A09
De Dietrich Flue Gas Overtemperature / Safety Thermostat

Boiler locks out with A09, often after long operation at high temperature

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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 undersized flue on De Dietrich condensing boiler, Faulty flue gas thermostat, Fan speed too low, Heat exchanger partially blocked causing high flue temperature

How to Fix / Troubleshooting

Safety: Do not continue to operate the boiler with a flue overtemperature fault. Risk of flue gas leakage and damage.

Steps:

  • Check flue routing: Ensure the flue is installed according to De Dietrich specifications (length, bends, diameter). Excessive length or too many bends can cause overheating.
  • Inspect flue terminal: Confirm that the terminal is not obstructed and that the terminal type matches the boiler configuration (room‑sealed or open flue).
  • Clean heat exchanger: A dirty heat exchanger can cause high flue temperatures. This requires removal of the burner and brushing/flushing the exchanger, which should be done by a qualified technician.
  • Reset thermostat: Some models have a manual reset flue thermostat. Once cooled, press the reset button. If it trips again, do not bypass or bridge it.

Technician tasks: A professional should check combustion, fan speed, and replace the flue gas thermostat if defective.

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Repair Difficulty

Professional Required 5/5

Required Part

Flue Gas Thermostat
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