Rinnai Error Code 15: Rinnai Code 15 - Hot Water Temperature Too High
How to Fix Rinnai Code 15
- Step 1: Check that multiple hot water taps aren't running simultaneously, overwhelming the unit's capacity.
- Step 2: Reduce the temperature setting on the Rinnai unit to a lower setting (e.g., 120°F or 49°C).
- Step 3: If the error persists after these steps, contact a qualified HVAC technician to diagnose and repair the unit.
How to Reset Rinnai Code 15
- Turn off the unit using the power button on the front panel.
- Turn off the gas supply valve.
- Turn off the circuit breaker supplying power to the unit. Wait 30 seconds.
- Restore power at the breaker, then open the gas valve.
- Turn the unit back on. If the error returns, the underlying issue needs repair.
Resetting clears the code from the display but does not fix the underlying fault. If the code returns after reset, proceed to the diagnostic steps above.
What Does Rinnai Error Code 15 Mean?
Rinnai error code 15 indicates that the hot water temperature inside your tankless water heater has exceeded its safety limit. This triggers a safety shutdown to prevent scalding and potential damage to the unit. Several factors can cause this, but it usually boils down to a problem with the temperature sensors, the flow rate of water through the unit, or issues with the burner firing rate. When the water gets too hot, the unit locks out and displays this error code, preventing further operation until the issue is resolved. This is a safety feature, so it's important to address the underlying cause rather than just resetting the unit repeatedly. Ignoring this error could lead to scalding water or permanent damage to your Rinnai tankless water heater.
Common Causes
- Scale buildup on the heat exchanger restricting water flow and causing overheating.
- Faulty thermistor(s) providing incorrect temperature readings to the control board.
- Low water flow rate through the unit.
- Malfunctioning gas valve causing excessive burner firing.
- Defective control board.
Diagnostic Steps
| If... | Check | Expected |
|---|---|---|
| Unit displays code 15 | Measure the resistance of the inlet and outlet thermistors at room temperature. | Values should be within the range specified in the Rinnai service manual for the measured temperature. Significant deviation indicates a faulty thermistor. |
| Thermistors appear within spec | Check the incoming water flow rate while a hot water tap is open. | Flow rate should meet the minimum requirement specified in the Rinnai installation manual for the unit's BTU rating. |
| Flow rate is adequate | Monitor the gas valve operation during startup. Observe if the burner flame modulates correctly or if it remains at a high firing rate. | The gas valve should modulate the flame down after initial ignition to maintain the setpoint temperature. |
Applicable Rinnai Models
- CU(gas)
- HE(dual_fuel)
- HE++ Series(gas)
- I-SERIES(gas)
- NCB(gas)
- NPE(gas)
- RE(gas)
- REP(gas)
- RL(gas)
- RU(gas)
- RUCS(gas)
- RX(gas)
- RXP(gas)
- Rinnai NCB Series(gas)
- Rinnai NPE Series(gas)
- Rinnai R Series(gas)
- Rinnai RL Series(natural_gas)
- Rinnai RU Series(natural_gas)
- Rinnai RUR Series(gas)
- Rinnai VSP(gas)
- V(gas)
Replacement Parts
| Part | OEM Number | Est. Cost | Where to Buy |
|---|---|---|---|
| Inlet Thermistor | 813297001 | — | Find Part |
| Outlet Thermistor | 813297001 | — | Find Part |
| Gas Valve | — | — | — |
Related Codes on Same Models
Other codes that affect the same Rinnai model families as 15.
| Code | Description | Severity |
|---|---|---|
| 02 | Stagnant Water Detection | Warning |
| 10 | Air Supply/Exhaust Blockage | Warning |
| 11 | No Ignition | Warning |
| 12 | Flame Failure | Warning |
| 14 | Thermal Fuse | Warning |
| 16 | Over-Temperature | Warning |
| 19 | Electrical Grounding | Warning |
| 25 | Condensate Overflow | Warning |
| 29 | Condensate Neutralizer | Warning |
| 31 | Inlet Thermistor Failure | Warning |