Honeywell RM7800 Fault 13: Airflow switch on in standby, what to check?
My Honeywell RM7800 burner control is giving a Fault 13, "Airflow Sw. On", right after a cycle finishes. The fan is off, but the module thinks the airflow interlock is closed. What should I check first?
Fault 13 on a Honeywell RM7800 series relay module indicates that the combustion airflow interlock is detected as closed during STANDBY, when the fan should not be running and the switch should be open. This prevents the module from trusting the switch to prove airflow during the subsequent purge cycle. It often occurs after a previous cycle while the fan is still winding down.
Likely causes and checks
1Air switch contacts stuck or welded closed
- Points to it
- The airflow switch contacts may be physically stuck or welded in the closed position, making the module believe there is airflow even when the fan is off.
- Check
- With the fan stopped and power isolated, check the switch contacts for continuity. Also, visually inspect the switch for any jumpers left from previous maintenance work.
2Air still moving through the boiler with the fan off
- Points to it
- Residual airflow or natural draft through the boiler, especially in windy conditions or with a tall stack, can cause the airflow switch to close even when the fan is not operating.
- Check
- Read the pressure at the switch sensing point with the fan off and compare it to the switch's setpoint and the burner manufacturer's specified draft limits.
3Fan running when it should be off
- Points to it
- The fan may be inadvertently running during STANDBY due to a faulty motor contactor, starter auxiliaries, or a VSD run command that remains active.
- Check
- Listen for the fan running in STANDBY. Check the fan contactor, starter auxiliaries, and any VSD run command. Confirm that the fan stops completely when the relay module drops its motor output.
4Wiring error feeding the airflow input
- Points to it
- An incorrect wiring connection could be supplying line voltage to the interlock input, making the module detect a closed circuit even if the switch itself is open.
- Check
- A qualified technician should perform a voltage check on the interlock terminals with the controller off. If unexpected voltage is present, trace the wiring to identify and correct the source of the feed.
Next check: Start by checking the airflow switch contacts for continuity and inspecting for any jumpers, as stuck contacts are the most common cause for this fault.
Related questions
Describe the fault or upload a photo, and InduQT checks it against the manufacturer manuals. Share the result here when it helps someone else.