Honeywell RM7800 Fault 20: Low Fire Switch Off, what to check?
My boiler's Honeywell RM7800 is throwing a Fault 20 after purge, right before ignition. It says 'Low Fire Sw. Off'. What should I be looking at first?
Fault 20 on a Honeywell RM7800 series relay module indicates that the low fire switch did not close in time after the prepurge cycle, preventing the burner from lighting at a safe, low input. This fault occurs while the module is driving the firing-rate motor back to low fire before ignition. It's crucial to distinguish this from faults 31 or 45, which relate to the low fire switch opening later, or fault 14, which concerns the high fire (purge) switch.
Likely causes and checks
1Actuator does not return to low fire
- Points to it
- The firing-rate motor actuator might not be moving to the full low fire position, which means the damper could be partly open or a modulating signal is still calling for a higher load.
- Check
- Observe the actuator's movement to confirm it returns to the low fire position. Verify the drive signal to the actuator and ensure no physical obstruction is holding the linkage.
2Low fire switch out of adjustment
- Points to it
- Even if the actuator reaches its mechanical end stop for low fire, the switch itself might not be making contact due to incorrect adjustment.
- Check
- Use the Run/Test switch to hold the system at low fire. Confirm that line voltage is present at the low fire switch input. Adjust the switch according to the burner manufacturer's specifications.
Next check: First, visually confirm that the firing-rate actuator is indeed returning to the low fire position after the purge cycle. If it's not, investigate the actuator's drive signal and linkage for any issues. If the actuator appears to be in the correct position, then proceed to check the low fire switch adjustment.
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