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Rinnai · Tankless water heater

58 Secondary heat exchanger bi-metal switch open

High severity Non-SENSEIRH180 hybrid

What the control is telling you

One of the two bi-metal switches at the rear of the secondary heat exchanger (where the air box joins it) has opened. Shows even when idle; fix the cause, then cycle power (hard reset).

Applies to: Non-SENSEI Series

Likely causes

  • Scale build-up inside the secondary heat exchanger
  • Untreated hard water supply
  • Loose, broken or damaged sensor wiring
  • Open bi-metal switch circuit

Test and repair steps

  1. Inspect the sensor harness from the secondary HX bi-metal switches to the PC board for loose, broken or damaged connections.
  2. Measure the circuit resistance at the PC board connectors; it should be under 1 ohm.
  3. This code points to scale inside the secondary heat exchanger: flush it per the owner's manual to prevent damage.
  4. Treat hard water to stop scale from forming and damaging the heat exchanger.
  5. After the problem is corrected, hard reset by turning power off and back on.

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Source: Rinnai Troubleshooting TWH Diagnostic Codes (100000652), p.51. Summarized in our own words; check the original manual for your model.

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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.