Navien E016: heat exchanger overheat, causes and fixes

Navien lists E016 as heat exchanger overheat. The unit shuts off the burner to protect itself when the water or metal inside gets hotter than it should.

Overheating almost always comes back to one thing: heat is not moving into the water fast enough. That usually means scale or low flow.

What to check first

1. Think about when the unit was last descaled. If it has been more than a year, or you have hard water, scale is the prime suspect.
2. Clean the cold water inlet filter. Close the cold water valve, relieve pressure at a hot tap, then remove and rinse the filter as shown in your manual.
3. Make sure the cold and hot isolation valves under the unit are fully open.
4. Turn the unit off with the power button, wait a few minutes, turn it back on and run hot water.

Common causes

Scale in the heat exchanger

Mineral scale coats the inside of the heat exchanger and acts as insulation. The fix is to descale a tankless water heater with a pump kit and vinegar or an approved descaler.

Low or interrupted water flow

A clogged inlet filter, a partly closed valve or a recirculation problem can reduce flow through the heat exchanger so the water overheats.

Sensor or control faults

If flow and scale are ruled out, a technician will check the temperature sensors, the high limit switch and the control board.

When to call a technician

Call a licensed installer if E016 returns after you have descaled the unit and cleaned the inlet filter. Stop resetting it at that point: repeated overheating can damage the heat exchanger.