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Navien · Tankless water heater

E785 Flow switch / sensor abnormal (with air handler)

Medium severity Navien NPE-2

What the control is telling you

The space heating flow switch or sensor signal is abnormal when an air handler is installed.

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

  • No flow in the space heating loop
  • Flow switch or wiring fault

Test and repair steps

  1. Check that the circulation pump runs and loop valves are open; purge air.
  2. Check the flow switch wiring at the H2Air controller.
  3. 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.