F9
Protherm
Flue Gas Overtemperature / Blocked Flue
F9 displayed, furnace stops and locks out during operation
Advertisements
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.
Advertisements
Possible Causes
Partially blocked flue or terminal, Collapsed inner flue liner on Protherm concentric flue, Excessive burner output due to incorrect gas setting, Failed flue gas NTC sensor reading too high
How to Fix / Troubleshooting
Safety first: Turn off power and gas. Do not operate the furnace with a suspected flue blockage; risk of carbon monoxide.
- Step 1 – Inspect outside terminal: Go outside and check the flue terminal. Remove any visible obstructions (ice, snow, leaves, nests). Ensure the terminal clearances meet local codes.
- Step 2 – Check flue routing: Inside, visually inspect accessible flue sections for disconnections, sagging, or crushed pipes. Correct any mechanical issues.
- Step 3 – Condensate check: On condensing Protherm furnaces, a blocked condensate trap can cause water to back up into the flue collector. Remove and clean the condensate trap and hoses.
- Step 4 – Sensor verification: If the flue is clear, test the flue gas NTC sensor as described for F7. Replace if faulty.
- Step 5 – Technician combustion check: A licensed technician should verify burner pressure and combustion values. Over‑firing can cause genuine flue overtemperature and must be corrected by adjusting or replacing the gas valve.
Advertisements
Repair Difficulty
Hard
4/5
Required Part
Flue System (Concentric Flue Kit)
Check Price on Amazon
We may earn a commission from links on this page.
Advertisements