Computer fans

Harrris used Ask the Experts™

I just want to know the correct way to connect the fans inside a PC.

They suppose to blow the air from inside the case to outside ?
or vice versa ?

How the air flow must move inside the case ?

For example, in my pc there is a fan on the side of the case, parallel
with the motherboard, and it blows the air from inside the case to outside.
Is this the correct way ?

What about the fans to the back of the case. For example the fan inside
the power supply, or any other fan at the back of the case.

b.t.w. is there any way to see/visualize how the air moves inside the case ?

And does it really make significant difference in temperature by adding fans,
or changing their position ?
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Top Expert 2009
"Is this the correct way?" ==>  No.   You should generally have front-mounted and side-mounted fans blowing IN, and rear-mounted and top-mounted fans blowing OUT.   Fans can absolutely make a HUGE difference in the temperature inside the case !!

"... is there any way to see/visualize how the air moves inside the case ? "  ==> No.  You could "visualize" it with the right physical modeling software and careful measurement of the velocities of the various fans; but that's far beyond the scope of most users to produce ... and the modeling software you'd need costs thousands of $$ :-)

Hi Harrris

There are several different concepts, and it all depends on the case design.
Here's a reasonably concise explanation:
Note the change from the early ATX Power Supplies (that sucked air in from the rear down into the case) to the later revision that sucked air from the case to the outside air.  Old computers usually have clumps of dust on the outside of the power supply air intake grille.

Take this image as an example of a filtered grille in the side of the case ducted right down onto the processor's heatsik fan to draw cool air from outside directly onto the heatsink:

Most processors' heatsink fans blow down onto the heatsink fins (, when it might seem more logical to draw the hot air OFF the heatsink.  In a case without such a duct, success depends on how much cooler the air inside the case is than the air in between the fins of the heatsink.  It also depends on having enough of it available within suction reach of the processor heatsink fan to push it down into the vanes.  If there is an additional case extractor fan close to the processor, then it could be working against the processor fan.  So good airflow is where each flow source and direction doesn't conflict and disturb the other one.  There is usually a sensor on the motherboard very close to the processor that measures the air temperature and would trigger an alarm either from the BIOS or from thermal monitoting software.

Here's another design that has a fan pulling air up into the case through a foam filter at the bottom of the case.  All that's happening there is that air is being drawn upwards through the case by the power supply unit's fan and expelled out the rear, and is being supported by the bottom fan because there are no other gaps in the case.  It cools components en-route as it is temporarily diverted around the obstructions.  The air obviously gets a lot hotter as it passes hot components, and the processor happens to be right up near the top.

I'm not convinced this is a good design despite the designer's boasts : "The Nexus Breeze case has no other ventilation holes because these holes would interrupt the created airflow".  Yeah, but the hottest air all ends up around the processor, so it depends on just how fast the power supply unit fan can get it out so it doesn't hang around the processor too long.  They have obviously tested the transfer rates of both fans and balanced them for that particular case.  Whether or not the airflow is any more efficient than the classic single extractor fan layout like this is only something you have to take their word for:

Of course it IS true that too many air holes lessen the effective suction and airflow direction in a design like this.  In a design where the expeller fan is supplement by a fan at the lower front, any "gills" in the side panels could either draw air in or have it forced out through them depending on the positions in relation to both other fans, and the size of the gill holes.  There's no easy answer to that one.  If air is drawn in through them, then it might help to mix cool air with the already warmer air being sucked upwards through the case.  Instead of a direct "current" of air flowing right through the case and being temporarily diverted by the occasional obstacle, a front intake fan or side gills creates more of a dispersed "breeze" of cooler air from several sources. A front mounted intake fan often has the added bonus that it pushing cool air over the hard drive(s).

Obviously you want to reduce the surface area of all obstacles, and a good place to start is by replacing the flat IDE Ribbon Cable with round ones.

The general concept, and classic design, is that you should have a lot more suction out of the top rear than you have being pulled in by a fan in the bottom front, or you just end up with increasingly warmed-up air just eddying inside the case.  If you create too much air current into the case through a fan or gap at the bottom front of the case, you end up sucking in too much cat hair and navel fluff as well as air ;-)  I use the gauze from disposable surgical masks as filters, and stick circles of the stuff over the outside of the intakes with blu-tac.  It's so pervious to air that it doesn't obstruct airflow, but the fibrous nature of it catches everything.  Got to change it regularly though, because it clogs up quickly.

Here's a good (but slightly small) diagram showing the sources and directions of a combined airflow:

Better ones on Page 13 of this document:

Take a look at some of the whacky experiments here:
expecially this one:
I would submit that the airflow arrows attributed to the side panel fans located so close to each other would be impossible.

A good discussion here which is worth a read:

This guy has done a fair amount of observations on his own case:

Your case design?
I would say that the direction of airflow from the fan in the side of the case depends ENTIRELY on how it is lined up in relation to the processor fan.  If they are physically opposing each other, then your case fan is probably cancelling out the action of the processor fan that needs to push air down into the heatsink fins.  If the fans are a fair distance apart then it MIGHT be supplementing the cool air flow.

