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How to get a cool CPU

Wednesday, August 22, 2001 by TheDoc

There have been many articles written about the perfect case configuration for optimum cooling. This article isn't one of them. What it will give you are ideas for tweaking your own case. While every computer configuration is not the same the basic laws of physics do apply in every "case"; heat rises and warm air circulating in your computer case, if not exhausted, will only make things warmer. Also resistance is not only futile...it should be eliminated. This article will explore a computer case from stock to tweaked in order for you to take some basics to your own system.

This is also not the definitive guide. It is a source for ideas for your own case. This isn't "the way" to do it. It's another example that I have perfected in my own setup after many, many trials, partial successes, and failures. Adapt it to your own needs taking what is best in this article.

The Basic Case

24 " OCD-33333 tower supplied by AMK Computers
450 watt Powerwin power supply unit (PSU)
Abit KT7A-Raid with a 1.13Ghz T-bird
768Mb SDRAM Cas3 133mhz
Globalwin WBK38 lapped heatsink with 92mm Fan
Matrox G450 Video card dual head
Aopen modem
SB Live! value sound card
30 GB Maxtor Diamond HD
internal 100mb zip
52x CD
HP 8200i CDRW

Problem Area Number 1: back plate grills block half of the airflow. Back pressure is created in the fan blades and the exhuast fans are not at optimum efficiency.

You can really see evidence of this problem by the top arrow. I've included a second arrow indicating a secondary problem area. While this may seem a good idea to have more air holes it does actually present a potential problem. Once mounted the fans can exhaust warm air out of the case but inside the case an area of lower pressure may be created thus air would potentially be drawn back into the case. As your powersupply is directly below this then warm air from the power supply could be drawn back into the case. As you also want to channel the air from the lower front of the case to the top rear this may also disrupt and degrade the efficiency of the airflow channel. While air will always find its way into your case from nooks, cracks and cranny's it is best to seal off the obvious ones.

Problem Area Number 2: No airflow onto the motherboard. Solution: fans in the side of the case blowing onto the motherboard peripheral card area. The bottom fan will blow air directly onto the PCI card area: your modem, video card, nic card, sound card, etc. The top fan will feed air from outside of the case directly to the heatsink. Remember that OUTSIDE air is always cooler than INSIDE air.

You can order a case pre-modified with a few custom fan holes or choose to do it yourself with a hole saw or lastly, a dremel. While I can be fairly handy with powertools I chose to get the case pre-modified with a few basic fan modifications. It takes the room for error out of cutting. If I botched it I may have marred or damaged my new case with no one to blame but myself. You may choose to leave the sides exposed like you see above but I prefer to filter the air being drawn into the case. I have a cat which is fond of rubbing up against the side and each time the computer is on the case "vacuums" the cat. It didn't take me long to ponder what a pile of cat hair might do to the computer.

Solution: Filters.

There are filters that can bought to fit the size of the fan holes which are very nice. I chose the Home Depot "ghetto" route. I went to Home Depot and picked up a range hood filter. It's a filter with aluminum mesh and charcoal that goes above your stove in the exhust vent. I also picked up a roll of self adhesive velcro tape. The hook side of the velcro when onto the case and the fuzzy side onto the filter and instantly I had my own home made removable filter.

The filter aluminum mesh which catches the larger airborne debris: birds, feathers, cat hair...cat.

The reverse side showing the charcoal which catches a lot of dust. You can see the "hook" side of the velcro which basically is peel and stick.

Same thing with the fuzzy side of the velcro. Peel and stick it to the side of your case. I removed the top fan as I found that it was not doing anything to lower the temperatures. to the heatsink. The important lesson is that outside air has a direct path to the fan on the heatsink.

Now that's nice and neat. A "ghetto" racing look air cleaner that you can simply un-stick and replace with a cheap hardware store range hood filter.

Eliminating Resistance.

How should the air flow in your computer case? Most, if not all, guides agree that the optimum path for air is in through the bottom front of the case to be exhausted out of the top rear. Let's take a look at the computer with the guts in place.

Those 24 " towers do have a lot of room don't they? Let's take another look with the airflow pattern set up.

