Overclockers.com---Out, Out Damned Heat!

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"Out, Out Damned Heat!"

by Joe Citarella

10/15/98


There is no question that heat is the overclockers enemy. In chasing this demon, many employ the number one talisman – the 80 mm case fan. This is one area where the prevailing logic seems to be “the more the better”.

Well, it ain’t necessarily so.

I have two passions in this area:

1. To get my case to 10 degrees over ambient or better, and
2. To do it without having it sound like a 747 on takeoff.

Dear friends, both are achievable at reasonable cost and effort.

First, some General Rules:

1. Heat Rises – don’t fight it.
2. If you move 60 cfm INTO the case, you have to move 60 cfm OUT of the case.
3. A big slow fan can move lots of air quieter than smaller fast fans.
4. Local hot spots must be corrected with small fans.
5. If you bolt a fan directly onto a metal case, it will make more noise.
6. If you put objects directly in front of a fan, it will be noisier and lose cfms.
7. Aim fans at heat sources.

OK, now let’s put these Rules to work!

A typical case has a major problem: In order to get air into the case, you need an unobstructed hole the size of the fan’s opening in the bottom front of the metal case. If you use a 120 mm fan, this is about a 4.7 inch circle. Why use a 120 mm fan? Because it can move 50-60 cfm INTO the case at 30 dBa (this is really quiet). To move the same with an 80 mm fan would involve a noise level of around 45-55 dBa – not good. So, you say, use 2 80 mm fans with low noise; Well, two low noise fans are noisier than one, and noisier than one large one. Besides, why cut 2 small holes instead of one big one?

So how do you cut this hole? Metal hole saw, tin snips, jig saw with metal blade, etc. It doesn’t have to look pretty because nobody will see it anyhow. Or, you can always take it to a machine shop and let them do it.

(By the way, I assume if you do this you empty the case first. In addition, after this metal hacking, SCRUPULOUSLY clean up using a vacuum cleaner. Metal shaving are notoriously bad things to get on pcbs).

So you measure the fan’s opening, cut the hole, then drill 4 holes in order to mount the fan on the case. BUT, you are going to bolt the fan onto the case using rubber grommets (A grommet is a rubber doughnut which insulates the bolt from the case) in each hole – this will insulate the fan’s noise-making vibrations from the metal case so it won’t act as a loud speaker.

If you don’t want to use grommets (or maybe you’re afraid to go into a hardware store and say “grommets”), another approach is to insulate the screws where they come in contact with the fan body. I did this by using rubber tape, wrapping one layer around that part of the screw which contacts the fan. But then you have to insulate the bolts and screw head from the fan also, and I was back to gr*****s. Another trick is to glop some silicon glue on the screw body and wait for it to dry. This is probably the best approach, but I hate to wait.

GENERAL RULE 8: Mount all fans using rubber grommets!

So now we have 60 quiet cfm coming into the case – NOT! If you don’t have openings in the plastic front of the case equal to the fan’s opening, you will not get maximum air into the case and you will create noise. So, figure out how much area you need and start drilling a large hole or a lot of small holes either in the case front or along the sides. Try to be neat because you will see the results of your efforts all the time. (If you don’t care about looks, leave the front of the case off – real techno-chic.)

OK, we have 60 cfm coming in the front door; now we have to get it out.

The Power Supply will exhaust some of this air, and some will escape through the holes you find in the back of the case. But, if you really want to lose this filthy, fetid, fiery air mass efficiently and quietly, you need an additional fan so that the Power Supply plus the second fan will exhaust the 60 cfm coming into the case.

Now remember hot air rises, right? Sooo, this exhaust fan should be at, what?, the TOP of the case. The most efficient way to do this is to cut a hole in the TOP of the case and mount an 80 mm fan in this hole. This acts like a chimney, and in conjunction with the PS fan, exhausts 60 cfm out of the case.

Of course, if it rains in your house, this is not cool. Or if you habitually place drinks on top of your case, one of them could be very expensive.

NOTE: you do not want this fan to fight the PS fan; if you were to use a 60 cfm fan in the “chimney”, the PS fan would not be as effective in cooling because the “chimney” fan could overpower the PS fan and suck air back into the case through the PS.

Let’s assume the PS fan is 25 cfm; you would be safe to get a 20-30 cfm low noise 80 mm fan to work with it in scavenging hot air out of the top of the case. Alternatively, you could use a temperature controlled fan so that it will speed up when the temp rises. The intake fan would result in positive pressure inside the case, ensuring that hot air is driven out of the case, not sucked back in.

Most people will not cut holes in the top of a case, so then you have to figure out how to locate a scavenging fan at the top. Here you could experiment with the nifty Radio Shack blowers (really NIDEC Gamma 28s). You could mount one of these near the top of the case aimed out. They are very quiet and move a decent amount of air. If you use them, you should cut some openings for the outlet so that air escapes easily.


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"Out, Out Damned Heat!"


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Or, you could try a “Power Supply Fanwich”. You could mount a second fan on top of the power supply fan, although this is difficult to do with an ATX PS. But you could add a second fan either inside the PS or directly outside on the PS vents, thereby boosting its exhaust. This can be a noisy approach and one I have not fully explored, but I thought I’d mention it anyway.

So, 60 cfm in, 60 cfm out.

Next, vacuum the case out!

General Rule 9: The more metal shavings left in the case, the higher the repair bill.

Now you put the guts back in, power up the machine and start feeling around for hot spots. Off the bat there are two areas, drives and video cards. There are a number of products geared to solving local hot spots by moving hot air out into the exhaust flow. So the objective here is to move stagnant hot air pockets into the chimney to be exhausted.

General Rule 10: Fans inside the case move air within the case.

Each case will have its own unique problems in this area. As a rule of thumb, look at the cards in your case. If they look like a pyramid (ie longest cards in top slots) you’re creating a big hot pocket. Arrange cards so that the longest are on the bottom, the shortest on the top. Look at your drive cables; these big fat obstructions should be moved OUT of the air flow. Obstructions such as these will materially affect your case temp.

Now I have not covered the CPU because I’m assuming you have taken care of that – there are a host of articles on CPU cooling and products available, and I assume you’ve covered this base effectively. But –

General Rule 11: The cooler the CPU, the cooler the case.

A Final Thought:

There’s nothing like a good hardware store for finding the little odds and ends that make projects like these fun.

PS: So what’s in my case? I have a 120 mm front fan 51 cfm at 29 dBa, metal cut away to improve airflow, a Radio Shack 9cfm blower aimed at my SL2W8 running at 450, an AAVID PII chip cooler, and the Power Supply fan. My case does not get more than 10 degrees over ambient. I’m still experimenting with noise control and now find that the power supply fan is the loudest. I will be trying to sound isloate the power supply and probably change this fan for one that’s thermally controlled.


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