Honeywell RM7800 Fault 32 during modulation, what to check?
My Honeywell RM7800 is tripping on Fault 32, 'Airflow Switch', but only when the burner modulates to a lower fire. It seems to run fine at high fire. What should I check first?
Fault 32 on a Honeywell RM7800 series relay module indicates the combustion airflow interlock was not in the required state. This fault can occur during PREPURGE, ignition trials, or RUN. When it trips during modulation or at low fire, it often points to issues with the airflow switch setpoint or the air pressure differential.
Likely causes and checks
1Air pressure falls near the switch setpoint while firing
- Points to it
- The air pressure differential across the switch may be too close to its setpoint, causing the switch to chatter or drop out when the burner modulates to a lower fire rate.
- Check
- Measure the differential pressure at the airflow switch across the entire firing range of the burner. If the low-fire differential is very close to the switch's setpoint, it can lead to intermittent trips.
2Fan or air path problem
- Points to it
- Issues with the combustion air fan or obstructions in the air path can lead to insufficient airflow, especially when the burner is modulating. Changes in fan noise or VSD faults can be indicators.
- Check
- Inspect the fan and intake for any problems. Also, check the fault history of any Variable Speed Drive (VSD) controlling the fan for related issues.
3Wiring or dirty contacts on the interlock
- Points to it
- Intermittent electrical contact due to loose wiring, corrosion, or dirty contacts on the airflow interlock switch can cause the module to falsely detect a loss of airflow.
- Check
- With the burner sequence held in PREPURGE (using the Run/Test switch), confirm that line voltage is present at the interlock input. Clean or replace the switch contacts if they appear dirty or corroded.
Next check: Start by measuring the differential pressure at the airflow switch across the burner's firing range to see if the pressure at low fire is too close to the switch's setpoint.
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