HVACopedia

Navien · Boiler

E279 External system return sensor not detected

Medium severity Navien NHB-HNavien NFB-H

What the control is telling you

The external system return sensor is not connected or not read. Manual action: check sensor connections.

Applies to: Navien NHB-H (NHB-055H/080H/110H/150H) and NFB-H fire-tube (NFB-175H/200H) condensing boilers

Likely causes

  • System return sensor missing or unplugged at the RETURN terminal
  • Broken wire
  • SH Control Method set to Sys Return without a sensor

Test and repair steps

  1. Check the sensor wires at the RETURN terminal of the CNT terminal strip.
  2. Cascade systems require the system return sensor on the main unit.
  3. If no system return sensor is used, do not select Sys Return Temperature as SH Control Method.
  4. Check Sys Return Temp in Status Info; replace the sensor if open.

Settings for these units

Save it for the next call

Star this code in the HVACopedia app and keep the whole library on your phone for jobs with no signal.

Open the app

Source: Navien NHB-H Installation and Operation Manual, 11.14 Understanding Error Codes, p.119; Navien NFB-H Installation Manual, 11.7 Understanding Error Codes, p.116; NHB-H User's Information Manual (V1.0 Apr 2024), 5.2, p.24. 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.