Any update on this? I am having the same issue
Do a web search for the terms | mikey pipes navien | and watch a few of his videos. A former Navien fan-boy he has installed hundreds of Naviens, but has pretty much given up on trying to get them to fix this.
MANY water tube heat exchanger type Naviens leak exhaust internally at the gasket at the top of the heat exchanger, which if bad enough or long enough degrades the control board and plastic parts inside the cabinet. Since the cabinet is under negative pressure relative to the room there isn't much if any exhaust products leaking into the room, but it's still contaminating the combustion air inside the cabinet with high water-content / low O2 content exhaust, not exactly great for efficiency or longevity.
There's a good reason Navien voids the warranty if the exhaust vent is installed too close to the intake vent on the house, but there's no excuse for shipping products that cross-contaminate internally. It appears to be a manufacturing quality control issue (improperly torqued screws?), but may also be a design flaw.
