Navien · Tankless Water Heater Serious
E406 — Heat Exchanger Scale / Lime Buildup
The unit detected an abnormally high temperature rise across the heat exchanger relative to flow — classic scale-buildup signature. The board logs this as a scale warning and may lock out.
- Keep using it?
- Limited use only
- DIY difficulty
- Moderate
- Verified against
- navieninc.com ↗
How to fix it
- Close isolation valves; connect hoses to the service ports.
- Circulate 3-4 gallons of white vinegar or a tankless descaler with a submersible pump for ~3 hours.
- Flush thoroughly with fresh water; clean the inlet filter screen.
- Open isolation valves and verify normal flow at a hot tap.
- Power up; if E406 persists, test inlet/outlet thermistors against spec.
- Install a softener or scale-prevention device if water is over ~7 gpg hardness.
Why it happens
Typical signs: E406 displayed · Reduced hot water output · Rumbling or kettling from unit · Long heating delay · Unit cycling on overheat
- Calcium / lime scale in heat exchanger Hard water without softening or annual descaling is the dominant cause.
- Restricted flow Clogged inlet filter or partially closed valves reduce flow and raise delta-T.
- Failed inlet or outlet thermistor Wrong delta-T calculation triggers false scale code.
Parts you may need
If diagnosis points to a failed part, these are the common replacements. As an Amazon Associate we earn from qualifying purchases.
More Navien tankless water heater codes
Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician. Verified against manufacturer documentation: navieninc.com.