Honeywell 7800 Series fault 13: Airflow Sw. On
In STANDBY the relay module expects the combustion airflow interlock to be open, because the fan has not been asked to run. Fault 13 means the module saw that circuit closed at a point where no air flow should exist, so it cannot trust the switch to prove air during purge.
Diagnose this with AI
Try Free Boiler Diagnostic Upload a keypad photo, the fault history or a parameter export from your Honeywell 7800 Series. InduQT compares the evidence and suggests the next useful checks.
When it happens
- STANDBY, before the call for heat starts the fan
- Often seen right after a previous cycle, while the fan is still running down
Where it sits in the burner
Supervised functions: Combustion air proving, Limit and interlock supervision.
| Component | Signals to look at | How it fails |
|---|---|---|
| Combustion air proving switch | Combustion air proving switch, Combustion air pressure | Pressure or position switch out of adjustment; Welded, jumpered or miswired contact; Restricted combustion air path |
| Combustion air fan (FD / ID) | Combustion fan running, Combustion fan speed, Combustion air pressure, Fan motor current, Fan vibration | Restricted combustion air path; Fan motor or drive fault; Bearing wear or misalignment |
| Burner management controller | Burner sequence state, Controller supply voltage | Controller internal fault; Supply voltage, frequency or grounding problem; Configuration or commissioning data changed; Electrical noise on signal wiring |
Likely causes and checks
1Air switch contacts stuck or welded closed
- Points to it
- Fault appears on every start; the switch reads closed with the fan stopped.
- Check
- With the fan stopped and power isolated, check the switch contacts for continuity and inspect for a jumper left from previous work.
2Air still moving through the boiler with the fan off
- Points to it
- Intermittent, worse in windy weather, with a tall stack or when another fan shares the breeching.
- Check
- Read the pressure at the switch sensing point with the fan off; compare with the switch setpoint and the burner maker’s draft limits.
3Fan running when it should be off
- Points to it
- Fan can be heard in STANDBY; motor contactor or fan VSD stays on.
- Check
- Check the fan contactor, starter auxiliaries and any VSD run command; confirm the fan stops when the module drops the motor output.
4Wiring error feeding the airflow input
- Points to it
- Line voltage present on the interlock input with the switch open.
- Check
- Table 3 groups 11, 12 and 13 together and directs a voltage check on the interlock terminals with the controller off; a qualified technician should trace any unexpected feed.
Telling it apart
- Fault 13 is the switch closed when it should be open (STANDBY). Fault 23 and fault 32 are the opposite: air not proven when it should be.
- Faults 11 and 12 (running or lockout interlock on at the wrong point) share the same checks; read which message appears before resetting.
Evidence to collect
- Photo of the display with code 13 and the sequence status
- Whether the fan is audibly running in STANDBY
- Air switch model, setpoint and sensing-line routing
- Pressure at the sensing point with the fan off
Work on fuel trains, burners, pressure vessels and electrical equipment must be done by qualified personnel following the manufacturer’s documentation and local codes. Never bypass a safety device or interlock.