Honeywell 7800 Series fault 19: Main Flame Ign.
The pilot was proven, the main fuel valve opened, and then the flame signal was lost during the Main Flame Establishing Period (MFEP) or within the first 10 seconds of RUN. The transition from pilot to main flame is where this burner is failing.
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
- MAIN IGN (MFEP)
- First 10 seconds of RUN
Where it sits in the burner
Supervised functions: Main fuel delivery, Flame supervision, Ignition and pilot.
| Component | Signals to look at | How it fails |
|---|---|---|
| Main safety shutoff valves | Main fuel valve output, Valve proof of closure | Fuel valve not opening fully; Valve seat leakage or slow closing; Welded, jumpered or miswired contact |
| Fuel supply and regulator | Main gas pressure, Oil pressure | Fuel supply pressure droop; Regulator out of adjustment or sticking |
| Flame scanner (UV / IR) | Flame signal | Scanner no longer sighted on the flame root; Dirty or fogged scanner sight path; Detector or amplifier ageing; Electrical noise on signal wiring |
| Pilot gas train | Pilot valve output, Pilot gas pressure | Regulator out of adjustment or sticking; Fuel valve not opening fully; Fuel supply pressure droop |
| 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
1Main fuel not reaching the burner in time
- Points to it
- Pilot holds, main flame never appears; valve recently replaced or rewired.
- Check
- Confirm the main valve opens when energised and that the manual main cock is open; check valve wiring.
2Pressure sag when the main valve opens
- Points to it
- Main flame lights then dies; gas pressure drops sharply at light-off.
- Check
- Record gas pressure at the burner during light-off; check regulator response and supply capacity.
3Pilot too small to light the main flame
- Points to it
- Rough or delayed main light-off.
- Check
- The checkout publication’s pilot turndown test checks that the smallest pilot that holds the flame relay will still light the main burner smoothly; repeat it.
4Detector sighted on the pilot only
- Points to it
- Flame signal falls when the interrupted pilot shuts off, even though main flame is burning.
- Check
- Measure the flame signal with main flame alone. The checkout publication uses 1.25 Vdc as the minimum flame signal for a reliable pilot or main flame on the 7800 test jack (65-0229-2, Flame signal and pilot turndown tests).
Telling it apart
- Fault 19 is at main light-off; fault 16 or 28 is earlier (pilot trial) and fault 17 is later (after 10 s of RUN).
Evidence to collect
- Code 19 and sequence status
- Gas pressure trend during light-off
- Flame signal with pilot alone and main alone
- Pilot type (interrupted or intermittent)
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.