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Navien · Combi boiler

E777 Low water cutoff (LWCO) activated

High severity Navien NCB-HNavien NFC-H

What the control is telling you

The external LWCO device has opened (low water in the system). NFC-H manual lists the reset type as: Auto reset.

Applies to: Navien fire-tube combi boilers, NFC-H series (NFC-250/200H, NFC-250/175H) and NCB-H combi boilers (NCB-250/150H, NCB-240/130H, NCB-240/110H, NCB-190/080H, NCB-190/060H)

Likely causes

  • Low system water level
  • External LWCO wiring
  • LWCO fault

Test and repair steps

  1. Inspect the LWCO wiring. The boiler puts out 24 VAC on the LWCO terminals (CNC3).
  2. Make sure the system water level/pressure is correct.
  3. Top up the system with make-up water as required.
  4. Test the LWCO per its own instructions (annual inspection may be required).

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Source: Navien NCB-H Installation & Operation Manual, 11.7 Understanding Error Codes, p.129-130; Navien NFC-H Installation Manual, 11.7 Understanding Error Codes, p.127-128. Summarized in our own words; check the original manual for your model.

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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.