Case Airflow: Positive or Negative Pressure?

[4Ryan6]

If I go by fans only, my setup would list as a negative pressure system, however my case is seriously performance modified and I've additionally cut 2 80mm size intake holes in the bottom panel of the case, which actually allows an equal airflow pressure, because even though the intake holes have no powered fans they are allowing unobstructed fresh air being pulled directly into the case.

My power supply is not a case cooling/heating factor because it's mounted outside the case itself so it's fans are not factors in my setup at all. My case modifications have definitely improved the overall cooling over the stock case configuration which is basically standard accepted case design today, with very little changes over the years but still the old same ole same ole layout.

Basically what matters is the end result and does your case airflow no matter the fan count, allow the cooling you're really after.

 

[matta85]

I got the Antec Six Hundred case with 200mm exhaust at the top, 120mm exhaust at the rear top, psu 140mm at the rear bottom and a 120mm intake in the front where the hdd is. I have'nt installed gpu yet but when I do I'm thinking of installing a 120mm intake on the side. I have CM Hyper 212 + cpu cooler with a 120mm fan facing downwards. Installing a gpu (thinking of hd 6850) I belive it will reduce the intake on the cpu fan since it is facing the direction of the gpu. Anyone know if a 120mm fan on the side would be enough or should I change the direction of the fan for the cpu?

In advance thank you for your advice on this matter.

 

[ScrewySqrl]

I got the Antec Six Hundred case with 200mm exhaust at the top, 120mm exhaust at the rear top, psu 140mm at the rear bottom and a 120mm intake in the front where the hdd is. I have'nt installed gpu yet but when I do I'm thinking of installing a 120mm intake on the side. I have CM Hyper 212 + cpu cooler with a 120mm fan facing downwards. Installing a gpu (thinking of hd 6850) I belive it will reduce the intake on the cpu fan since it is facing the direction of the gpu. Anyone know if a 120mm fan on the side would be enough or should I change the direction of the fan for the cpu?

In advance thank you for your advice on this matter.

change the fan on your cooler to blow toward either the top or rear exhaust fans

 

[matta85]

change the fan on your cooler to blow toward either the top or rear exhaust fans

Okey thanks man =)

Hehe not quite the answer I was hoping for cause then I have to remove the the cooler, add new thermal paste and try to set it the other way.. But hell if it's worth it. Now I have 460mm (140mm + 120mm + 200mm) exhaust and only 120mm intake (in front)

Do you think I should add a 120mm on the side for cooling down the gpu and get more air into the case?

 

[ScrewySqrl]

sure. I'd add teh side fan as an intake

 

[Floatsup]

sure. I'd add teh side fan as an intake

I also agree, but I would put a filter in too. I have pets, and petfur/dander are an issue at my place.

 

[B3AR]

I vote positive, it seems to work for my case. Ive got the Corsair Obsidian 700D (not awesome for air cooling) but I have the three stock 140 mm fans going (1 intake at the bottom (filtered), 1 exhaust at the rear, and 1 internal blowing over hard drives) and three additional 120 mm fans up top blowing air in (filtered), which cools most of my motherboard. Air gets pushed out by that 1 stock 140 mm fan, the video cards, and the grill around the videocards. I might be doing it wrong, but my temps are good.

 

[das_stig]

Was in the positve camp, but have just experimented with my case and have now use negative to drop temps by a fair margin.

1 x 120mm intake for hdd cage
5 x 120mm exhaust, 1 x rear behind cpu, 2 x top, 2 x side panel
1 x 120mm on psu fitted at bottom of case, intake

Side and top fans rpm ast lowest speed by fan controller, rear fan controlled by bios via header.

