Quiet PCs Don’t Have to Suck
I was recently called upon to build a PC for my partner; the only requirements were that it be reasonably powerful, capable of playing reasonably new games reasonably well, and be reasonably quiet. Oh, and it had to be cheap. I kind of like the results.
I started in a most unusual place - the heatsink. I was able to get an enormous all-copper “flower” heatsink, new, for nine bucks, shipped. It’s the fairly well-known Zalman one, as marketed by PC Toys. Since it only fits Socket 478… I was stuck with some variety of P4 for a processor. Not a big deal.
I hunted around for cheap P4 chips in a 478-pin package, and came across an SL726 for a ridiculously cheap price. It’s a Mobile Pentium 4 chip, but in an FC-MPGA4 package, i.e. socket 478, with a nominal speed of 3.06GHz. I chose it mainly because it’s one of the coolest-running PIVs at any speed, with a maximum power dissipation of only 70 watts.
CPU and heatsink out of the way, I purchased a cheap Matsonic MS9377C+ motherboard to hold everything. 512MB of PC2700 RAM came out of the parts drawer, as did a Radeon 9600 and a pretty nice 3Com NIC. A 10K RPM, 120GB hard drive was laying around, and got added to the pile. Up in the attic was an old PII machine in a big, ugly case that has room for eleven 80mm and two 120mm fans; everything but the 56x CD-ROM drive were pulled, and the new bits installed. A 350W power supply was found online on sale, and I replaced the Matsonic board’s dodgy-looking Northbridge cooler with a finned aluminum one that sort of matches the giant CPU cooler.
The Matsonic board doesn’t support SpeedStep, so by default the Pentium chip runs at 1.6GHz. That’s a little slow for gaming, so some fiddling with the BIOS settings soon produced a 2.4GHz speed. XP was installed, and everything’s up and running fine.
As an aside, the whole thing was built to be cheap, but also to offer an easy upgrade path, should more performance be needed in the future. The board supports an 800MHz FSB, and hyperthreaded CPUs, and 8X AGP should be usable for a while to come.
With the CPU fan running at full speed, and a single 80mm exhaust fan on the top of the case going, the CPU temperature, ever after hours of heavy usage, never gets about 66F - that’s two degrees below ambient. 
The northbridge runs a little warmer, at 80F, and the hard drive runs at a reasonable 95F, peak. It’s not very quiet, though; the big 120mm fan on the CPU cooler moves a lot of air, and generates a lot of noise.
It came with a speed controller, that allows you to adjust the fan speed. I hadn’t initially installed it, so it was running at full power, but decided to throttle it back a bit. So, while keeping an eye on the temperature monitor on the screen, I unplugged the fan from the motherboard header, and connected it to the speed controller. Before attaching the controller to the motherboard, I checked the screen again, expected the CPU temperature to rise… and it did. Eventually.
Twelve whole degrees.
Even after an several hours, including some CPU-intensive benchmarking and so on, it’s still running, passively cooled, at 78F. The Northbridge actually got the warmest, running now at an even 90F, but that’s still very good. The whole thing is almost perfectly silent; the only fans are a pair of 80mm Panaflos, one in the PSU, and one at the top of the case that could probably be done away with. Everything passes my decidedly unscientific “touch it without burning yourself test” with flying colors, with no pumps, no radiators, no reservoirs, no water to leak, no heatpipes, just one enormous flower of copper. I’m pretty pleased, myself. A 2.4GHz PIV isn’t a powerhouse, by any means, but for a passively-cooled CPU, it’s not bad at all. For around $165 plus parts from here and there, it would have been hard to do better, I think.
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You know it’s not actually possible for it to be running below ambient, right? The thermocouple must be a bit out of calibration, or it’s slightly cooler wherever the computer is sitting than where you’re measuring ambient.
On an old passively cooled Apple, they ended up needing to add a chimney to the computer so that there would be enough convection. If the fan on the top is blowing out (or if you can make the other fan blow in) you could put a chimney on the top (cardboard tube, 3″PVC pipe a couple feet long, whatever) and get rid of the fan without hurting airflow. Possibly a bit more “Make:” than you’re going for.
Yes, hence the smiley.
The computer’s on the floor, next to an outside wall, and the thermometer’s across the room, at eye-level on an inside wall.
The fan on top is blowing out; I could add a chimney, but I’m not sure how much that would really help. I’m not opposed to computer hacking, but not quite such a geek that I go all hardware-hax0r just for the heck of it. Also, I’m opposed on general principle to making power supply fans suck air in. As it is, air comes into the case pretty much everywhere (all the fan grilles all over) and exits out the top and top-rear. If I was going to change anything, I might move the exhaust fan off the top, make it an intake fan on the front, blowing over the hard disk, and let the PSU fan handle the job of venting the case.
It’s hard (for me) to estimate the heat dissipation of an underclocked CPU, but I’d guess it’s still around 50W. Some of that’s going into the power supply now, but I’m not sure I want all of it to.