Honeywell RM7800 Fault 14 during PREPURGE, what to check?
I'm getting a Fault 14 on my Honeywell RM7800 during the PREPURGE cycle. The module drives the firing-rate motor to high fire, but it never proves the high fire switch. What should I check first?
Fault 14 on a Honeywell RM7800 series relay module indicates that the high fire switch did not close within the allowed time during the PREPURGE cycle. This means the module commanded the firing-rate motor to high fire, expecting proof from the switch, but never received it. This fault is distinct from Fault 20, which relates to low fire not being proven before ignition.
Likely causes and checks
1Actuator does not reach high fire
- Points to it
- The firing-rate actuator or damper may not be reaching its full open position, preventing the high fire switch from being engaged. This can manifest as the damper stopping short or the motor humming/stalling.
- Check
- Observe the actuator and damper movement during the PREPURGE cycle. Verify the drive signal to the actuator, inspect the linkage for any issues, and check for mechanical stops that might be impeding full travel.
2High fire switch cam out of adjustment
- Points to it
- The actuator might be reaching its full travel, but the cam that actuates the high fire switch is misadjusted, preventing the switch from making contact. This often occurs after maintenance involving linkage or motor work.
- Check
- Use the Run/Test switch to hold the sequence at high fire. Confirm line voltage at the high fire switch input. Adjust the cam according to the burner manufacturer's specifications to ensure the switch makes contact when the actuator is at high fire.
3Linkage slip between actuator and damper
- Points to it
- There might be a slip in the linkage connecting the actuator to the damper, meaning the actuator moves to high fire, and the switch makes, but the damper itself is not fully open, leading to insufficient air flow during purge.
- Check
- Mark the actuator shaft and the damper to visually check for any slip at couplings or set screws. This will confirm if the damper position accurately reflects the actuator's position.
Next check: If the actuator moves but neither the high fire nor low fire switch ever makes, investigate for a wiring fault as it often affects both switches.
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