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Navien · Boiler

E736 Cascade communication fault

Medium severity Navien NHB-HNavien NFB-H

What the control is telling you

The main and sub boilers lost communication. Manual action: check the cascade cable.

Applies to: Navien NHB-H (NHB-055H/080H/110H/150H) and NFB-H fire-tube (NFB-175H/200H) condensing boilers

Likely causes

  • Ready-Link cable loose or damaged at CNK1
  • Sub unit not registered / IDs lost
  • Sub unit powered off

Test and repair steps

  1. Warning: Turn off power to all boilers before reconnecting cables.
  2. Check the Ready-Link communication cables at the CNK1 terminal on each boiler (up to 16 boilers).
  3. Power all boilers on, then on the main unit run Cascade Status and Settings > 1. Cascade System Setting to reassign IDs.
  4. Use 2. Cascade System Removal on a sub with a network problem to drop it from the cascade.
  5. Check 7. Cascade Info for the status of each unit.

Settings for these 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.

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