Navien Error Code E517: Water Flow Switch Failure

The E517 error on your Navien boiler or water heater means the water flow switch did not close within 10 seconds after a call for heat or hot water. The flow switch is a safety device that confirms water is actually moving through the heat exchanger before the burner fires. If the switch doesn't close, the unit locks out to prevent overheating or damage to the heat exchanger. This is commonly caused by a stuck or failed flow switch, but can also happen when the system is low on water pressure, has air trapped in the pipes, or the pump isn't running. Some Navien models use a magnetic reed switch; others use a Hall effect sensor. On combi units (NCB), the internal circulator must be running for flow to occur. On tankless water heaters (NPE, NPN), the flow switch is triggered by the pressure drop across an internal orifice when a hot water tap is opened. You'll need to check the system pressure, bleed air from the unit and zone piping, and verify the pump is getting power. If those are all fine, the flow switch itself is likely defective and needs replacement.