> And if you need a good fan that’s quiet enough for the CPU, you’re looking at 4U.
Depends on the CPU, I imagine. I'm using one with a 65W TDP. I'm hopeful that I can cool that quietly with air in 2U, without having to nerf it with lower BIOS settings. Many NASs have even lower power CPUs like the Intel N97 and friends.
> You may even be able to get away with passive cooling at the 65W range.
I saw there's a "passive" Dynatron A43, which even claims to handle up to 155W. My understanding is that most/all server motherboards will have the socket oriented so the fins are front-to-back and the RAM is off to the side. And then you have chassis fans blowing air front-to-back, so I think they basically double as the CPU fan. (Which is also what the older motherboard that came in my CSE-813M did.) I air-quoted passive because I think it needs those chassis fans, but there's not one on the CPU anyway. And I'm not sure I completely trust the A43's rating, but with this setup I think it'd be fine for my 65 W TDP CPU at least.
On the other hand, I'm using a cheap gaming motherboard with fins sideways, RAM blocking the front-to-back airflow. My gut says that Dynatron A43 wouldn't do well. I don't understand why this orientation is desirable for desktops; my conspiracy theorist side says they make the consumers ones this way so they won't eat into the rack-mounted server market share. I am kinda tempted to get a server motherboard for this and IPMI (and/or at least serial port-accessible BIOS), but I started by looking at budget NASes and things have already spiraled a bit from there.
Oh yes, you can definitely get away with much less for something like that or an ARM, Ryzen embedded chips, etc. The 4U is more for full scale desktop CPUs like the i9-12900k I am running (like an NH-D15 sink/fan). You may even be able to get away with passive cooling at the 65W range.