I have a NVR2108HS-4KS2, it is fanless and doesn't have any PoE ports. I guess those with PoE ports could need a better cooling.
I recently replaced a very noisy NVR with built in PoE ports with the mentioned Dahua's NVR and a 8-ports PoE switch from Tenda TEF1110P-8-102W fanless as well.
It's...