate the boiler with a known flue or fan fault. Risk of flue gas spillage.
- Check flue terminal: From outside, ensure the Viessmann flue terminal is clear of obstructions (ice, leaves, covers).
- Listen for fan: On startup, listen for the fan running. If silent, the fan may be seized or not powered.
- Inspect condensate trap: A blocked condensate trap can cause backpressure. Remove and clean the trap, ensuring the hose to the drain is clear.
- Air pressure switch tubing: Check silicone tubes from the fan/combustion chamber to the air pressure switch for splits, kinks, or water ingress.
- Professional testing: A technician should test fan voltage, measure air pressure switch operation, and verify flue integrity. Replace the fan assembly or air pressure switch as required.
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Repair Difficulty
Professional Required
5/5
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