Honeywell 7800 Series fault 15: Flame Detected
The relay module checks for flame before every start. Fault 15 means it saw a flame signal during STANDBY. Either fuel is still burning in the furnace, or the detector, amplifier or wiring is producing a signal without a flame. Both are treated as unsafe, so the module locks out.
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
- STANDBY (faults 9 and 18 are the related codes for STANDBY with a shutter check and for PREPURGE)
Where it sits in the burner
Supervised function: Flame supervision.
| Component | Signals to look at | How it fails |
|---|---|---|
| Flame scanner (UV / IR) | Flame signal | Scanner no longer sighted on the flame root; Detector or amplifier ageing; Electrical noise on signal wiring; Dirty or fogged scanner sight path |
| Flame amplifier / flame input | Flame signal | Detector or amplifier ageing; Electrical noise on signal wiring; Wrong detector, amplifier or configuration |
| Main safety shutoff valves | Main fuel valve output, Valve proof of closure | Valve seat leakage or slow closing; Fuel valve not opening fully; Welded, jumpered or miswired contact |
Likely causes and checks
1Real flame after shutdown
- Points to it
- Flame visible through the sight glass after the cycle ends; burning oil drips or a leaking valve.
- Check
- Look into the furnace; if flame persists, close manual fuel valves and have the safety shutoff valves tightness-tested.
2Infrared scanner seeing hot refractory
- Points to it
- Only after long high-fire runs; IR detector; clears once the furnace cools.
- Check
- The checkout publication warns that IR detectors can respond to hot refractory; re-sight the scanner so it sees the flame, not glowing brick.
3Failed UV tube or amplifier
- Points to it
- Flame signal present in a cold, dark furnace; amplifier is old or a non-self-checking type.
- Check
- Measure the flame signal with the burner off and the scanner covered; replace the detector or amplifier if a signal remains.
4Noise on the flame lead
- Points to it
- Signal flickers with ignition or nearby motor starts; detector wires share conduit with power wiring.
- Check
- Route the flame detector leads separately from ignition and power wiring as Table 3 directs.
Telling it apart
- Fault 15 (STANDBY) and fault 18 (PREPURGE) are both flame where none should be; fault 9 is the shutter-check version during STANDBY.
- Fault 51 (Flame Too Strong) is a signal above the valid range during firing, not flame at the wrong time.
Evidence to collect
- Code and sequence status
- Flame signal reading with the burner off
- Detector type (UV, self-checking UV, IR, flame rod) and amplifier model
- Whether flame is visible after shutdown
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.