HVACopediaRinnai · Tankless water heater
58 Secondary heat exchanger bi-metal switch open
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
- Inspect the sensor harness from the secondary HX bi-metal switches to the PC board for loose, broken or damaged connections.
- Measure the circuit resistance at the PC board connectors; it should be under 1 ohm.
- This code points to scale inside the secondary heat exchanger: flush it per the owner's manual to prevent damage.
- Treat hard water to stop scale from forming and damaging the heat exchanger.
- After the problem is corrected, hard reset by turning power off and back on.
Settings for these units
RUCS/RUS unit clearances · Non-SENSEI
| Location | RUCS indoor to combustibles | RUCS indoor to non-combustibles | RUS outdoor to combustibles | RUS outdoor to non-combustibles |
|---|---|---|---|---|
| Top | 6 in. (152 mm) * | 2 in. (51 mm) * | 12 in. (305 mm) | 2 in. (51 mm) |
| Back | 0 in. | 0 in. | 0 in. | 0 in. |
| Front | 6 in. (152 mm) | 6 in. (152 mm) | - | - |
| Front (panel) | - | - | 24 in. (610 mm) | 0 in. |
| Front (exhaust) | - | - | 24 in. (610 mm) | 24 in. (610 mm) |
| Sides | 2 in. (51 mm) | 1/2 in. (13 mm) | 6 in. (152 mm) | 1/8 in. (3.2 mm) |
| Ground/bottom | 12 in. (305 mm) | 12 in. (305 mm) | 12 in. (305 mm) | 2 in. (51 mm) |
| Vent | 0 in. | 0 in. | - | - |
0 in. from vent components and condensate drain line.
Service clearance 24 in. in front of the heater; closet installs need 6 in. (152 mm) in front.
RUCS/RUS water quality limits · Non-SENSEI
| Contaminant | Maximum level |
|---|---|
| Total hardness | Up to 200 mg / L |
| Aluminum | Up to 0.2 mg / L |
| Chlorides | Up to 250 mg / L |
| Copper | Up to 1.0 mg / L |
| Dissolved CO2 | Up to 15.0 mg / L or PPM |
| Iron | Up to 0.3 mg / L |
| Manganese | Up to 0.05 mg / L |
| pH | 6.5 to 8.5 |
| TDS | Up to 500 mg / L |
| Zinc | Up to 5 mg / L |
Scale build-up shows as code LC# on a controller (LC on the status monitor without one); flush the heat exchanger.
ScaleCutter parts: 103000038 (system), 103000039 (refill).
Models that show this code
59 models in Non-SENSEI, RH180 hybrid
Other codes on the same units
- 41 Freeze protection / outdoor air temperature sensor fault
- 51 Inlet water temperature thermistor fault
- 52 Gas valve / modulating solenoid valve (POV) signal fault
- 54 High exhaust gas temperature
- 57 Burner overheat bi-metal switch open
- 61 Combustion fan motor fault
- 63 Recirculation low flow
- 65 Water flow control device (servo) fault
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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.
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.