Case Airflow: Positive or Negative Pressure?

Case airflow should err on the side of...

  • Positive

    Votes: 164 49.5%
  • Negative

    Votes: 167 50.5%

  • Total voters
    331
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Findail

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Based on comments in another thread, I now have a question about the ideal way to set up airflow through a case. It seems that there are two camps in this debate -- those that think the air pressure in the case should be positive, and those that think it should be negative. For the sake of clarification, I'll define what I think each of those terms mean:

Positive - there is more fresh air coming into the case than there is hot air exiting the case. Basically, the CFM from intake fans exceeds the CFM from exhaust fans.

Negative - there is more air exiting the case via exhaust fans than fresh air entering the case via intake fans and vents. The CFM for exhaust fans exceeds the CFM from intake fans.

Now I know that very few of us have taken the time to run silicon caulking on all the seams of our computers to make them completely air tight, so there is more involved here than just the fan CFM -- positive pressure cases won't explode over a period of time and negative pressure cases won't implode. Also, the direction of airflow within a case has a lot to do with the effectiveness of the cooling, but let's ignore that for now.

The real question is this, I suppose. Is it better to pull air through the system or to push it? Or does it not really matter? Anyone have links to some good testing on this, or some empirical data to back up theories?

EDIT: I turned this into a poll with a slightly different flavor. Balancing a system's airflow exactly is probably impossible, so when designing your fan setup, should you make it more negative, or more positive?
 

anticupidon

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first hot air tends to expand and cool air to do the reverse
in my opinion CFM for exhaust fans exceedes CFM from intake fans a negative pressure
second every case has its own airflow
i modded a few cheapo cases adding a top exhaust fan...and i saw temperatures dropdown around 5 10 c why top exhaust fan following principes of termodinamic hot air expands and goes up and cool air goes down so the top exhaust fan comes in the right way
to have a positive pressure intake CFM higher than exhaust CFM in my opinion will cause a temperature drop but after a period of time temp will raise because hot air stays inside ..due to low exhaust CFM
plese feel free to reply to this thread more opinion more to learn
ps i did not provided links to feed up my theory but i will
 

anticupidon

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i almost forget
more intake fans more cool air but..more dust also the invisible enemy
my opinion 2 best scenarios
1 intake CFM equals exhaust CFM
good cable management
2 negative pressure exhaust CFM higher than intake CFM
good cable management
keep the flames to a minimum level :tongue:
 

chuckshissle

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Intake is always better and it has bigger effect than exhaust, especially directed towards the components needed to be cooled. Usually there's one in front of the case blowing air in towards the hard drive cage and pass it to the motherboard. The warm air exits out the back fan and PSU. Adding a side panel fan drawing air in to the cpu cooler and gpu coolers will have a significant decrease in temperature in those components and as well as the ram and north bridge. It's good to have a filter but it's better to have not to has less air restriction. But of course you need to clean the pc occasionally.
 

yakyb

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i have always been a fan of negative pressure as i feel it will leave less standing air in the case

whether there is any scientific proof of this im not sure ive never read a test of it if anyone has a link to one id be grateful

youd have to bear in mind tho that for any pressure change to occur from outside pressure the whole case would have to be air tight as even gaps between cd drives would pull in air to balance out so unless you plan on spending ages sealingeverything your not going to see any difference

also as a side note things cool better witht the side off so you would be better off putting a mesh on your side panel
 

nastymat

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I like to look at things in the extreme to get an understanding of the benefits.

Imagine there was only a tiny exhaust, the temperature would get very hot quickly. Pumping cool air in would not lower the temperature much.

Now imagine there was a huge exhaust fan on the top of a case and only a small input fan.. the hot air would get extracted instantly. The air going in would get hot quick but not stay in long.

Stand in a hot room and open no windows.. it's only going to get hotter. No open a window that wind blows in.. in front of the window it's nice.. elsewhere it's still hot.. no open a door to allow the air to circulate out.. room cools down nicely..
 

croc

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Your desktop is not a computer room... If it were, then positive pressure would be the obvious answer as the input air is highly filtered... So positive pressure tends to keep dirt out. In a desktop I'd just look for a balanced cfm environment with effecient input filtering and enough exhaust to prevent suck back through the exhaust fans. In short, I think that a balanced airflow would be a good thing, but I'd lean a bit towards some positive pressure and some good filtration at the input.

