Honeywell 7800 Series fault 23: Airflow Switch
Fault 23 means that during PREPURGE the module did not see the combustion airflow interlock closed. Without proven air the furnace cannot be purged, so the module locks out before any fuel valve opens.
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
- PREPURGE
Where it sits in the burner
Supervised functions: Combustion air proving, Purge.
| Component | Signals to look at | How it fails |
|---|---|---|
| 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 |
| Combustion air proving switch | Combustion air proving switch, Combustion air pressure | Pressure or position switch out of adjustment; Restricted combustion air path; Welded, jumpered or miswired contact |
| Air damper, actuator and linkage | Air damper position, Firing rate | Damper or valve linkage slip; Actuator or feedback fault; Air/fuel ratio drift |
Likely causes and checks
1Fan not producing enough air
- Points to it
- Fan slow to start, overload trips, belt slipping, or VSD not reaching purge speed.
- Check
- Confirm the fan runs at full speed in purge; check motor current, belts and VSD settings.
2Air path restricted
- Points to it
- Gradual onset; dirty intake screen, closed inlet damper or blocked flue.
- Check
- Inspect the air intake, fan wheel and damper; Table 3 directs checking the fan intake for blockage.
3Air switch setpoint or sensing line
- Points to it
- Air flow is good but the switch does not make; water in the sensing tube.
- Check
- Measure the differential at the switch during purge and compare with its setpoint; clear and re-route the sensing tube.
Telling it apart
- Fault 23 is air not proven during PREPURGE; fault 32 is the general airflow interlock fault and fault 13 is air proven when it should not be.
- Faults 21 and 22 (running or lockout interlock in PREPURGE) share the same checks.
Evidence to collect
- Code 23 and sequence status
- Fan motor current and speed in purge
- Differential pressure at the air switch during purge
- Air switch setpoint
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.