Code 016 (or E016) = Overheat protection. A safety thermistor in the burner or heat-exchanger area detected temperature above safe limit and locked out the unit. Serious code — won't restart until the underlying cause is addressed.
Almost always: scaled heat exchanger
By far the most common cause. Scale insulates the heat exchanger from water flow. Burner heat can't transfer fast enough; metal temperature rises until the safety trips. Fix: full descale. In severe cases, heat exchanger is permanently damaged and needs replacement.
Other causes
Restricted combustion air
Blocked intake (lint, debris) or undersized intake venting. Inspect and clean the air inlet filter.
Restricted exhaust
Bird nest, ice, or undersized vent. Inspect exterior termination.
Low water flow
If GPM drops below threshold mid-draw (clogged inlet screen, partially closed isolation valve), exchanger heats faster than water removes heat. Clean inlet water filter.
Recirculation pump failure (NPE-A2)
If the internal pump fails during a recirc cycle, hot water traps in the exchanger. Code 016 throws within minutes. See pump page.
Combi boiler: heating-side issue
On NCB / NFC, if the heating-side circulator pump fails or air-lock develops in the heating loop, hot water can't move away from the exchanger. Code 016 results. Verify heating circulator and purge air.
Reset procedure
- Power off completely
- Wait 30 minutes to cool
- Power back on, attempt Level 1 reset
- If code returns immediately, do NOT keep resetting — underlying cause still present
If Code 016 persists
Stop troubleshooting DIY and call a Navien Service Specialist. Repeated overheat events cause cumulative damage to the heat exchanger. 1-800-519-8794.
Prevention
- Annual descaling — non-negotiable in hard water
- Water softener above 11 grains per gallon
- Annual inlet filter cleaning
- Annual vent inspection
- For combi: keep hydronic loop air-purged and circulator pumps in good order
Warranty implications
Heavy scale = denied warranty claim. Keep descaling documentation. See warranty hub.
Bottom line
Code 016 is almost always the cost of skipping annual maintenance. Descale first; if it returns, call an installer. Don't cycle the reset repeatedly — accelerates exchanger damage.