F25
Glow-worm
Flue Gas Temperature Too High / Flue Sensor Trip
Boiler displays F25, burner shuts down, may be associated with flue issues
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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
Blocked or restricted flue, Faulty flue gas temperature sensor, Incorrect flue installation, Fan or air supply problem
How to Fix / Troubleshooting
Safety: Turn off power and do not operate the boiler if flue gas overheat is suspected. Flue work must be done by a qualified engineer.
- Check flue terminal: From outside, ensure the flue outlet is clear and not obstructed by objects or vegetation.
- Ventilation: Ensure any required ventilation openings around the boiler are not blocked.
- Engineer tasks:
- Inspect the entire flue system for blockages, incorrect slope, or dislodged joints.
- Test and replace the flue gas temperature sensor if faulty.
- Check combustion settings and fan performance.
Do not attempt to dismantle or modify the flue yourself; incorrect installation is hazardous.
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Repair Difficulty
Professional Required
5/5
Required Part
Flue Gas Temperature Sensor
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