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Navien · Tankless water heater

E736 Cascade communication abnormal

Medium severity Navien NPE-2

What the control is telling you

Units in a cascade are not communicating.

Applies to: Navien NPE-A2 (built-in recirculation) and NPE-S2 condensing tankless water heaters: NPE-150S2, NPE-180A2/S2, NPE-210A2/S2, NPE-240A2/S2

Likely causes

  • Ready-Link cable loose or damaged
  • IDs not assigned / unit removed
  • Cascade protocol mismatch with older NR/NP/NPE units

Test and repair steps

  1. Kill power, then inspect the Ready-Link cables in the J6 ports (left side of every front panel).
  2. On the main unit, open Service menu > 2. Cascade Status and Set, then run item 3. Cascade System Setting again. Then press and hold OK for 3 s on every sub unit so it gets an ID.
  3. When mixing older NR/NP/NPE units, set Cascade Protocol (item 11) correctly; the older unit must be the MAIN and the NPE-2 a SUB.
  4. A sub unit with a network problem can be excluded with 4. Cascade System Removal on that sub unit.
  5. Fix given phone Navien Technical Support, 1-800-519-8794.

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Source: Navien NPE-A2/S2 User's Information Manual (20049624A), 5.2 Understanding Error Codes, p.18. 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.