Quiet PCs, Ducting Air from Case Fan to Heatsink?

Milo_Mindbender asks: "While listening to the whine of my heatsink fan I was wondering. It seems like a good way to get quiet cooling for the CPU would be to mount a fan in the back of my case and run a duct of some kind (folded sheet metal or some kind of hose) from the back of the fan directly onto the top of a fan-less CPU heatsink. You should be able to get the same amount of airflow with a large slow (quiet) case fan as you do with a little noisy cpu fan...and the air being blown onto the heatsink would be cooler as well. This seems like a fairly obvious idea so I'm wondering if there's some reason why it wouldn't work, or if anyone has tried it and could tell us how it turned out." Yeah, but what about the heat in the rest of the system? Depending on the size of your enclosure (and what's in it), you may or may not need more than one fan. Has anyone tried something like this and can comment on how well it worked?

Comments

Already been done

[waytoomuchcoffee]

link

Problem and solution

[mfos.org]

You'd have two big problems.

1) Turbulence in the ducting would reduce your effeciency

2) Cooling the ducting itself

Your best bet would be to get a larger, 80mm heatsink and use a larger, quiter fan right on top of that. They are a little heavier and larger, so your mother board must be able to support it.

Dell did this

[bartb]

I have two dell PCs, the first is 4 years old, the second 2 years. They both use a plactic duct that is attached to a fan in the back and covers the CPU entirely.
The four year old is still really quiet, the other one is starting to make more noise. But that's because of a buggy fan on the video card...
-> maybe we can apply the same strategy there?.

Re:Dell did this

[walt-sjc]

Speaking of aircompressors, careful when blowing out computers. If you spin the little fans beyond their normal speed you can kill the little sleeve bearings... I've did this a couple times years ago before I learned better. If you must clean the fan, stop it from spinning with a screwdriver or something. Also, make sure you have a water trap on your air line or you can end up spraying a fine mist of water all over the inside of your computer.

Back on topic, lots of commercial machines / servers have this ducting. My compaq servers had it over the ram and CPU's and rather than blowing in from the out side, it sucked the air out. Your big heat generators are your RAM, CPU, and diskdrives.

My homebrew tower actually has one big-mother fan that takes up 3 5-1/4" drive bay slots (sits in the front of the opening) to cool the drives behind it in addition to the 2 other case fans, and power supply fans. With dual processors, 2G ram, 4 36G 10Krpm scsi drives, it would get bloody hot in there without all those fans.

While I haven't played with it much, the lmsensors package on linux can tell you temps of various things on the motherboard which could be useful in playing with cooling.

In trying to keep the noise out of the house, I built a cabinet in the garage for my servers and ran a duct from my central airconditioner to the cabinet. I run the furnace fan continually and the thermostat is never set over 75, so it keeps things nice (note that I have a thermostat controlled damper that shuts the vent when the heat kicks on.) I'm mentioning this because you have to keep in mind that you can have that you can have nice airflow in your case, but if your machines in a 95 degree room it's not gonna do much good.

I just got a system that works like that

[foonf]

Its kind of old, a FIC Neptune mini-NLX system. If you are at all familiar with the NLX chassis layout, the CPU is placed in the upper right corner of the motherboard, directly in front of the front case fan. There is a small duct focusing the fan on the CPU, and as there isn't even a place to plug a CPU-mounted fan in, you have to use a passive cooler and hope the case fan is sufficient. It was designed for Pentium II and first-generation celeron systems, apparently the motherboard can run coppermines though...so far, with a slow celeron, its been fine with no CPU fan.

I just looked at a similar thing

[MiTEG]

I generally leave my computer on, and recently I realized that the noise was nearly unbearable to those who were unacustomed to it. I also realized how pleasant it might be to have a near-silent system, so I took it upon myself to see what I could do to make my system noiseless.

I think the main problem with your plan is the amount of airflow required to properly cool a CPU with just a heatsync. Unless it is a Celeron300A or C3 or something of that nature, if you use a just a heatsync you'll need an 80mm fan running at least 2000 RPM. At that speed, the noise the air makes is quite noticeable, even if you choose to get one of the ultra-quiet brands.

I finally decided to just get a new heatsync/cpu fan combo for around $30 from QuietPC and I have to say that not only is it almost silent, it keeps my system cooler than the stock Intel CPU fan. The PSU is another source of noise, and I upgraded my 300W Antec to a 370W TTGI-350SS for around $40. TTGI isn't as well known a name as Enermax or Zalman, but I've found my PSU to be just as noiselss as advertised.

I don't want to discourage you, but I don't really consider heatsync/case fan combos viable for a silent PC with an Athlon XP or P4. It might be expensive to experiment, but then again having such a quiet machine is worth it. I take great pleasure in surprising my friends by turning my monitor on the show them that my computer is already on.

TMTOWTDI

[Etyenne]

Brand name computer maker (Dell, IBM, Compaq, etc.) do that on many model of business class PC.

You may not know, but there is a whole culture that had developped around cooling and case modification. People do air duct all the time out of cardboard, soft metal, acrylic, etc. There are many other option : mounting a larger fan right on your HS with an adapter, throttling your fan down (7volting, rheostat, voltage regulator [my favorite], PWM), using a quieter fan (Panaflo L1A are popular), etc. Check out the Case and Cooling Fetish forum of Arstechnica. 7 volts is another site I like very much.