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Positive or negative air pressure? (case fan cooling)

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a b ) Power supply
July 3, 2011 5:10:17 AM

hi there,is it better to have positive or negative air pressure for case cooling fans,e.g more intake vs less exhaust,i have a cooler master storm enforcer case for my gaming system(core I5 2500k..gtx 560 ti oc..cm hyper 212 cpu cooler),200mm intake fan at the front and 120mm exhaust at the rear,with the option of a 200mm fan at the top..
a b ) Power supply
July 3, 2011 5:17:39 AM

I personally like more exhaust than intakes
which creates a negative pressure
but alot of users favor the postive due to less dust getting in

I think the negative pressure reduces dead spots in the case
that too many intakes can cause

I like a straight front to back flow through case
with intakes low and exhausts higher up

Air cooling is kind of controversial and sometimes a "voodoo science"
difference cases need different setups

If you are going to do negative then make sure you have filtered
intakes
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a b ) Power supply
July 3, 2011 6:20:57 AM

I've given both sides a try, and I am one who prefers positive pressure. However, I don't practice it in excess. In my 4 systems, not a single one has more than roughly +40 CFM, with the majority hovering at about +20 CFM. And yes, I choose positive pressure mostly due to the dust issue king smp mentioned, with the key element being complete filtration of the intake fan ports.

As for dead spots within a case, that's not caused by positive pressure, but a lack of circulation. It can occur in both positive and negative pressure environments. The key to avoiding them is creating a clear path/flow of air from one point to another, or several points to others.

As for filtration of a negative pressure system, it's near impossible to do so completely. Negative pressure essentially creates a vacuum. Air will always seek the path of least resistance, which leaves unobstructed paths the easiest to enter through, while obstructed ones are tougher. A filter over an intake vent actually creates an obstructed path. Since you can't filter every crack and crevice of a case, they become the paths of least resistance. These are the places air can enter more freely, and with it, dust also enters.

I'll offer my Antec 300 case's setup as a positive pressure example. That case has the following fan slots:
2 front 120mm, 1 rear 120mm, 1 top 140mm, 1 side 120mm (My side port is sealed - I cut it out and installed a sealed window in the panel.)

PSU is an Antec EA430, w/ an 80mm fan (20-40 CFM, approx.)
Video card: HIS HD4850 w/ an 60mm fan (20-40 CFM, approx.)
The average of the two: about 60CFM of exhaust combined.
I cover negative flow rate with a single 66CFM 120mm front intake fan.
The rear 120mm fan: ~26CFM of exhaust.
Top 140mm fan: ~33CFM of exhaust.
I then cover them with another 66CFM 120mm front intake fan.
Positive CFM rating is 132, negative is about 120. That's a positive pressure of roughly 10 CFM, which can easily escape through any cracks/crevices and the unused 5-1/4" drive bay ports. At low-load, the PSU and GPU fans will not pull their average. That's why the front fans are on a controller. I can easily adjust them to a slightly slower speed to maintain a more balanced level of pressure.
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a b ) Power supply
July 3, 2011 6:35:38 AM

Personally, I'd take positive air pressure. Technically, negative provides more cooling if you exhaust as much hot air as you can in most cases, however, your computer becomes a dust magnet. While in positive air pressure, the case almost acts as a seal from dust. This can result in less cooling, but if you have a good case and your parts are running at fine/cool/stable temperatures, then why not just do positive?

I take a different approach, I have a crazy air filtration system in my house with almost all hardwood floor so my air itself has almost no dust, so even with negative air pressure it takes years for even a insignificant invisible layer of dust to accumulate on anything. But for you, if you want the best cooling, by a air filter and place it near your PC and let it run 24/7, whenever you leave the room, turn it up to high, and when you are using it, turn it down. The lower dust levels (assuming the filter is good and large enough for the room), will allow you to use negative pressure with no filters.
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a b ) Power supply
July 3, 2011 7:02:05 AM

RazberyBandit said:
I've given both sides a try, and I am one who prefers positive pressure. However, I don't practice it in excess. In my 4 systems, not a single one has more than roughly +40 CFM, with the majority hovering at about +20 CFM. And yes, I choose positive pressure mostly due to the dust issue king smp mentioned, with the key element being complete filtration of the intake fan ports.

As for dead spots within a case, that's not caused by positive pressure, but a lack of circulation. It can occur in both positive and negative pressure environments. The key to avoiding them is creating a clear path/flow of air from one point to another, or several points to others.

As for filtration of a negative pressure system, it's near impossible to do so completely. Negative pressure essentially creates a vacuum. Air will always seek the path of least resistance, which leaves unobstructed paths the easiest to enter through, while obstructed ones are tougher. A filter over an intake vent actually creates an obstructed path. Since you can't filter every crack and crevice of a case, they become the paths of least resistance. These are the places air can enter more freely, and with it, dust also enters.