So the more I think about it, your desktop should be treated more like a computer room....
 

leo2kp

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I'm no pro on airflow, but I think location is a big problem too. There are hot-pockets of air that form in certain places in the case if airflow isn't done well and that can hurt things too. Like someone said before, hot air can be trapped on the top of the case if there isn't an exhaust on the top.

If you look at some of the case designs, for example the TT Armor, you can tell quite quickly that they favor negative pressure. The basic Armor case (without the huge side fan) has a single intake but 4 exhausts, considering the PSU as an exhaust as well. You can also see this in the way they've set up the front faceplate. Every drive cover has a filter from top to bottom (with or without an intake fan behind it) promoting air to be sucked in through the filters due to the negative pressure of the exhaust fans.

In my opinion, negative pressure just makes more sense. Assuming most cases have a decent area for air intake in the front, you can use the suction effect from the exhaust fans to draw in cool air from the open vents in the front while the fans blow air out the back. If you are exhausting air, then more air must be coming in from somewhere. Same goes with positive pressure, however you may not be blowing hot air out where you would like it to be blown and you can create pockets of hot air inside the case. It's probably better to use more exhaust fans. Air out = air in. BUT that's not to say that there is no place for intake fans. There should be strategically placed intake fans for things like hard-drives, video cards, and processors. I'm a big fan (lol) of side intake fans because they blow cold air directly on the hottest points of the computer, and I think hard-drive cooling is also a definite must for any enthusiast, and you should also strongly consider a top exhaust of some sort.

Point being, it's all about strategy, eliminating hot-pockets, getting cold air to flow over your stuff, and getting hot air out. However you do it is up to you.


That's my 2c =)
 

No1sFanboy

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I agree that intake fans only real purpose is to promote airflow in particular areas. If you had only exhaust fans you would still move as much air; it's not like you're creating a vacuum.
Think of a wind tunnel, they only exhaust. Now if computers were shaped like a tunnel you have no need for intake fans.
 

Findail

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Wow. Seems to be a real mix of opinion here. So why not turn it into a poll?

I refined the question a bit. I think we'd all agree that a perfectly balanced system would be best. However, without knowing exactly how much that PSU fan is pulling out, or that GPU cooling fan, or a thousand other factors, it is nearly impossible to balance perfectly. So the new question would be, do I want to err on the side of positive or negative?

For example -- I have a 120mm exhaust on my PSU, and a 60mm exhaust on my GPU. I have no idea what CFM they are pumping out. I also have a 120mm case fan for exhaust that I can't find CFM specs for (Zalman quiet fan). So let's just use millimeters to start with. I've got 300mm of exhaust fan.

Now, I've got a 120mm intake (front bottom) that can pull in 45 CFM. Two more 80mm will go in a side panel as intake, pumping 25.64 each. In millimeters, I have 280mm of intake. CFM might be higher, though, as the fans on intake are better than the fans on exhaust. To sum up, I have:

300mm exhaust vs. 280mm intake, so I should be running negative. Is that preferable to positive, considering there's a lot of air pumping through that case?
 

warezme

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I am no thermodynamics expert....,

but air can be treated like water flow. Its not so much which way you run as long as the flow rate is positive in either direction and the higher the flow rate the more efficient it is.
 

jonkc

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I think negative for the reasons stated above, My case is a Tsunami Dream with 1 120mm intake, 1 120mm exhaust as well as a 90mm side exhaust and the power supply as an exhaust.

I was also thinking about adding a PCI slot cooler, but wanted a couple of opinions on how effective they are...
 

shadowmaster625

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You MUST have positive airflow, or else you'll pull in dust from every little crack and crevice in the case. Of course this is assuming you're using filtered air inputs. If you do not care about dust, then you want negative airflow because it creates more of a pressure gradient which allows for better cooling at the points where the air pressure is highest (which is wherever your fans are pointing)
 

ejay

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You’ve hit on what is probably the most important issue in extending motherboard and processor MTBF. This is a particularly relevant topic for system builders because of diminishing margins and/or bothersome warranty costs. It is also relevant to the DIY enthusiasts who seek to extend the useful lifetime of their equipment and maximize performance. For the air-cooling aficionado, the prudent selection of cases and fans becomes an important issue. From my own experience, flow-through is the key to a cool running box and for minimizing the heat envelope of the case. Creating a “wind-tunnel” trumps either positive or negative interior case pressures, provided there are no significant differences between them (>20% accumulative CFM). The relatively low output of GPU exit-air solutions and PSU fans can help to reduce the heat envelope but pale in comparison to push/pull affects of the much larger CFMs provided by 92mm or 120mm case fans.

