Navien Overheating / Temperature Overshoot Without Code

Navien water heater display shows normal operation but actual outlet water temperature is much hotter than the setpoint.

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Safety Warning

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

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Possible Causes

Faulty outlet temperature sensor reading low, Sticking gas valve not modulating down, Internal mixing valve stuck or scaled, Incorrect sensor placement or poor contact with pipe

How to Fix / Troubleshooting

Safety: This condition is a scald hazard. Immediately lower the setpoint to a safe level (e.g., 120°F / 49°C) and warn occupants.

Steps:

  • 1. Verify with thermometer: Measure hot water temperature at a faucet with a reliable thermometer to confirm overshoot.
  • 2. Inspect outlet sensor: Ensure the outlet thermistor is firmly attached to the pipe and insulated as required.
  • 3. Descale unit: Scale can cause localized overheating; perform a descaling flush.

If water remains significantly hotter than the setpoint, shut down the unit and contact a Navien technician to test sensors, gas valve modulation, and mixing valve operation.

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Repair Difficulty

Hard 4/5

Required Part

Outlet Temperature Sensor (Thermistor)
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