Honeywell RM7800 Fault 51: Flame signal too strong during ignition, what to check?
I'm getting a Fault 51 on our Honeywell RM7800 burner control module. It happens during the ignition trial, right when the spark is on. What should I check first for a flame signal that's too strong?
Fault 51 on a Honeywell RM7800 series relay module indicates an untrustworthy flame signal that is too high to be genuine. This often points to ignition interference, an incompatible amplifier, or a failing detector. Since it occurs during the ignition trial while the spark is active, ignition noise is the most likely cause.
Likely causes and checks
1Ignition noise on the flame lead
- Points to it
- The flame signal lead can pick up electrical interference from the ignition wiring, especially when the igniter is energized, leading to an artificially high flame signal reading.
- Check
- Verify that the F lead is routed separately from ignition wiring and is in its own conduit, as directed by Table 3 in the installation instructions.
2Amplifier and detector mismatch
- Points to it
- If the amplifier or detector has recently been changed, an incompatibility between these components can cause incorrect flame signal readings.
- Check
- Confirm that the installed amplifier and detector are compatible with each other and with the RM7800 module. Also, reseat the amplifier to ensure a proper connection.
3Detector failing
- Points to it
- A faulty flame detector can send an abnormally high signal, even when the actual flame is small or non-existent, leading the module to register a 'flame too strong' fault.
- Check
- Perform the flame detector checkout procedure. If the reading remains out of the acceptable range, replace the detector or the amplifier.
Next check: Given that the fault occurs specifically when the igniter is energized, the first step should be to check the routing of the F lead to ensure it is separated from ignition wiring and in its own conduit.
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