Step by step let's go from the top to the bottom. At the top rear of the case there are two 80mm. Sunon fans set to exhuast. Remember the stock grills with series of "hamster-like" air holes? A hole saw or dremel can take care of that.

Quite the difference. All of the resistance to airflow is removed. Your fans work more efficiently at drawing out the warm air in your case. Below you can see a total of four 80mm. fans with one hidden. The hidden fan is in the power supply. You ask why did I stack two fans on the power supply? While stacking the same type fans has been shown to lessen the load on each fan but not greatly improve airflow I changed out the original power supply fan. I replaced the noiser 80mm fan in the power supply with a lower rpm/cfm quiter fan. We'll get to how I control all of these fans later on but to foreshadow I was on the quest for silence. You'll also notice that I placed another 80mm. fan below the power supply. This fan draws warm air off the heatsink out of the case.

I tried several methods to construct a housing for the 80mm. external PSU fan. I started off with cardboard simply wrapped around the square fan to form a tube. A snip on all four corners created a flange for me to use masking tape to affix the fan and shroud. The warmth of the case kept loosening the tape that fan kept dropping off. Electricians tape was used next which only lasted three or four times as long before loosening a falling off. Plus I thought the cardboard may present a fire hazard.

I finally made a trip back to the neighbourhood hardware store and picked up a 36" x 13" section of heating duct aluminum. It can be cut with a minimum of difficulty with a standard pair of sharp household scissors though I would recommend a heavier industrial pair or tin snips. I would also recommend making a cardboard template first to get the measurements correct and to see how it will adapt to your case.

The second item I picked up was aluminum tape. This is about 6 feet to the left of the shelf I found the heating duct aluminum. You use that to secure your new fan housing around the fan and to the case. Sticks like duct tape and is the same colour as the aluminum. You can use duct tape but I find the aluminum duct tape much cleaner in appearance and sticky residue. Duct tape can leave a fair amount of the adhesive behind on your case once removed. You can choose to drill holes but I found the tape to be less work and far less permanent. And who really looks at the back of your computer case anyway.

What isn't shown is the front intake. This is your own personal choice. Some computer enthusiasts have gone the total step to modify their cases with fan intakes straight through the front of the case bezel. This is a very cool mod but at the least you should do the same removal of those "hamster air holes" at the front of your case as was done in the rear. This cuts down on resistance.

The best path is a straight one.

Above is a closer shot of the bottom section of the computer. You'll see the front bezel intake fan at the bottom right. At the top left you'll see the exhaust fan that pulls warm air out from around the heatsink area. There is also an aluminum shroud towards the bottom centre of the computer. This is another 92mm. fan modification that works well with computers with a bit of clearance underneath. My 24" tower has casters so it sits about 2" off the ground. I explored a bottom "ghetto" duct. It was a cardboard doohickey that drew outside air into the case and channeled it straight up the case following the chimney line between the rear of the power supply and rear of the drives in line with the RAM and southbridge chip.

I found that this duct was more efficient than the front intake fan alone at:
  • cooling the motherboard area.
  • pushing warm air from the bottom PCI card/heatsink to the top of the case to be exhausted
  • indirectly providing some cooling to the drive bay components. (hard drive/zip/CDRW/CD)

This was the evolution of an idea that began in the Case Temperature Research article and after much laughing and jabs that my cardboard ghetto duct may catch fire. I first cut a hole in the position I wanted. This was basically at the base of the case "chimney".

The picture below of the top half of the computer contines to show airflow travel.

Then I went back to my sheet of aluminum duct metal and, after building a few cardboard templates, came up with the following fireproof contraption.

And it sits into the case thusly.

If you are asking the question of the ghetto duct blocking the front intake fan remember that the ghetto duct fan is far more efficient at providing direct cooling. It channels the air onto the motherboard/RAM area. The air then travels upwards to the top of the case between the rear of the power supply, to be drawn out and also to the top of the case where the two 80mm. fans exhaust the heat. It also takes the warm air coming from the other side of the heatsink that isn't drawn out the rear of the case.