I'll offer my Antec 300 case's setup as a positive pressure example. That case has the following fan slots:
2 front 120mm, 1 rear 120mm, 1 top 140mm, 1 side 120mm (My side port is sealed - I cut it out and installed a sealed window in the panel.)

PSU is an Antec EA430, w/ an 80mm fan (20-40 CFM, approx.)
Video card: HIS HD4850 w/ an 60mm fan (20-40 CFM, approx.)
The average of the two: about 60CFM of exhaust combined.
I cover negative flow rate with a single 66CFM 120mm front intake fan.
The rear 120mm fan: ~26CFM of exhaust.
Top 140mm fan: ~33CFM of exhaust.
I then cover them with another 66CFM 120mm front intake fan.
Positive CFM rating is 132, negative is about 120. That's a positive pressure of roughly 10 CFM, which can easily escape through any cracks/crevices and the unused 5-1/4" drive bay ports. At low-load, the PSU and GPU fans will not pull their average. That's why the front fans are on a controller. I can easily adjust them to a slightly slower speed to maintain a more balanced level of pressure.

thank you for your reply,do you think if i had a 200mm front intake(110cfm) a rear 120mm exhaust(40cfm)and the psu at the bottom(antec 650watt true power)and my gtx 560 ti exhaust, is this a good positive flow(pressure)??
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a c 91 ) Power supply
July 3, 2011 12:39:45 PM

Just compare two cases, one with positive air pressure (Silverstone Fortress FT02) and one with negative air pressue (Coolermaster ATCS 840) and you will get your answer. Most cases opt for a positive air pressure setup. Now its also not about the number of fans. The placement matters. Consider a case like the Storm Sniper. The side 230mm intake creates almost no significant temperature difference. Here is an example where negative pressure works.
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a c 87 ) Power supply
July 3, 2011 4:08:58 PM

The computer will show different results (temps) under positive pressure, and under negative pressure. In both instances, I am talking about slight differences in pressure; less than 15% different.

Personally, I prefer a slight positive pressure; Intakes no more than 15% higher than Exhausts. Again, each computer will behave different under +ve and under -ve pressure.

Measuring the differential air pressure is simple enough using a manometer. http://www.chm.davidson.edu/vce/gaslaws/pressure.html - Is this required? Certainly not for a computer.

Instead, try various combinations of fan speeds and note down the resulting temps. - lean towards the settings that give you lower temps. - even this may not be necessary if the temps are well within acceptable ranges.

Theory is fine and required in order to grasp the principles of technology, but may not be required to be applied too deeply in some instances. One of my favorite expressions is "In theory, the bumble bee cannot fly!" (I have used this on some occasions at corporate meetings with the 'Big Three' in Detroit).

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a b ) Power supply
July 4, 2011 3:14:22 AM

all the temp testing i have done with my pc's in the past has shown more exhaust is better, particularly at the upper rear of the case. having an exhaust fan at the lower front i have found can have negative results.
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a b ) Power supply
July 4, 2011 3:55:18 AM

iam2thecrowe said:
all the temp testing i have done with my pc's in the past has shown more exhaust is better, particularly at the upper rear of the case. having an exhaust fan at the lower front i have found can have negative results.



^+5 I agree completely

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Anonymous
a b ) Power supply
August 15, 2011 7:57:48 PM

well that's just logic. Cool air is moving down and hot air is moving up. If the down opening is blocked by an exhaust fan temp will raise. And of course when there is no cool air the hot air isn't moving since there is no where to go :) 
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December 1, 2011 5:32:09 AM

blackhawk1928 said:
Personally, I'd take positive air pressure. Technically, negative provides more cooling if you exhaust as much hot air as you can in most cases, however, your computer becomes a dust magnet. While in positive air pressure, the case almost acts as a seal from dust. This can result in less cooling, but if you have a good case and your parts are running at fine/cool/stable temperatures, then why not just do positive?

I take a different approach, I have a crazy air filtration system in my house with almost all hardwood floor so my air itself has almost no dust, so even with negative air pressure it takes years for even a insignificant invisible layer of dust to accumulate on anything. But for you, if you want the best cooling, by a air filter and place it near your PC and let it run 24/7, whenever you leave the room, turn it up to high, and when you are using it, turn it down. The lower dust levels (assuming the filter is good and large enough for the room), will allow you to use negative pressure with no filters.


:I agree with positive air pressure concept where there is a single air intake port with a dust filter. With a negative air pressure in the case, dust collects or enters through all the openings in the case.
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a b ) Power supply
December 11, 2011 1:23:36 AM

Best answer selected by mrclownface.
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