Navien Flow Sensor Replacement — What This Part Does
The flow sensor (also called flow meter) sits in the water path inside your Navien tankless heater and spins a turbine whenever water moves through the unit. The sensor sends a signal to the control board so it knows you’ve opened a hot-water tap and can fire the burner. When debris, scale, or wear stops the turbine from spinning freely or the sensor from outputting a valid signal, the board doesn’t see demand and the burner won’t fire or will fire erratically.
Most flow-sensor failures come from mineral buildup or contamination that jams the turbine, a worn sensor assembly that no longer reads correctly, or loose and corroded wiring at the connector. Less often, restricted inlet flow from a clogged filter keeps water velocity too low for the sensor to respond. Navien’s own troubleshooting confirms that a non-spinning or obstructed sensor is the most common cause of flow-related fault codes.
Signs It Needs Replacing
- Flow-sensor or flow-meter error code on the display The unit shows a model-specific fault code indicating the flow sensor is not reading water movement correctly.
- No ignition when you open a hot-water tap The burner does not fire even though water is flowing, because the board doesn’t detect demand.
- Intermittent or unstable burner firing The flame lights briefly then cuts out, or cycles on and off as the sensor signal drops in and out.
- Unit runs but delivers no hot water Water flows through but stays cold because the board never receives a valid flow signal to start heating.
- Sensor turbine visibly jammed or covered in scale If you can inspect the sensor assembly, you see debris or mineral crust preventing the turbine from spinning.
How to Replace It
- Turn off gas supply at the manual shutoff valve upstream of the heater and switch off or unplug electrical power at the breaker or disconnect.
- Close the inlet cold-water isolation valve below the unit and open a hot-water tap downstream to relieve pressure in the system.
- Locate the flow-sensor assembly on the inlet water path inside the front service cover, typically near the heat exchanger or inlet manifold.
- Disconnect the sensor wiring harness by unplugging the connector, noting the orientation for reassembly.
- Use a wrench or socket to loosen the retaining nut or screws holding the sensor body in the water path, then pull the sensor straight out of its housing.
- Inspect the old sensor turbine and bore for debris or scale buildup, then clean the bore with a soft brush if needed.
- Install the new flow-sensor assembly with new O-rings or gaskets (transfer from the replacement kit or purchase separately), seat it fully, and torque the retaining hardware to snug.
- Reconnect the wiring harness, push the connector until it clicks, and tuck any slack away from moving parts or hot surfaces.
- Open the inlet isolation valve slowly, check for leaks around the sensor body, then restore gas and power and run a hot-water tap to verify ignition and stable operation with no fault codes.
The Part You Need
| Part | Notes |
|---|---|
| Navien flow sensor assembly (flow meter) | Amazon | Model-specific. Find your exact part number on the unit’s rating plate (model and serial number) and cross-reference with Navien parts diagrams or contact a Navien-certified service provider for the correct sensor for your chassis series. |
| Flow-sensor O-ring or gasket kit | Amazon | Often included with the sensor assembly. If sold separately, verify size and material from your unit’s service manual or parts list. |
When to Call a Pro
Navien recommends that any work involving gas connections, burner control, or internal water-path components be performed by the original installer or a Navien-certified service technician. If you are not comfortable isolating gas and water, working inside the unit’s pressure boundary, or interpreting model-specific fault codes, call a qualified pro. After sensor replacement, a technician can verify proper flow-signal voltage, check for upstream restrictions at the inlet filter, and confirm that the control board is reading the new sensor correctly under full-demand conditions. For gas line, burner, or igniter work, or if you ever smell gas, stop and call a licensed technician.