The front intake fan only provides volume of fresh air into the case rather than direct cooling. There have been discussions as to the effectiveness of the front intake fan but it is important to remember to try to balance CFM in with CFM out.

I didn't hear you! Did you say NINE fans?!

You aren't an air cooled overclocker unless you have the "Spruce Goose" of computers. Yes I seven 80mm. fans and two 92mm. fans in my case with two 40mm. card coolers and a 60mm. hard drive cooler. Oh yes...three 92mm. fans. I forgot to include the heatsink fan. More fans is not necessarily better. For one the noise isn't horrendous...it's just prominent. Also there is a theory that I support that a kabillion fans in a computer case can actually be counter productive if not configured correctly.

Call a doctor.

The Digital Doc 5 (picture courtesy of the Macpower website) has been reviewed many times so I won't go to far into it. In a nutshell it is a unit that can control up to eight fans on/off by temperature. You can set an temperature of each fan control separately to turn on the power to the fan when a specific temperature is exceeded. There are eight power leads and eight thermistors with this unit that fits nicely into an available drive bay. These thermistors can reach every part of your case to measure temperature changes and thusly turn the appropriate fan on or off to control intake and exhaust. It looks like this in the drive bay.

The display is the only problem area of the DD5 as it can be a bit difficult to see on any angle except straight on.

I have found that putting thermistors in every corner of my case is a load of crap. I have put the thermistor leads in 4 places.

The heatsink

On the power supply.

And in my video card heatsink and taped onto the top of my hard drive.

Without the long and boring explanation with charts and graphs I found that placing a thermistor at the top of the case, front and rear, only measured a 2 degree Celsius temperature change. Not much of a range to control fans when heatsink temperature changes were over a far greater range. Thermistors placed at the bottom of the case were effected by airflow thus shut off fans prior to the fans having a cooling effect on the components.

The setup is simple.

As the video card heatsink warms it causes the thermistor to turn on the lead that is to the cooling fan on the video card.

If the hard drive becomes too warm it warms the thermistor which in turn turns on a fan below the hard drive. The hard drive cools from the bottom side through to the thermistor on the top side. The thermistor (and hard drive) cools down...the fans shuts off)

The power supply warms up and the thermistor warms up. It turns on the external fan which draws more air through the power supply thus cooling it off and the thermistor which then turns off the fan.

The heatsink has five thermistors gently placed in it. They don't block air flow through the fins to a great degree. These five thermistors control the leads to the two 80mm. upper rear fans, the 92mm. chimney duct, the 92 mm. front intake, and the backplate heatsink exhaust fan. The heatink begins to warm and one top 80mm. fan turns on. The heatsink warms another degree or two and the second fan turns on. As temperature rises more and more fans turn on to control the heat. Conversely as temperatures fall in the heatsink fans begin to shut down.

The Digital Doc 5 or a fan bay bus is a great addition. It allows you to generate extremely good air flow when needed for cooling and to shut off the noise when not needed.


  • Airflow and fan placement has some basics. Try to balance air flow into the case with airflow out of the case.
  • During normal activity a computer may only need one exhaust fan running to maintain a good system temperature so there may not be a need for four, five, or twenty fans running ALL the time. If you aren't sure on how to do the modifications ask someone to show you or do them for you. A bit of teaching time can prevent a lot of mistakes.
  • If you do have or are going to buy a Digital Doc 5...trust me on the thermistor placement. If you don't believe me...put the leads all over your case and record temperatures without fans on during periods of load and inactivity. Then try controlling temperatures in these zones with fans controlled by the DD5. You'll probably find that you can get most of the fans working like you wanted them to but not all. Do experiment but heed my advice. Also don't cram the leads into the fins. Place them in there and gently push them in till semi-firmly in place. Pushing them too far into the fins will cause them not to function.
  • Changing out a power supply fan will mean breaking the seal and most likely voiding your warranty. If you are going to do it...unplug it first.
  • Finally...if you are going to drill holes in your case. Take everything out of it and make sure the case is good and clean of chips and burrs afterwards. Metal shavings and electrical bits just don't like each other.

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