HVACopedia

Rinnai · Tankless water heater

34 Combustion air thermistor (Non-SENSEI) / Lime detecting thermistor (RX/CX)

Medium severity Non-SENSEIRH180 hybridSENSEI RX/CX

What the control is telling you

On Non-SENSEI units, the combustion air temperature thermistor reports a fault or abnormal combustion conditions; on SENSEI RX/CX units, the lime detecting thermistor on the hot water pipe is faulty.

Applies to: Non-SENSEI Series; SENSEI RX/CX Series

Likely causes

  • Loose, broken or damaged thermistor harness
  • Thermistor resistance out of spec or failed thermistor
  • Wrong gas type or improper gas conversion (Non-SENSEI)
  • Incorrect DIP switch settings or manifold gas pressures (Non-SENSEI)
  • Debris in burner, manifold or heat exchanger fins (Non-SENSEI)
  • Inadequate clearances or venting out of spec (Non-SENSEI)
  • Foreign material on lime thermistor (RX/CX)

Test and repair steps

  1. Non-SENSEI Inspect the thermistor harness to the PC board, remove the thermistor from the fan motor housing, and test it with a meter set for 20K ohms (resistance should fall when warmed).
  2. Non-SENSEI 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.
  3. Non-SENSEI Confirm the unit matches the gas type on the rating plate; if it was ever converted, confirm the model-specific procedure in the Gas Conversion manual was followed.
  4. Non-SENSEI Confirm clearances around the unit and vent terminals, and that DIP switches match the Gas Conversion or Installation manual.
  5. Warning: Only technicians qualified on gas systems should continue.
  6. Non-SENSEI With a manometer, verify forced low and high fire manifold pressures per the Technical Data Sheet inside the front cover.
  7. Warning: Shut off gas and water supply to the unit before continuing.
  8. Non-SENSEI Inspect burner manifold, burner assembly and heat exchanger copper fins for debris or blockage, and confirm venting clearances and lengths meet spec.
  9. Non-SENSEI If code 34 remains, replace the thermistor.
  10. SENSEI RX/CX Inspect the lime thermistor harness to the PC board, then remove the thermistor from the hot water pipe.
  11. SENSEI RX/CX Test with a meter set for 100K ohms. Typical: 70.6–85.0K at 59°F, 36.5–42.8K at 86°F, 20.0–22.8K at 113°F, 11.5–12.8K at 140°F, 2.7–3.0K at 221°F.
  12. SENSEI RX/CX Clean any foreign material off the thermistor, reinstall it on the hot water pipe, and replace it if code 34 continues.

Settings for these units

Save it for the next call

Star this code in the HVACopedia app and keep the whole library on your phone for jobs with no signal.

Open the app

Source: Rinnai Troubleshooting TWH Diagnostic Codes (100000652), p.39–40. 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.