Location of fans and case design are critical elements. Matched 120mm case fans, one pulling in air from the front, and the second in the rear pushing out, is a good place to start. Beware of cases with bottom mounted hard drive cages turned 90 degrees; this arrangement generally reduces the flow-through of the front mounted fan – look for the “open throat” hard drive cage solution. Also, consider cases that have side panels with a CAG tube that accommodate at least a 92mm fan, and that also have a second air intake for the PCI and GPU slots. If a front 120 doesn’t come with the case, a side panel CAG 92mm and 80mm, coupled with a front 80mm, is a good match. With the proper case design and matched fans, I’ve seen processor and motherboard temperatures lowered by 20-30° F, even with an OEM HSF. A well-designed case and carefully matched case fans, can best even the most exotic air-cooled, processor HSF solutions. Of course, a well designed case with carefully matched fans, and the addition of a superior processor HSF such as the Zalman 9700, can enhance the probability of safe and effective over-clocking and/or extend the lifetime of the motherboard and processor. Just my 2 cents.
 

piasabird

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On airflow there are a lot of different things going on. I like a case where the Air intake flows directly around the Hard Drives. Whether there is a fan there or not is up to you. I have seen some cases where the fan is behind the Hard Drives so the fan noise is not as bad.

The Power supply also makes a negative air flow.

The size of the Air Holes can actually make a fan seem louder.

The Case can be like an echo chamber and actually aplify the noise of the fan. Think of it like the body of a guitar.

Rubber Grommets on the mounting holes of the hard drive can help deaden noise.

So do any of you people have a Fan on top of the Case? It seems like hot air should rise so why not have an air duct on top so it can keep on going?
 

Problem

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I'm on the Positive pressure side of this debate. I have a Raidmax xB case with a 120mm fan in front blowing in across the hard drive, the side cover fan 80mm also blows in, and the 80mm exhaust fan is on the back panel of the case, with an 80mm fan in the PS. I modded the case to open up the air intakes on the front panel, and glued fine mesh nylon screening over the openings from the inside. I'd like a cool system that stops sucking dust through my floppy and dvd drives.

Problem

New System
asus P5W DH bios 2004
E4300 1.80 Ghz Retail HSF
2Gb Gskill F2-6400CL5D
250Gb Seagate 7200.10
EVGA GeForce 7600GS 256mb
XP Pro SP2 OEM
 

leo2kp

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With dust out of the equation, negative is still the best solution. Add dust and you potentially have a huge problem - if you don't plan on cleaning out your case. Many cases today are coming with air filters to help with that issue because they know that negative pressure is better for helping eliminate air pockets which is something you can't just clean out, like dust. If you have a lot of dust, it's your fault for being a slob. If your case is built for positive pressure and thus has air pockets, it's only your fault for buying the case, not designing it. You have a higher chance of overheating because of the air pockets than you do with a few specs of dust. Dust builds up over time and can be removed anytime at your convenience every month or so. Hot air pockets build up within seconds of turning on your PC and can't be removed from a badly designed case...think about it.

I still say negative pressure with filters and regular dusting (which you should do regardless).
 

Findail

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Findail, wheres the poll option for balanced airflow? :)
Hey, no fence riding! :lol:

Actually, if I put that option up, then everyone would probably pick it. So the poll question is based on the assumption that you can never perfectly balance your airflow. With that being the case, which way should you err -- positive or negative?

This thread really has my noggin going now. I've got a box full of fans and filters on the way in the mail, and a case that desperately needs some ventilation. It basically has low CFM airflow right now, regardless of positive or negative, and as a result my E6600 runs at about 38C under moderate load. I'd like to get that down to the high 20's before I start overclocking. Removing the side panel and pointing a fan into the system on low brings it down to 26C, so it's definitely airflow.

BTW, anyone ever use one of these aluminum mesh fan filters? I ordered three of them; they appeal to me because you can remove the mesh screen for cleaning without opening the case. I figure they're better than nothing...
 
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