Honeywell RM7800 Fault 19: Main Flame Ign. after main valve opens, what to check?
My Honeywell RM7800 is throwing a Fault 19: Main Flame Ign. The pilot proves fine, but as soon as the main fuel valve opens, the flame signal drops out. What should I check first?
Fault 19 on a Honeywell 7800 Series relay module indicates that the flame signal was lost during the Main Flame Establishing Period (MFEP) or within the first 10 seconds of RUN. This typically means the transition from pilot to main flame is failing. The pilot was proven, and the main fuel valve opened, but the main flame did not establish or was immediately lost.
Likely causes and checks
1Main fuel not reaching the burner in time
- Points to it
- The main fuel valve may not be opening correctly or there might be an obstruction preventing fuel from reaching the burner. This is common if the valve was recently replaced or rewired.
- Check
- Confirm that the main fuel valve opens when energized and that any manual main cock is fully open. Also, check the wiring to the main fuel valve.
2Pressure sag when the main valve opens
- Points to it
- Opening the main fuel valve can cause a sudden drop in gas pressure, leading to the main flame lighting briefly and then dying. This points to an issue with the gas supply or regulator response.
- Check
- Record the gas pressure at the burner during light-off to observe any sharp drops. Check the regulator's response and ensure the supply capacity is adequate.
3Pilot too small to light the main flame
- Points to it
- If the pilot flame is too weak or improperly sized, it may not be able to reliably ignite the main burner, resulting in a rough or delayed main light-off.
- Check
- Perform the pilot turndown test as described in the checkout publication. This test verifies that the smallest pilot flame capable of holding the flame relay can still smoothly light the main burner.
4Detector sighted on the pilot only
- Points to it
- The flame detector might be positioned such that it only 'sees' the pilot flame. When the interrupted pilot shuts off, the flame signal drops, even if the main flame is burning.
- Check
- Measure the flame signal with only the main flame burning. The 7800 test jack (65-0229-2) should show a minimum flame signal of 1.25 Vdc for a reliable main flame.
Next check: First, confirm that the main fuel valve opens when energized and that the manual main cock is open. Also, check the valve wiring, especially if it was recently replaced or rewired.
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