Honeywell 7800 Series fault 17: Main Flame Fail
Fault 17 means the burner lit and ran normally, then the flame signal dropped out during RUN, at least 10 seconds after the main flame was established. Because light-off succeeded, look for something that changes while firing: fuel pressure, air flow, firing rate or the detector’s view of the flame.
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
- RUN, at least 10 seconds after main flame was established
- Often at load changes, high fire or when other fuel users start
Where it sits in the burner
Supervised functions: Flame supervision, Main fuel delivery, Firing-rate positioning.
| Component | Signals to look at | How it fails |
|---|---|---|
| Fuel supply and regulator | Main gas pressure, Oil pressure | Fuel supply pressure droop; Regulator out of adjustment or sticking |
| 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 |
| 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 |
| Air damper, actuator and linkage | Air damper position, Firing rate | Air/fuel ratio drift; Damper or valve linkage slip; Actuator or feedback fault |
| Fuel valve actuator and linkage | Fuel valve position, Firing rate | Air/fuel ratio drift; Damper or valve linkage slip; Actuator or feedback fault |
Likely causes and checks
1Fuel pressure dips while firing
- Points to it
- Trips at high fire or when another large gas user starts; low gas pressure events near the trip.
- Check
- Trend burner gas (or oil) pressure across a full modulation cycle; compare with the burner’s required pressure at high fire.
2Main valve partially closing or chattering
- Points to it
- Valve actuator noise, voltage dips on the valve circuit, or a trip that coincides with vibration.
- Check
- Inspect main valve wiring and terminations; check the valve actuator and supply voltage during firing.
3Detector loses sight of the flame at some firing rates
- Points to it
- Flame signal good at low fire but weak at high fire, or the reverse.
- Check
- Record the flame signal at low fire, high fire and while modulating, as the checkout publication suggests. 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).
4Air/fuel ratio drift makes the flame unstable
- Points to it
- Trips follow a tune-up, linkage work or seasonal air temperature change; flame looks lifting or smoky.
- Check
- Combustion-test across the firing range and check linkage for slip.
Telling it apart
- Fault 17 is loss of flame after 10 seconds of RUN. Loss during the main trial or the first 10 seconds of RUN is fault 19.
- A falling flame signal over weeks before the first trip points to detection; a sudden trip with a pressure dip points to fuel supply.
Evidence to collect
- Code 17 with the fault history (cycle and hour of each lockout)
- Gas or oil pressure trend around the trip
- Firing rate or damper position at the trip
- Flame signal at low and high fire
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.