HVACopediaNavien · Boiler
E777 Low water cutoff activated
What the control is telling you
The external LWCO input opened. Manual action: check supply water.
Applies to: Navien NHB-H (NHB-055H/080H/110H/150H) and NFB-H fire-tube (NFB-175H/200H) condensing boilers
Likely causes
- Low water level at the external LWCO
- LWCO wiring or factory jumper removed without a device
- Faulty LWCO
Test and repair steps
- Check system water level/pressure and refill if needed.
- Check LWCO Input Status in the Input/Output Status menu.
- Check external LWCO wiring at CNC3 (boiler supplies 24 VAC). The factory jumper must stay in place if no LWCO is used.
- The external LWCO must be mounted at least 6 in above the top of the heat exchanger.
- Test the LWCO per its manufacturer.
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.