Navien Tankless Water Heater E406 — Heat Exchanger Scale / Lime Buildup
Navien Tankless Water Heater · Error code E406
What E406 Means
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.
Symptoms
- E406 displayed
- Reduced hot water output
- Rumbling or kettling from unit
- Long heating delay
- Unit cycling on overheat
Common Causes
Hard water without softening or annual descaling is the dominant cause.
Clogged inlet filter or partially closed valves reduce flow and raise delta-T.
Wrong delta-T calculation triggers false scale code.
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.
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
Verified against manufacturer documentation: navieninc.com
Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician.