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Rinnai · Boiler

310 Freeze protection thermistor fault

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

What the control is telling you

Fault detected in the freeze protection thermistor circuit.

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

Likely causes

  • Loose, broken or damaged thermistor wiring/connectors
  • Thermistor unplugged (reads -40)
  • Damaged or failed thermistor
  • Faulty PC Board

Test and repair steps

  1. Inspect the thermistor harness from the sensor back to the PC Board for loose, broken or damaged connections.
  2. Caution shut off the water supply and drain the system down before going further.
  3. Open Performance Data 05 and compare the displayed temperature with a thermometer at the same spot. Within 5° = sensor OK; more than 5° off = continue; a reading of -40 means unplugged or poor connection.
  4. Remove the thermistor and check it for damage.
  5. Disconnect and measure at the thermistor side on a 50K ohm range; resistance should drop when warmed by hand and rise against ice. 150K and smaller: CN7 pins 7 and 10. 175K and higher: CN7 pins 7 and 14.
  6. Compare readings against Appendix C Ambient Temperatures in the source manual (p.17 references it); replace the sensor if it is off by more than 5°F.
  7. Cross-check the readings with the controller's performance data; if correct, reinstall the thermistor.
  8. Fire the unit again; if code 310 persists, replace the thermistor.
  9. If the code returns after a new thermistor, replace the PC Board.

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Source: Rinnai Troubleshooting I-Series Plus Boiler Diagnostic Codes (800000375), p.17. Summarized in our own words; check the original manual for your model.

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