HVACopediaRinnai · Tankless water heater
31 Burner thermocouple out of range
What the control is telling you
Burner thermocouple reading is outside its allowed range: above 30 mV (possible over-firing) or below 8 mV (low gas pressure or low-Btu gas) during combustion.
Applies to: RH180 (Hybrid Tank-Tankless Non-SENSEI Series); Non-SENSEI Series
Likely causes
- Manifold gas pressures set incorrectly for altitude
- DIP switches set incorrectly
- Over-firing from high manifold pressure
- Blocked heat exchanger fins
- Low gas pressure or low Btu gas content
- Failed thermocouple
Test and repair steps
- Check low and high fire manifold pressures and set them for your altitude per the Service Manual or Technical Data Sheet inside the front cover.
- Confirm all PC board DIP switches are set correctly for altitude (see Technical Data Sheet).
- Measure thermocouple output during combustion; normal is 20–27 mV. Code 31 triggers above 30 mV or below 8 mV.
- Above 30 mV verify high fire manifold pressure. If within spec, remove manifold and burners and check heat exchanger fins; clean or replace the heat exchanger if clogged.
- Below 8 mV verify forced low fire gas pressure. If within spec, contact Rinnai Technical Support.
- If the fault persists, replace the thermocouple.
Settings for these units
RUCS/RUS unit clearances · Non-SENSEI
| Location | RUCS indoor to combustibles | RUCS indoor to non-combustibles | RUS outdoor to combustibles | RUS outdoor to non-combustibles |
|---|---|---|---|---|
| Top | 6 in. (152 mm) * | 2 in. (51 mm) * | 12 in. (305 mm) | 2 in. (51 mm) |
| Back | 0 in. | 0 in. | 0 in. | 0 in. |
| Front | 6 in. (152 mm) | 6 in. (152 mm) | - | - |
| Front (panel) | - | - | 24 in. (610 mm) | 0 in. |
| Front (exhaust) | - | - | 24 in. (610 mm) | 24 in. (610 mm) |
| Sides | 2 in. (51 mm) | 1/2 in. (13 mm) | 6 in. (152 mm) | 1/8 in. (3.2 mm) |
| Ground/bottom | 12 in. (305 mm) | 12 in. (305 mm) | 12 in. (305 mm) | 2 in. (51 mm) |
| Vent | 0 in. | 0 in. | - | - |
0 in. from vent components and condensate drain line.
Service clearance 24 in. in front of the heater; closet installs need 6 in. (152 mm) in front.
RUCS/RUS water quality limits · Non-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 or PPM |
| 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 |
Scale build-up shows as code LC# on a controller (LC on the status monitor without one); flush the heat exchanger.
ScaleCutter parts: 103000038 (system), 103000039 (refill).
Models that show this code
59 models in Non-SENSEI, RH180 hybrid
Other codes on the same units
- 19 Electrical grounding / DC circuit short
- 25 Condensate pump failure or clogged drain
- 30 Flammable vapor sensor activated or faulty
- 32 Outlet water temperature thermistor fault
- 33 Heat exchanger thermistor fault
- 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)
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Source: Rinnai Troubleshooting TWH Diagnostic Codes (100000652), p.36. 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.
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.