My Navien NPE 240A tankless heater is about 8 - 9 years old. I've owned the house for 2 years. Twice in the last couple of months, while we have been away, the recirc pump air vent (aka air separator) has leaked, and sprayed water inside the cabinet. This caused my GFCI breaker in the garage to trip, knocking out the water heater, my garage 'frig-freezer, water softener, etc. Each time it happened, I had turned the water heater off, and closed the main water supply valve before leaving the house, but forgot to open a hot water faucet to prevent pressure build-up. On other occasions, when I remembered to open a hot water faucet, there were no problems.
The house is plumbed with PEX, mostly up in the attic space. I believe that what happened is, the attic space became warm during the day, causing pressure to rise in the recirc lines, and eventually pop a seal in the air vent assembly. Thankfully it happened there in the garage, and not in anywhere in the attic space plumbing.
When we got home, the heater cabinet was dripping water all over the floor in the garage. My local Navien service guy checked it out the next day, drained and flushed the heater, got it running again, saw no leaks anywhere, and so we called it good for $195. A month later, same problem. Again I had forgotten to open a hot water faucet while scrambling to depart for a short trip. This time I opened the Navien cabinet myself, and found the recirc pump air separator (Navien Installation Manual item 4-1, aka air vent) leaking away from under the red plastic vent cap. I removed the cap, and found that its O-ring was split, and the vent assembly was leaking away. Obviously the pressure had deformed an internal seal in the assembly this time. The next day, my local Navien fixer replaced the leaking air vent assembly, for another $185. So, almost $400 total within a month, to replace a $30 part.
Our installation does not include an expansion tank kit. Nor does it include a drip leg on the gas line. My local Navien guy told me that these are standard issue for any installations that they do, and the tank would probably have prevented the problem from occurring. His quote to make the improvements: $386.
