HVACopediaNavien · Combi boiler
E278 External (system) supply sensor not detected
What the control is telling you
The system supply sensor is not detected. Occurs when the heating control method is set to system supply temperature. NFC-H manual lists the reset type as: Alarm.
Applies to: Navien fire-tube combi boilers, NFC-H series (NFC-250/200H, NFC-250/175H) and NCB-H combi boilers (NCB-250/150H, NCB-240/130H, NCB-240/110H, NCB-190/080H, NCB-190/060H)
Likely causes
- System sensor not installed or disconnected
- Wiring fault
- Control method set to Sys Supply without a sensor
Test and repair steps
- Check Status Information > 8. Sys Supply Temp.
- Check the system sensor wiring at the PCB.
- Without a system sensor, go to Space Htg Operation > 5. SH Control Method and pick Supply or Return Temperature.
- Code keeps returning? Phone Navien Technical Support, 1-800-519-8794.
Settings for these units
Minimum installation clearances (indoor) · Navien NCB-H
| Side | Minimum |
|---|---|
| Top | 9 in (229 mm) |
| Back | 0.5 in (13 mm) |
| Front | 4 in (100 mm) |
| Sides | 3 in (76 mm) |
| Bottom | 12 in (300 mm) |
Indoor wall-hung only; ambient must stay above 32°F (0°C).
Do not install on carpet; leave room for service.
Water quality limits (DHW and system fill) · Navien NCB-H
| Contaminant | Max allowed |
|---|---|
| Total hardness | 200 mg/l (12 grains/gallon) |
| Aluminum | 0.05 to 0.2 mg/l |
| Chloride | 250 mg/l |
| Copper | 1.0 mg/l |
| Iron | 0.3 mg/l |
| Manganese | 0.05 mg/l |
| pH | 6.5 to 8.5 |
| Sulfate | 250 mg/l (DHW table); 205 mg/l (fill table) |
| Total dissolved solids | 500 mg/l |
| Zinc | 5 mg/l |
| Chlorine (fill water) | 4 mg/l |
Scale damage from poor water is not covered by warranty.
Manual shows sulfate 250 mg/l on p.23 and 205 mg/l on p.29 - check local guidance.
- Minimum installation clearances (indoor) · Navien NFC-H
- Water quality limits (DHW and system fill) · Navien NFC-H
Models that show this code
7 models in Navien NCB-H, Navien NFC-H
Other codes on the same units
- E109 Fan motor fault
- E110 Exhaust blockage / abnormal air pressure (APS)
- E205 Heating supply temperature sensor fault
- E218 Heating return temperature sensor fault
- E279 External (system) return sensor not detected
- E302 Low water pressure (internal LWCO)
- E351 Auto feeder (make-up water) valve fault
- E352 High water pressure
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Source: Navien NCB-H Installation & Operation Manual, 11.7 Understanding Error Codes, p.129-130; Navien NFC-H Installation Manual, 11.7 Understanding Error Codes, p.127-128. Summarized in our own words; check the original manual for your model.
Homeowner looking at this code? See what it means for you and when to call a technician.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.