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Navien · Boiler

E777 Low water cutoff activated

High severity Navien NHB-HNavien NFB-H

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

  1. Check system water level/pressure and refill if needed.
  2. Check LWCO Input Status in the Input/Output Status menu.
  3. Check external LWCO wiring at CNC3 (boiler supplies 24 VAC). The factory jumper must stay in place if no LWCO is used.
  4. The external LWCO must be mounted at least 6 in above the top of the heat exchanger.
  5. Test the LWCO per its manufacturer.

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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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