HVACopediaNavien · Boiler
E278 External system supply sensor not detected
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
- Check the sensor wires at the SUPPLY terminal of the CNT terminal strip.
- Cascade systems require the Navien system supply and return sensors on the main unit.
- If no system sensor is used, make sure SH Control Method and DHW control method are not set to Sys Supply.
- Check Sys Supply Temp in Status Info; replace the sensor if open.
Settings for these units
Installation clearances (indoor) · Navien NHB-H
| Side | Minimum clearance |
|---|---|
| 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) |
| Vent to combustibles | 0 in |
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
| Side | Minimum clearance |
|---|---|
| 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) |
| Vent to combustibles | 0 in |
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
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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.
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.