I was kind of excited about this build but then I saw your comment and remembered that was a much better option.

4-6 months ago I built a new workstation for work. I had used one of those Corsair closed loop water coolers with the prior build, so I set one up on this guy. A month or so later my workstation was running really sluggishly, and I realized it was drastically throttling the CPU because of heat. I installed some software to spin up the fans on the cooler to keep it down under 100C, but now it'll get kind of loud when I run much heavy CPU.

Now, this is a pretty heavy duty workstation, 64GB of RAM and 3 displays. But, if I were doing a new machine for home to be quiet, I think it'd be a NUC. Then I'll put the box that has the 6 drive ZFS array in a closet and call it good.

If you want something slightly more modifiable, Compulab makes passively-cooled screwdriver-openable boxes at basically all levels that it makes sense.

Their highest end is available with ECC ram and a discrete graphics card: https://fit-iot.com/web/product/airtop2-build-to-order/

And the lowest end fitlet2 can be configured with an atom in a reasonable configuration for around $300 (it's $130-$200 for case/cpu/motherboard depending on which CPU you configure).

I bet water circulation is broken. Check pump. You may have some air in pipes, turn your PC upside down and back (no joke).

I had looked at the NUCs a few months ago, and just remembered how hard it is in their product page to find something that will support 3 monitors. Looks like only the highest end models will do. I'd be tempted to build my own again, especially for my home office where I can put it in a closet next to my desk. The NUC product page lets you select requirements and then seems to give you a list of products that don't match.

Gigabyte also has their BRIX products, which are similar to the NUC.

I was at a hotel this weekend and at check-in they had a monitor with a "ThinkVantage slotted in the back of the monitor, that might be a nice setup.

Ha, the entire team at my previous job were using NUCs. Nifty little buggers. In the summer it would heat up quite a lot (not yet to the point of damage I suppose) and I couldn't even turn it off without burning myself if the PC froze.

Eventually I just bought a cheap USB desktop fan and ran it facing the NUC.

I'm using a NUC for a quiet home PC that I do mostly browsing but also some light web development on. Although it's mostly silent, I haven't been too impressed with the performance of it. It was one of the top of the line models in 2015, an i7-5557U. I was surprised to find it's quite a bit slower than my 2013 Macbook Pro 2.4 Ghz i5. On the Geekbench profile the MBP got almost 3x the single-core score of the NUC and 1.7x for multi-core. Real world performance of the older macbook is noticably faster.

It has made me think that instead of getting a NUC, for a quiet desktop system I should have just gotten a second used MBP and ran it permanently docked in clamshell mode with the monitors and keyboard attached. (with the added benefit of being able to go portable when I want to)

Neither the NUC nor my MBP is completely silent but for my purposes I find that I seldom tax either of them enough to where the fans become audible enough to be annoying. Still, I do find the difference in performance between them to be apparent just in things like iteration time on web development and IDE responsiveness.

Without ECC, that isn't a very industrial PC.

FWIW, most industrial PCs I've seen do not use ECC. In many case they use CPUs that do not support it anyways.