The processor fan is not the only consideration.  Most modern graphics cards will have their own fan, and the same issues about cancelling its air flow to the chip on it apply.

How to visualise and measure the airflow, etc?

If your motherboard supports a software temperature monitoring utility (eg. Intel Active Monitor), AND IF the thresholds are set to default, then if it alarms out or shows a warning into the amber or red for any areas or on-chip sensors, then the airflow is wrong.   A significant difference in termperatures between different sensors might indicate if warm air is circling and stagnating.  If you know your motherboard and processor model, then you can find out the optimum and maximum permissable temperature ranges for them.  If any of the sensors are fast approaching this threshold, then airflow is probably wrong.

If the board doesn't support such a software monitoring utility (or if there is not one on your motherboard CD or manufacturer's site), then a couple of digital thermometers dangled into the case through holes into strategic areas (without touching any components) would be a reasonable substitute.  What about those probes to check if your chicken is thoroughly cooked inside?  Just DON'T let it touch any components!!

Airflow indicators?
People study at universities to qualify in this field, and use miniatute digital anemometers and other expensive equipment.  Here's a cheap tip though.

While working at the new £530 Million N.E.C. Wafer Fabrication Plant about 10 years ago, I witnessed the cleanroom commissioning engineers using the most primitive of testing apparatus in an environment where a human hair would have been like a tree trunk in terms of air pollutants, and would have wrecked the etching or photo-lithography processes.  I mean, this place ran about 10 times cleaner than the cleanest of surgical operating theatres in terms of air cleanliness.  Nobody gets in without walking over tacky mats, dressing in a cleanroom suit, walking over another tacky mat, and finally through an "air shower". ;-)

The extensive floor area was made from perforated tiles and mesh grilles just like this:

Air was sucked down into the sub-fabrication plant by hugely powerful fans behind filament filters and water "air scrubbers" and baffled by partitions which created separate wind tunnels that were very hard to walk in while dressed in the bunny suit.  The air was then returned to the fabrication floor above through ceiling filters.  If the air suction through the floor perforations wasn't powerful enough, it didn't pull it down vertically and was less efficient.

The testing equipment used for this, instead of the proper equipment ( was .... a bent coat hanger with something like a length of red unwaxed dental floss attached to it !!  Well, this has got me thinking about how you could visualise airflow inside your case.

Some cases have plexiglass (perspex) sheeting for the sides so you can see all the neon fans and ide cables.  If you could gate a couple of pieces the right size, you could tape them into place temporarily.  Cut the hole for the fan with a heated knife (watch the fumes) or with a hand-held jigsaw or Dremel.

Several ways to see the air flow:

1. Liquid Nitrogen "smoke" (dangerous and expensive - could freeze and crack components or your fingers):
Dry-Ice (frozen solid carbon dioxide) may work the same, is less dangerous, and is more readily available, but could leave dampness inside the case.

2. Real smoke like an incense stick, but this could leave tarry contaminants that may damage a motherboard and components.

3. Some "flagella" flexible enough to yield in a wind, but return to shape again without any wind.  Something that would emulate the way seaweed moves in a current.

Obviously I don't advise 1 or 2, but how about using some short pieces of nylon fishing line or similar stuff.
Stick them vertically to vacant spaces on the motherboard with blu-tac.
Put on your plexiglass sides and power up.
You might just be able to determine the direction and power of the airflow visually.

Personally I wouldn't go to those extremes.  You can get a good feel for how hot the air is that blows out the rear of the computer.

Top Expert 2013
here is the visualisation
front  |                    |   Back
         |                    |
 AIR   |                    | AIR
  IN    | ===>          | OUT   ==>
         |                    |
         |                    |
         |                    |
the idea is to create a stream front front to back; so a fan on the front should blow air in, and on the back it blows out.  if the fan on the side is blowing out, and you want to add a fan, you can best put it on the fron t for air intake, to equalise in - and output
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Top Expert 2007
Put the intake fans at the lowest point in the front and the output fans at the highest point in the rear( Heat rises) Ensure that there are as little as possible obstructions to air flow by cables and such. Is the fan on the side of the case directly above the CPU fan. If so which direction is the CPU fan blowing. Down onto the heatsink or up through the heatsink. If the side fan is pulling air out then I would say the CPU fan should be pulling air through the heatsink and exhausting towards the side fan.
In olden days it was considered bad practice to use a fan to evacuate the casing, because then there would be no control over the incoming air; it goes in where it can and brings dust. So, consequently, fans were used to take outside, cool air into the casing, with a filter included in the air flow...

Generally that is still a good idea. Fill the box with filtered air from outside and let it vent out through adequate openings (or blow-out fans; just keep the box under "pressure"). The air flow inside is in part another matter. Ensuring that the inside air is cool (good air flow from outside) is step one, blowing air over (from) the heatsink fins is step two. Which way heatsink fans blow is a matter of fin/heatsink design. I'd guess "blowing" into the fins is the better method, but that's a guess...
I suggest just splitting points equally between all contributors.
Thank you rindi and Computer101

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