So here you can see they used a rubber coupling on the exhaust side. I have concerns this material may not be durable or even allowed. However, the installation manual is very vague. Any feedback welcome.
Navien Model NPE-240A2 (NG)
Here is an excerpt and a link to the manual.
Photos
Is the coupling on the intake or exhaust? The excerpt you have is referring to intake.
I would think the rubber coupling would be OK on the intake. Not sure about the exhaust pipe. BUT, I believe Fernco rubber couplings have a 140 degree temperature rating, same as schedule 40 PVC.
Bingo! There was my error. Late night report writing.
Digging deeper thanks to you…I found this approved materials for the exhaust
I do not think that coupling is approved. 
What do you think? It says “recommended”.
I doubt that coupler is rated for airtight or gastight applications.
From your install link…
Perfect…thank you very much!
