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optimizing air flow by blocking vents?

R

Red Squirrel

[H]F Junkie
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Nov 29, 2009
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I've always had the belief that air should be forced to go a specific path through the PC in order to get the best cooling. Basically want to ensure that the heat generators get fresh air at all times and the air that passes by ends up exhausting. Want to avoid "vortexes" or rough air flow. So idealy there should be air entering the front of the case, cooling any hard drives that are there, this air should then be forced to go over the motherboard and cards (especially video cards) and then go up to the cpu, then out the top of the case, and some of it out the power supply.

So basically, fan(s) in front sucking air in, fan(s) on top sucking air out, and PSU fan. Also, video card vents for the video cards. Any other holes should be taped up.

Or is it better to just leave them all open?

What about fans on the side blowing on the motherboard/cards, is this good or bad? It sounds good at first, but then, I wonder what it's doing to the rest of the air flow because the air is coming in perpendicular to the rest of the air flow.
 
It depends on how much air you have taken in and how much exhausted. In other words, whether you have a positive pressure or negative pressure setup. If you have more exhaust than intake (negative pressure) like 90% of the people who are using strictly air cooling and don't know what they are doing, its best to leave the vents open. The case manufacturers put them there so that if any additional air should be needed, cool air will be brought in at the hot spots. If you start blocking off those vents, then intake fans get louder as they have to artificially spin faster to provide more air, and air will also be sucked in through every little crevice (e.g. between PCI slots and DVD drives) creating little pockets of air and dust everywhere. Worse yet is when the exhaust fans can't get the air they need from those little crevices or vents anymore due to dust or being blocked off and no additional air is being supplied to them. The stronger fans will win against the weaker ones (e.g. PSU fans) to the point they move little air through the fan and simply throw remaining hot air about and not exhaust it.

The ideal air cooling setup is a front to back cooling setup just like how servers are setup with a preference for positive pressure. There are 2 reasons why:

1. The intake fans are forcing a stream of cold air directly onto components forcing the air to move in a linear pattern from front to back. This means the air doesn't have any chance to be heated up by other components, such as is the case with intake fans on the bottom who's air gets heated by the hard drives and graphics cards first. With more intake than exhaust, the excess pressure will force the air out any crevices in the back and through removed or slotted PCI slot covers taking heat with it.

2. Dust buildup is considerably less. Dust will buildup where there is the most friction between air and material, creating a static charge large enough to keep it stuck to things.
If air has to be sucked in through crevices, eventually the crevices get caked up with dust making the cooling situation even more difficult as mentioned above. Places where there is no air movement at all will build up dust rather than dust being forced out with the air flow.

Now keep in mind, the front to back cooling arrangement does not necessarily apply to watercooling setups where the heat is pumped to a different location before being removed. However, it is still necessary to have enough venting if your watercooling arrangement is a negative pressure setup rather than have it suck in air from crevices.
 
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The front to back air flow only works if there's an unobstructed path between the front intakes and the CPU heatsink. In my experience if you have the usual bottom mounted front intake and the mid/top mounted exhaust the airflow is already screwed up by the graphics card and hard drives in between the intake and the CPU.

Note that this doesn't apply to rackmount servers since they usually have a straight front to back airflow where the air doesn't have to take 2 or 3 turns to get from the intake to the exhaust.

IMO the best rules of thumb for case air cooling are

1. Short and direct path from fresh air intake to component - side intakes that directly blow onto CPUs and graphics cards lower temperature significantly more than a good theoretical front to back flow.

2. If you have a tower style HSF a side intake that blows down into the area directly in front of the HSF, or a front intake that is inline with the tower's air flow works best.

3. Positive air pressure with quality dust filtering on all intakes - eliminates dust almost completely.
 
Cool so from what I read, think I will go with 2 front intakes and 2 side intakes, and I'll leave the exhaust ports open for air to passively move out. Most of the top of the case is exhaust which works nicely as heat rises.

The front has a filter but the side does not, I may rig something up. My current pc has no filters and it's nasty. I could probably make a kitten with all the cat hair. :p The thing with filters is I'll have to make sure to vacuum them out regularly.
 
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