Honeywell RM7800 Fault 17: Main flame failure in RUN, what to check?
My Honeywell RM7800 is tripping on Fault 17, Main Flame Fail. The burner lights off fine and runs for a while, but then the flame signal drops out during the RUN cycle, sometimes at high fire. What should I check first?
Fault 17 on a Honeywell RM7800 series relay module indicates a main flame failure during the RUN cycle, at least 10 seconds after the main flame was established. This means the burner successfully lit, but the flame signal was lost later. Common causes include issues with fuel pressure, the main valve, the flame detector's view, or air/fuel ratio drift.
Likely causes and checks
1Fuel pressure dips while firing
- Points to it
- The burner's fuel pressure might be dropping, especially at high fire or when other large fuel users activate, leading to an unstable or lost flame.
- Check
- Trend the burner's gas or oil pressure across a full modulation cycle and compare it to the required pressure at high fire for the burner.
2Main valve partially closing or chattering
- Points to it
- The main fuel valve might not be staying fully open or could be vibrating, causing intermittent fuel supply issues.
- Check
- Inspect the main valve's wiring and terminations. Check the valve actuator and its supply voltage during burner firing for any anomalies.
3Detector loses sight of the flame at some firing rates
- Points to it
- The flame detector might have a good view of the flame at certain firing rates (e.g., low fire) but loses it at others (e.g., high fire), or vice versa.
- Check
- Record the flame signal at low fire, high fire, and while modulating. Ensure the signal is above the minimum reliable level of 1.25 Vdc at the 7800 test jack.
4Air/fuel ratio drift makes the flame unstable
- Points to it
- Changes in the air/fuel ratio, possibly due to recent tune-ups, linkage work, or seasonal temperature shifts, can make the flame unstable and prone to dropping out.
- Check
- Perform a combustion test across the entire firing range and inspect the linkage for any signs of slip or improper adjustment.
Next check: Start by trending the burner's fuel pressure across a full modulation cycle, especially noting any dips at high fire or when other large fuel users start.
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.