Fireye NX6100 F11 False Flame after shutdown, what to check?
My Fireye NX6100 burner controller is locking out with an F11 False Flame fault. It happens after the burner shuts down, and I'm seeing a visible flame persist in the furnace. What should I check first?
The Fireye NX6100 F11 False Flame fault indicates the controller detected a flame signal when no flame should be present, such as after shutdown or during purge. This is a safety lockout because the controller cannot distinguish a real flame from a faulty signal. Your observation of a persistent flame after shutdown points to a specific cause.
Likely causes and checks
1Fuel still burning after shutdown
- Points to it
- This is the most common cause when a visible flame persists after the burner cycle. It means fuel is still reaching the burner, possibly due to after-drip from oil or a leaking safety shutoff valve.
- Check
- Visually inspect the furnace for any remaining flame. If a flame is present, manually close the fuel valves and arrange for the safety shutoff valves to be tightness-tested. If your system has a valve leak test, that can also help diagnose this issue.
2Failing UV tube or self-check detector
- Points to it
- A failing flame detector can generate a false signal even in a dark, cold furnace, leading to an F11 fault. This can sometimes be preceded by F66 events, which indicate the controller's internal dark test failed.
- Check
- With the burner off, cover the flame scanner. If the controller still registers a flame signal, it indicates a fault with the detector or its input circuit.
3Detector seeing ignition spark
- Points to it
- If the F11 fault occurs specifically during the pre-ignition phase when the spark is active, the flame scanner might be incorrectly positioned and detecting the ignition spark as a flame.
- Check
- Re-sight the scanner to ensure it cannot view the ignition spark. Also, check the routing of both the scanner cable and the ignition cable to ensure proper separation.
4Noise pickup on the flame input
- Points to it
- Electrical interference from other components, especially motors or ignition systems, can induce a false flame signal on the scanner input, leading to intermittent F11 faults.
- Check
- Verify the proper termination of the scanner cable's screen. Ensure the scanner cable is routed separately and kept away from power and ignition wiring to minimize electrical noise.
Next check: Given your description of a visible flame after shutdown, your immediate next step should be to visually confirm the flame persistence and then close the manual fuel valves. Following that, arrange for tightness testing of the safety shutoff valves.
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