HVACopediaRinnai · Tankless water heater
Water flow fault (no code displayed) No-code water flow fault
What the control is telling you
Used when the unit shows no code and water flow is the suspected problem.
Applies to: SENSEI and Non-SENSEI series
Likely causes
- Low inlet water flow
- Clogged inlet water filter
- Water flow turbine binding
- Faulty water flow control sensor
- Faulty water flow servo motor
- Reversed or cross-connected hot and cold lines
Test and repair steps
- Check and clean the inlet water supply filter.
- Confirm the minimum activation water flow rate is met.
- Confirm hot and cold lines are not reversed or cross-connected.
- Check the building plumbing and fixtures for a cross-connection.
- Check that the water flow turbine spins freely.
- Measure the water flow control sensor resistance and compare it to the technical data sheet behind the front cover.
- If the display is blank and clicking comes from the unit, disconnect the water flow servo motor. If the display comes on, replace the servo.
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
- RUCS/RUS unit clearances · Non-SENSEI
- RUCS/RUS water quality limits · Non-SENSEI
- Water quality limits - RU/RUR/CU · SENSEI
- Clearances - RU/RUR/CU · SENSEI
Models that show this code
78 models in Non-SENSEI, SENSEI
Other codes on the same units
- Perf data Performance data viewer (live readings and counters)
- Priority light Priority light on (this controller is in charge)
- S Venturi reset sequence: step S (SENSEI)
- SE Cascade diagnostic display (error on secondary unit)
- SS Service Soon alert (also listed as 55)
- SS (55) Code SS (55)
- t Venturi reset sequence: step t
- °C/°F Temperature scale switch (Celsius / Fahrenheit)
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Source: Troubleshooting Tankless Water Heater Diagnostic Codes, p. 60. 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.