Honeywell RM7800 Fault 23: Airflow not proven in prepurge, what to check?
My industrial burner with a Honeywell RM7800 is locking out with Fault 23 during prepurge. The module isn't seeing the combustion airflow interlock close. What should I check first?
Fault 23 on a Honeywell RM7800 series relay module indicates that the combustion airflow interlock did not close during the prepurge cycle. This prevents the furnace from purging and locks out the system before any fuel valves open. This fault is specific to airflow not being proven during prepurge, distinguishing it from other airflow-related faults.
Likely causes and checks
1Fan not producing enough air
- Points to it
- The fan may not be running at full speed during prepurge due to issues like slow starting, overload trips, a slipping belt, or incorrect VSD settings.
- Check
- Confirm the fan runs at full speed during the purge cycle. Check the motor current, inspect the belts for slippage, and verify the VSD settings.
2Air path restricted
- Points to it
- Obstructions in the air intake, fan wheel, or flue can prevent sufficient airflow. This often develops gradually.
- Check
- Inspect the air intake screen, the fan wheel, and the damper for any blockages or restrictions. Ensure the inlet damper is fully open.
3Air switch setpoint or sensing line issues
- Points to it
- Even if airflow is adequate, the air switch might not be making contact, possibly due to an incorrect setpoint or a problem with the sensing tube, such as water ingress.
- Check
- Measure the differential pressure at the air switch during the purge cycle and compare it to the switch's setpoint. Also, clear and properly re-route the sensing tube to ensure it's free of obstructions or moisture.
Next check: Begin by checking the fan's operation to ensure it's producing enough air at full speed during the prepurge cycle, as this is the most common cause for Fault 23.
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