Fireye NX6100 (Nexus) fault F15: Combustion air pressure detected when it should not be
F15 means the main combustion air pressure switch registered air when the controller expected none. Subset 1 means air was seen at the wrong time; subset 2 means the switch still showed air more than three minutes after the burner turned off. The manual also notes F15 appears if the air-switch option (Opt 6.x) is not set.
Diagnose this with AI
Try Free Boiler Diagnostic Upload a keypad photo, the fault history or a parameter export from your Fireye NX6100 (Nexus). InduQT compares the evidence and suggests the next useful checks.
When it happens
- Status 01: burner off, before the fan starts
- After shutdown, while the fan runs down
Reading the subset
| Subset | What it tells you |
|---|---|
| 1 | Air pressure registered when it should not be present |
| 2 | Air pressure still registered more than 3 minutes after the burner turned off |
Where it sits in the burner
Supervised function: Combustion air proving.
| Component | Signals to look at | How it fails |
|---|---|---|
| Combustion air proving switch | Combustion air proving switch, Combustion air pressure | Welded, jumpered or miswired contact; Pressure or position switch out of adjustment; Restricted combustion air path |
| Combustion air fan (FD / ID) | Combustion fan running, Combustion fan speed, Combustion air pressure, Fan motor current, Fan vibration | Restricted combustion air path; Fan motor or drive fault; Bearing wear or misalignment |
| Burner management controller | Burner sequence state, Controller supply voltage | Configuration or commissioning data changed; Controller internal fault; Supply voltage, frequency or grounding problem; Electrical noise on signal wiring |
Likely causes and checks
1Switch contacts stuck or a jumper left in
- Points to it
- Every start; switch reads closed with the fan stopped.
- Check
- Check the switch with the fan off and isolated; look for jumpers from previous work.
2Fan still running or windmilling (subset 2)
- Points to it
- Fan keeps turning after shutdown; shared breeching or strong stack draft.
- Check
- Confirm the fan stops; read pressure at the sensing point with the fan off.
3Option parameter not set
- Points to it
- New installation or after parameter changes.
- Check
- Have the commissioning engineer confirm the Opt 6.x air switch settings.
Telling it apart
- F15 is air when there should be none; F14 is no air when there should be.
Evidence to collect
- Subset (1 or 2)
- Whether the fan is turning with the burner off
- Air switch wiring and any recent parameter changes
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.