Navien NPE-2 Code E445
Bypass mixing valve fault. The bypass mixing valve blends water that went through the heat exchanger with water routed around it, which is how the heater holds a steady temperature at the tap when demand changes. This code says it is not doing that reliably. Navien's instruction is to contact technical support.
Part replacementThe verdict
Replace a part
A moving part in the water path with a moderate price. The symptom while you wait is temperature that will not settle — hot, then less hot, then hot again as the unit tries to correct with a valve that is not responding.
Typical cost: $200–$600
US estimate for parts and labor. Prices vary by region and by who does the work — see how we estimate costs.
What the manual says
The meaning above is transcribed from the manufacturer's diagnostic documentation for the NPE-2 — not averaged across the brand.
Source: Navien NPE-2 User's Information Manual (p. 17), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- Note whether the temperature swings at the tap and at which flow rates.
- Contact your installer or Navien technical support.
Frequently asked questions
Could this be why I get a cold water sandwich?
It can contribute, though most cold water sandwiches are plumbing rather than a failed valve. If yours started at the same time as this code, they are probably related; if it has always done it, look at the pipe run and the inlet filter first.
Code E445 on other Navien series
The same code number can mean something different on another series — make sure you're reading the page for your exact unit:
Don't confuse this with
One other Navien NPE-2 code points at a valve too. They are different components in different places, and ordering a part against the wrong one is the most common way this fault gets expensive: