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Navien · Combi boiler

E736 Cascade communication fault

Medium severity Navien NCB-HNavien NFC-H

What the control is telling you

Communication between boilers in a cascade system has failed. NFC-H manual lists the reset type as: Alarm.

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

  • Loose or damaged RS-485 cascade cable
  • IDs not assigned
  • Unit dropped from the cascade

Test and repair steps

  1. Check the cascade communication cables between units.
  2. Check Installer menu 4. Cascade Status and Set > 6. Cascade Info for which unit is missing.
  3. Re-run 1. Cascade System Setting on the Main unit to reassign IDs, or use 2. Cascade System Removal on a faulty Sub unit.
  4. Code keeps returning? Phone Navien Technical Support, 1-800-519-8794.

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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.

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.