E279 External system return sensor not detected

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

Installation clearances (indoor) · Navien NHB-H

Bracket gives 5/8 in (16 mm) from the wall; studs at 16 in on centre.

Common vent installs: 36 in above the top and 24 in in front are recommended for service.

Installation clearances (indoor) · Navien NFB-H

Do not install on carpet; indoors only.

Remove the wood pallet and refit the 6 screws to keep the cabinet airtight.

Models that show this code

6 models in Navien NHB-H, Navien NFB-H

Other codes on the same units

• E110 Exhaust blockage
• E205 Heating supply temperature sensor
• E218 Heating return temperature sensor
• E278 External system supply sensor not detected
• E291 Return temperature higher than supply
• E302 Low water pressure
• E352 High water pressure
• E353 Water pressure sensor fault