HVACopediaNavien · Tankless water heater
E785 Flow switch / sensor abnormal (with air handler)
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
- Check that the circulation pump runs and loop valves are open; purge air.
- Check the flow switch wiring at the H2Air controller.
- Fix given phone Navien Technical Support, 1-800-519-8794.
Settings for these units
Installation clearances (indoor / outdoor) · Navien NPE-2
| Side | Indoor | Outdoor |
|---|---|---|
| Top | 9 in. (229 mm) | 36 in. (900 mm) |
| Back | 0.5 in. (13 mm) | 0.5 in. (13 mm) |
| Front | 4 in. (100 mm) | 24 in. (600 mm) |
| Sides | 3 in. (76 mm) | 3 in. (76 mm) |
| Bottom | 12 in. (300 mm) | 12 in. (300 mm) |
Vent clearance to combustibles is 0 in.
Outdoor: use the Navien Outdoor Vent Kit; keep 3 ft from an eave to the top of the vent terminals; mild climates only.
Water quality limits and flushing schedule · Navien NPE-2
| Hardness | grains/gal (mg/L) | Treatment | Flush frequency |
|---|---|---|---|
| Soft | 0-4 (0-60) | None | None |
| Moderately hard | 4-7 (61-120) | None required | Once a year |
| Hard | 7-9 (121-160) | Softener or Navien PeakFlow recommended | Once a year |
| Very hard | 9-12 (161-200) | Softener or PeakFlow highly recommended | Residential once a year; commercial twice a year |
| Extremely hard | >12 (200) | Softener or PeakFlow required | - |
Other max levels: hardness 200 mg/L, chloride 250, copper 1.0, iron 0.3, manganese 0.05, sulfate 250, TDS 500, zinc 5, chlorine 4 mg/L; aluminum 0.05-0.2 mg/L; pH 6.5-8.5.
Flushing is advised when no treatment device is installed; use white vinegar or a tankless descaler.
Models that show this code
7 models in Navien NPE-2
Other codes on the same units
- E615 Input and memory abnormal
- E736 Cascade communication abnormal
- E740 Outdoor temperature sensor abnormal (with air handler)
- E760 Flushing / service alarm
- E762 Vent pipe not properly inserted
- E773 Wrong gas type detected
- E782 Main / panel communication abnormal
- E788 Gas type mismatch: DIP switch vs panel setting
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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.
Homeowner looking at this code? See what it means for you and when to call a technician.
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