HVACopediaRinnai · Tankless water heater
33 Heat exchanger thermistor fault
What the control is telling you
The heat exchanger temperature thermistor or its circuit is reading abnormally.
Applies to: SENSEI Series; SENSEI RX/CX Series; Non-SENSEI Series
Likely causes
- Loose, broken or damaged thermistor harness
- Scale build-up on the thermistor
- Thermistor resistance out of spec
- Failed thermistor
Test and repair steps
- Inspect the thermistor harness from thermistor to PC board for loose, broken or damaged connections.
- Caution: Shut off the water supply and drain the system before continuing.
- With the hot water lines empty, remove the thermistor and clean off any scale or deposits.
- With a meter set to read 20K ohms, measure across the thermistor plug; resistance should drop when the bulb is warmed and rise when cooled.
- 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.
- If readings are good, reinstall the thermistor, checking that its small O-ring is intact and lightly lubricated.
- Restore water supply and check for leaks at the thermistor.
- Fire the unit again; if code 33 returns, replace the thermistor.
Settings for these units
RSC supply connection sizes · SENSEI
| Connection | Size |
|---|---|
| Gas | 3/4 in. NPT |
| Cold inlet | 3/4 in. NPT |
| Hot outlet | 3/4 in. NPT |
| Condensate drain | 1/2 in. NPT |
| Pressure relief valve | M33 thread |
The PRV and its adapter come in the box; PRV must be rated 150 psi and at least the heater Btu/h.
Condensate drain line must stay at least 1/2 in. over its whole length.
RSC water quality limits · 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 |
| 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 |
Damage from water above these limits is not covered by warranty.
Hard water should be treated; use the Service Soon (SS) reminder (parameter 03) for flushing intervals.
- RSC clearances to combustibles · SENSEI
- RSC temperature settings available · SENSEI
- RX supply connection sizes · SENSEI RX/CX
- RX water quality limits · SENSEI RX/CX
- RX water treatment and flushing by hardness · SENSEI RX/CX
- RX clearances to combustibles · SENSEI RX/CX
Models that show this code
89 models in Non-SENSEI, RH180 hybrid, SENSEI RX/CX, SENSEI
Other codes on the same units
- 25 Condensate pump failure or clogged drain
- 30 Flammable vapor sensor activated or faulty
- 31 Burner thermocouple out of range
- 32 Outlet water temperature thermistor fault
- 34 Combustion air thermistor (Non-SENSEI) / Lime detecting thermistor (RX/CX)
- 35 Tank temperature sensor fault
- 38 Exhaust gas temperature sensor (CO sensor on RH180)
- 41 Freeze protection / outdoor air temperature sensor fault
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Source: Rinnai Troubleshooting TWH Diagnostic Codes (100000652), p.38. 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.