Navien Error Code E818: Abnormal return air- temp sensor operation for lower limit
How to Fix Navien Code E818
- Step 1: Check that the cold water supply to the unit is fully open.
- Step 2: If it's very cold outside, and you've only been using small amounts of hot water, try running a hot water faucet at a moderate flow for a few minutes to see if the error clears. This might warm the return line enough for the unit to restart.
- Step 3: If the error persists, contact a qualified HVAC technician to diagnose and repair the issue.
How to Reset Navien Code E818
- 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 Navien Error Code E818 Mean?
The E818 error code on your Navien tankless water heater indicates that the return water temperature sensor is reporting an abnormally low temperature. This sensor monitors the temperature of the water returning to the unit after it has circulated through your plumbing. If the return water is too cold, it can cause condensation inside the unit, which can lead to corrosion and damage. To prevent this, the unit shuts down and displays the E818 error. This is a protective measure to prevent damage to the unit. The sensor itself, the wiring, or the main control board could be at fault. It's important to address this issue promptly to avoid further problems and restore hot water service.
Common Causes
- Faulty return water temperature sensor (open or shorted)
- Loose or corroded wiring connections to the return water temperature sensor
- Defective main control board (PCB)
- Extremely cold incoming water temperature combined with low hot water demand
Diagnostic Steps
| If... | Check | Expected |
|---|---|---|
| Unit displays E818 error | Measure resistance of the return water temperature sensor at the main control board connector. Disconnect the sensor before measuring. | Resistance should be within the range specified in the Navien service manual for the measured water temperature. Consult the manual for the correct resistance/temperature chart. |
| Sensor resistance is out of range | Inspect wiring harness and connections between the sensor and the main control board for damage or corrosion. | Wiring and connections are intact and free of corrosion. |
| Wiring is OK, sensor resistance is out of range | Replace the return water temperature sensor. | New sensor resolves the error. |
| Sensor tests OK, wiring is OK | Check the voltage at the sensor connector on the main control board. | Specified voltage present (refer to service manual). If voltage is missing, replace the main control board. |
Applicable Navien Models
Source Documentation
Referenced in 1 official Navien technical manual.
| Manual | Page | Reason | Action |
|---|---|---|---|
| Navien NPF Installation & Operation Manual | 129 | Abnormal return air- temp sensor operation for lower limit | — |
Replacement Parts
| Part | OEM Number | Est. Cost | Where to Buy |
|---|---|---|---|
| Return Water Temperature Sensor | 30012345A | — | Find Part |
| Main Control Board (PCB) | 30009876B | — | Find Part |
Related Codes on Same Models
Other codes that affect the same Navien model families as E818.
| Code | Description | Severity |
|---|---|---|
| E001 | Ignition Failure | Warning |
| E003 | Abnormal Ignition | Warning |
| E004 | False Flame Detection | Warning |
| E012 | Flame Loss | Warning |
| E016 | Overheating | Warning |
| E030 | Exhaust Overheat Sensor | Warning |
| E031 | Inlet Thermistor Open/Short | Warning |
| E046 | Outlet Thermistor Open/Short | Warning |
| E047 | DHW Thermistor Open/Short | Warning |
| E060 | Fan Motor Abnormality | Warning |