Honeywell RM7800 Fault 15: Flame detected in standby, what to check?
My Honeywell RM7800 burner control is locking out with Fault 15. It says "Flame Detected" during standby, but there shouldn't be any flame. What should I check first?
Fault 15 on a Honeywell RM7800 series relay module indicates that a flame signal was detected during the standby phase, before the burner is supposed to start. This is considered an unsafe condition and causes the module to lock out. The issue could be a real flame persisting after shutdown, the infrared scanner detecting hot refractory, a faulty UV tube or amplifier, or electrical noise on the flame lead.
Likely causes and checks
1Real flame after shutdown
- Points to it
- Fuel might still be burning in the furnace due to issues like burning oil drips or a leaking fuel valve, causing the flame detector to register a signal.
- Check
- Look into the furnace through the sight glass. If you see a persistent flame, manually close the fuel valves and arrange for the safety shutoff valves to be tightness-tested.
2Infrared scanner seeing hot refractory
- Points to it
- If an infrared (IR) detector is used, it can sometimes pick up the glow from hot refractory inside the furnace, especially after long high-fire runs, mistaking it for a flame.
- Check
- Observe if the fault clears once the furnace cools down. If so, re-sight the scanner to ensure it is aimed at the flame and not at glowing brickwork.
3Failed UV tube or amplifier
- Points to it
- A faulty UV tube or amplifier can generate a false flame signal even when there is no actual flame, particularly if the amplifier is old or not a self-checking type.
- Check
- Measure the flame signal with the burner off and the scanner covered. If a signal is still present, replace the detector or the amplifier.
4Noise on the flame lead
- Points to it
- Electrical interference from ignition sources or nearby motor starts can induce a false signal on the flame detector wiring, especially if the leads share conduit with power wiring.
- Check
- Check if the signal flickers with ignition or motor starts. Route the flame detector leads separately from ignition and power wiring, as directed in the installation instructions.
Next check: Given the potential safety implications, first visually inspect the furnace through the sight glass to confirm there is no actual flame present after shutdown. If a flame is visible, immediately close manual fuel valves.
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