E016
Navien Tankless Water Heater
Severity: CriticalWhat it means
Navien E016 is an over-temperature shutdown — the heat exchanger or outlet got hotter than the safety limit allows and the unit cut the burner.
By far the most common cause is scale (limescale) building up inside the heat exchanger in hard-water areas, which traps heat.
It's also caused by low water flow, a stuck/closed isolation valve, or a bad temperature sensor.
Hot water stops until it cools and the cause is dealt with.
Affected Models
- Navien NPE-180A, NPE-210A, NPE-240A (Classic)
- Navien NPE-180S, NPE-210S, NPE-240S
- Navien NPE-2 series
- Navien NCB-E combi-boilers
- Navien NPN series
Common Causes
- Limescale buildup in the heat exchanger from hard water — the number-one cause
- Low water flow through the unit (clogged inlet filter, partly closed valve, kinked line)
- Isolation/service valve left only partly open after maintenance
- Cold-water inlet filter screen plugged with debris
- Outlet or heat-exchanger temperature sensor reading wrong
- Recirculation loop set up incorrectly on combi/recirc systems
How to Fix It
-
Let it cool, then reset.
Power the unit off at the panel and give it 15-20 minutes.
Power back on and run a hot tap.
If E016 returns quickly, the cause is still there — almost always scale or flow.
If it stays clear for now, still descale soon; it will come back. -
Check the isolation valves and inlet filter.
Make sure both isolation (service) valves are fully open — handle in line with the pipe.
Then shut the cold isolation valve, unscrew the cold-water inlet filter, and rinse out the screen.
A plugged screen starves the unit of flow and overheats it fast. -
Descale (flush) the heat exchanger.
This is the real fix in hard-water areas.
Hook a pump and a bucket of white vinegar (or a commercial descaler) to the service valves and circulate it through the unit for 45-60 minutes, then flush with clean water.
Navien recommends doing this yearly — if it's never been done and you're on city water with scale, that's your E016. -
Check flow rate at the tap.
Navien needs a minimum flow to activate (around 0.5 GPM) and adequate flow to run safely.
A trickle-flow faucet, a clogged aerator, or a failing recirc pump can drop flow low enough to overheat.
Clean aerators and confirm the recirc pump (if fitted) actually runs. -
If it overheats with good flow and a clean exchanger, call service.
Descaled, valves open, filter clean, decent flow — and still E016? Then a temperature sensor or the control board is misreading.
That's a service-tech diagnosis and part swap.
Frequently Asked Questions
How often should I flush a Navien tankless to avoid E016?
Once a year is the standard recommendation, and more often if you have very hard water and no whole-house softener.
A yearly vinegar flush takes about an hour and is the single best thing you can do to keep E016 (and a shortened heat-exchanger life) away.
If you've owned the unit for years and never flushed it, do it now.