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Rinnai · Tankless water heater

35 Tank temperature sensor fault

Medium severity RH180 hybrid

What the control is telling you

The tank temperature sensor (thermistor) on the RH180 hybrid unit or its wiring is reading abnormally.

Applies to: RH180 (Hybrid Tank-Tankless Non-SENSEI Series)

Likely causes

  • Loose, broken or damaged sensor harness
  • Sensor resistance out of spec
  • Failed tank temperature thermistor

Test and repair steps

  1. Inspect the tank temperature sensor harness from thermistor to PC board for loose, broken or damaged connections.
  2. Caution: Shut off the water supply and drain the system before continuing.
  3. With water isolated, remove the thermistor from the unit.
  4. With a meter set for 20K ohms, measure across the thermistor plug; resistance should drop when warmed and rise when cooled.
  5. 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.
  6. If readings are good, reinstall the thermistor, checking that its small O-ring is intact and lightly lubricated.
  7. Restore water supply and check for leaks around the thermistor.
  8. Fire the unit again; if the code persists, replace the thermistor.

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Source: Rinnai Troubleshooting TWH Diagnostic Codes (100000652), p.41. Summarized in our own words; check the original manual for your model.

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