F25
Protherm
Flue Gas Overtemperature / Flue Blockage Detected
Boiler displays F25 and locks out, often accompanied by the fan running at high 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
Partially or fully blocked flue, incorrect flue installation or length, faulty flue gas temperature sensor, fan not providing correct airflow.
How to Fix / Troubleshooting
Safety: Risk of carbon monoxide. Do not operate the boiler until the flue system has been checked by a qualified technician.
Steps (technician only):
- Inspect the entire flue route, including the terminal, for obstructions, disconnections, or signs of condensation damage.
- Verify that the flue length and configuration comply with Protherm specifications.
- Check the flue gas temperature sensor for correct mounting and resistance. Replace it if faulty.
- Test the fan for correct speed and airflow. Replace it if underperforming.
After repair: Perform combustion analysis and confirm that F25 does not recur under full load.
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Repair Difficulty
Professional Required
5/5
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