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Navien · Boiler

E278 External system supply sensor not detected

Medium severity Navien NHB-HNavien NFB-H

What the control is telling you

The external system supply 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 supply sensor missing or unplugged at the SUPPLY terminal
  • Broken wire
  • Control set to use the system sensor (cascade, SH/DHW control method = Sys Supply) without one installed

Test and repair steps

  1. Check the sensor wires at the SUPPLY terminal of the CNT terminal strip.
  2. Cascade systems require the Navien system supply and return sensors on the main unit.
  3. If no system sensor is used, make sure SH Control Method and DHW control method are not set to Sys Supply.
  4. Check Sys Supply Temp in Status Info; replace the sensor if open.

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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.

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