F4
Saunier Duval
Fan / air pressure system fault
Display shows F4, fan does not start or stops unexpectedly, no ignition
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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 flue fan, Blocked or obstructed flue, Faulty air pressure switch, Damaged fan wiring or connector
How to Fix / Troubleshooting
Safety first: Isolate power and gas. Do not operate the heater with a known flue or fan fault.
Steps:
- Check flue terminal: From outside, inspect the Saunier Duval concentric flue terminal for obstructions (leaves, nests, ice). Clear any blockage.
- Listen for fan: Restore power and call for hot water. If the fan does not run at all, turn power off again and check the fan connector and wiring harness on the PCB.
- Inspect fan wheel: Remove the fan assembly cover and check that the impeller spins freely and is not fouled by debris.
- Check air pressure switch tubing: Inspect silicone tubes from the fan/venturi to the air pressure switch for splits, kinks, or water inside. Replace damaged tubing.
- If fan still fails: The fan motor or air pressure switch may be defective and should be tested and replaced by a qualified technician.
Warning: Never bypass the air pressure switch. This is a critical safety device to ensure proper flue gas evacuation.
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Repair Difficulty
Hard
4/5
Required Part
Flue Fan Assembly
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