HVACopediaRinnai · Tankless water heater
51 Inlet water temperature thermistor fault
What the control is telling you
The inlet (incoming) water temperature thermistor is open, disconnected or reading abnormally. The heater keeps running and flashes 51 alternately with the set point.
Applies to: SENSEI Series; SENSEI RX/CX Series; Non-SENSEI Series
Likely causes
- Loose, broken or damaged sensor harness
- Scale build-up on the sensor
- Sensor open or disconnected
- Sensor resistance out of spec / failed sensor
Test and repair steps
- Inspect the sensor harness from sensor to PC board for loose, broken or damaged connections.
- Caution: Shut off the water supply and drain the system before continuing.
- With the lines empty, remove the sensor and clean off any scale or deposits.
- With a meter set for 20K ohms, measure across the sensor plug; resistance should drop when warmed and rise when cooled.
- Typical values 11.4–14K ohms at 59°F, 6.4–7.8K at 86°F, 3.6–4.5K at 113°F, 2.2–2.7K at 140°F, 0.6–0.8K at 221°F.
- If readings are good, reinstall the sensor, making sure the small O-ring in the sensor well is intact and lightly lubricated.
- Restore water and check for leaks around the sensor.
- Fire the unit again; if code 51 returns, replace the sensor. The code clears on its own once the fault is corrected.
Settings for these units
RSC supply connection sizes · SENSEI
| Connection | Size |
|---|---|
| Gas | 3/4 in. NPT |
| Cold inlet | 3/4 in. NPT |
| Hot outlet | 3/4 in. NPT |
| Condensate drain | 1/2 in. NPT |
| Pressure relief valve | M33 thread |
The PRV and its adapter come in the box; PRV must be rated 150 psi and at least the heater Btu/h.
Condensate drain line must stay at least 1/2 in. over its whole length.
RSC water quality limits · SENSEI
| Contaminant | Maximum level |
|---|---|
| Total hardness | Up to 200 mg/L |
| Aluminum | Up to 0.2 mg/L |
| Chlorides | Up to 250 mg/L |
| Copper | Up to 1.0 mg/L |
| Dissolved CO2 | Up to 15.0 mg/L |
| Iron | Up to 0.3 mg/L |
| Manganese | Up to 0.05 mg/L |
| pH | 6.5 to 8.5 |
| TDS | Up to 500 mg/L |
| Zinc | Up to 5 mg/L |
Damage from water above these limits is not covered by warranty.
Hard water should be treated; use the Service Soon (SS) reminder (parameter 03) for flushing intervals.
- RSC clearances to combustibles · SENSEI
- RSC temperature settings available · SENSEI
- RX supply connection sizes · SENSEI RX/CX
- RX water quality limits · SENSEI RX/CX
- RX water treatment and flushing by hardness · SENSEI RX/CX
- RX clearances to combustibles · SENSEI RX/CX
Models that show this code
89 models in Non-SENSEI, RH180 hybrid, SENSEI RX/CX, SENSEI
Other codes on the same units
- 34 Combustion air thermistor (Non-SENSEI) / Lime detecting thermistor (RX/CX)
- 35 Tank temperature sensor fault
- 38 Exhaust gas temperature sensor (CO sensor on RH180)
- 41 Freeze protection / outdoor air temperature sensor fault
- 52 Gas valve / modulating solenoid valve (POV) signal fault
- 54 High exhaust gas temperature
- 57 Burner overheat bi-metal switch open
- 58 Secondary heat exchanger bi-metal switch open
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Source: Rinnai Troubleshooting TWH Diagnostic Codes (100000652), p.45. 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.