Honeywell 7800 Series fault 20: Low Fire Sw. Off
After purge the module drives the firing-rate motor back to low fire so the burner lights at a safe, low input, and waits for the low fire switch to prove it. Fault 20 means the switch did not close in time.
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, while driving to low fire before ignition
Where it sits in the burner
Supervised function: Firing-rate positioning.
| Component | Signals to look at | How it fails |
|---|---|---|
| Low fire and high fire position switches | Low fire position switch, High fire (purge) position switch | Pressure or position switch out of adjustment; Damper or valve linkage slip; Welded, jumpered or miswired contact |
| Air damper, actuator and linkage | Air damper position, Firing rate | Actuator or feedback fault; Damper or valve linkage slip; Air/fuel ratio drift |
| Fuel valve actuator and linkage | Fuel valve position, Firing rate | Actuator or feedback fault; Damper or valve linkage slip; Air/fuel ratio drift |
Likely causes and checks
1Actuator does not return to low fire
- Points to it
- Damper stays partly open; modulating signal still calling for load.
- Check
- Watch the actuator return; check the drive signal and that nothing holds the linkage.
2Low fire switch out of adjustment
- Points to it
- Actuator reaches its end stop but the switch does not make.
- Check
- Use the Run/Test switch to hold at low fire and confirm line voltage at the low fire switch input, as Table 3 describes; adjust to the burner maker’s setting.
Telling it apart
- Fault 20 is low fire not proven before ignition; faults 31 and 45 are the low fire switch opening later (RM7838B and others).
- Fault 14 is the high fire (purge) switch.
Evidence to collect
- Code 20 and sequence status
- Actuator position at the fault
- Recent linkage, cam or actuator work
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.