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Navien · Combi boiler

E279 External (system) return sensor not detected

Medium severity Navien NCB-HNavien NFC-H

What the control is telling you

The system return sensor is not detected. Occurs when the heating control method is set to system return 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 Return without a sensor

Test and repair steps

  1. Check Status Information > 9. Sys Return Temp.
  2. Check the system sensor wiring at the PCB.
  3. If no system sensor is used, set 5. SH Control Method to Supply or Return Temperature.
  4. Code keeps returning? Phone Navien Technical Support, 1-800-519-8794.

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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.

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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.