HVACopedia

Rinnai · Boiler

371 Indirect tank thermistor fault

Medium severity I-Series Plus 150K and smallerI-Series Plus 175K and higher

What the control is telling you

Fault in the indirect DHW tank thermistor used when the boiler controls an indirect tank by thermistor.

Applies to: I-Series Plus boilers — 150K BTU and smaller and 175K BTU and higher (Solo)

Likely causes

  • Parameter 28 or 29 set incorrectly
  • Thermistor unplugged (reads -40)
  • Loose, broken or damaged thermistor wiring
  • Damaged or defective tank thermistor

Test and repair steps

  1. Confirm Parameter 28 is at 'A' (default when an indirect tank thermistor is used).
  2. Confirm Parameter 29 is at 'b' (thermistor control of the indirect tank).
  3. Open Performance Data 30 and compare the tank temperature with a thermometer at the same spot. Within 5° = OK; more than 5° off = continue; -40 = unplugged or poor connection.
  4. Inspect the thermistor harness for loose, broken or damaged connections.
  5. Measure the tank thermistor resistance and compare against Appendix D: Tank Thermistor in the source manual.
  6. Replace the thermistor if damaged or defective.

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 I-Series Plus Boiler Diagnostic Codes (800000375), p.23–24. 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.