Navien water heater displays E016 and shuts down during operation, often after running for some time.

Safety: Turn OFF power and close the cold water and gas valves before servicing. Allow the unit to cool completely.

Steps:

• 1. Check water flow: Ensure all inlet shutoff valves are fully open. Clean any external inlet water filter or screen on the cold water connection to the Navien.
• 2. Descale the heat exchanger: If the unit has not been flushed in the last 12 months (or more frequently in hard water areas), perform a descaling procedure using a pump, hoses, and a bucket with approved descaling solution (e.g., food-grade vinegar or manufacturer-recommended cleaner). Connect to the service valves, circulate for 45–60 minutes, then flush thoroughly with clean water.
• 3. Inspect 3-way valve (combi models): If you have a Navien NCB/NCB-E/NCB-H combi boiler, a sticking 3-way valve can cause localized overheating. Listen for valve movement when switching between DHW and heating; if it seems stuck, a technician should inspect or replace it.
• 4. Check outlet temperature sensor wiring: Ensure the hot water outlet thermistor is firmly attached to the pipe and its wiring harness is secure.

If E016 persists after descaling and verifying flow, contact a technician to test sensors, 3-way valve operation, and confirm proper pump performance.