 

[Floatsup]

OK, got a new case today, and will be updating the pics. The case is this

http://www.newegg.com/Product/ImageGallery.aspx?CurImage=11-147-120-TS&ISList=11-147-120-Z01%2c11-147-120-Z02%2c11-147-120-Z03%2c11-147-120-Z04%2c11-147-120-Z05%2c11-147-120-Z06%2c11-147-120-Z07%2c11-147-120-Z11%2c11-147-120-Z08%2c11-147-120-Z09%2c11-147-120-Z10&VSList=11-147-120-V01&S7ImageFlag=1&Item=N82E16811147120&Depa=0&WaterMark=0&Description=Rosewill%20Smart%20One%20ATX%20Mid%20Tower%20Computer%20Case%2c%20Black%20interior%2c%20comes%20with%20three%20Fans-1x%20Front%20Blue%20LED%20120mm%20Fan%2c%201x%20Rear%20120mm%20Fan%2c%201x%20Top%20120mm%20Fan%2c%20Option%20Fans-2x%20Side%20120mm%20Fan

Fantastick Case for the money. Will be posting pics on teh build. Waiting on UPS for the second delivery.

 

[d1g1t4l3ch0]

Negative is my vote.

Based on everything I've read so far and based on the fact that most premium cases are built with negative in mind. My case which is an Antec Twelve Hundred has 3 front 120mm fans which give 39CFM at low speed, 2 rear 120mm fans which also give 39CFM at low speed, and a top 200mm fan which gives 82CFM at low speed. I found that the ratio between intake and exhaust is 1.367. The CFM from intake is 39 * 3 = 117 and the CFM from exhaust is 39 * 2 = 78 + 82 = 160. You can clearly see that the case was designed for negative pressure. Of course this does not include any other factors than the rated CFM and with the front filters on will probably further lower the CFM on the front fans. Also, as mentioned before in this thread, the case isn't perfectly air tight so the actual ratio may be more close to 1:1 than calculated, but still; negative pressure.

 

[Griffolion]

Try and keep it fairly even, i give my computer a quarterly dust out anyway so the invisible enemy get's pwned under my roof.

 

[cjanecki84]

I am a FIRM believer in positive case pressure. For one reason only, dust management. Everything else can be managed either inside the case or outside the case, but if dust gets in it's hard to get it out.

The biggest argument i am reading is that Hot air stagnates in high pressure situations, but this is incorrect. Unless you actually did silicon caulk any air gaps in your case, hot air will escape through cracks, vents, and exhaust fans, granted that this may cause a slightly higher temp inside the case (no more than a few degrees), but your system shouldn't be pushed to the point that a few degrees may make a huge difference, and if it does, put your CPU in a refridgerator.

This few degree rise in tempurature is no match to the amount the temp will rise as your components slowly get cakes with dust. Yes you blow out your system weekly, but that doesnt stop dust and oil from coming to rest on your components and baking themselves to your Mobo and Cpu. Positive pressure, with filters on intake fans keeps much of the foreign material from entering your system giving you a longer life for your parts, and lower temps over the life of the equipment.

Now onto my example, I have a NZXT Hades Mid tower. Two 140's up top, one 120 in the back (on a Corsair H50), one 120 on the PSU (i believe is intake because of the dust on the filter but i cant remember for sure), and one 250 on the side panel (all filtered) for ~400 to 450 cfm of Intake. Now for exhaust I have one 250 in the front, and a cyclone under the video card for about 280 cfm's of exhaust. At these levels I can feel the positive pressure exiting the vents. My Case temps hover around 27 F, room temp. Now i am a minimalist. Only one Internal HD with one SSD, and no internal roms. But my case stays clean and cool

 

[19wish]

Here is my positive setup. 11 intake fan, 2 exaust fan the top and the one next to the cpu.

By niynewish

 

[cjanecki84]

Oh and for those of you wondering, you can find CFMs for almost every fan by google'ing the part number on the information label. Rule of thumb take the largest identifying number and plug it into google.

 

[simpleWho]

Hello, my first post, but here is my take on it.

If exhaust pushes out more air than the intake can feed into the case, then (hypothetically speaking), after some time, there will be no more air left for the exhaust to push out. In other words, exhaust fan can not continue to operate at the same CFM despite continuing to rotate at the same RPM; there just isn't air to push out. In other words, it will be rotating without pushing any air out. In other words, we can say the exhaust fan efficiency will go down over time and will not recover.

On the other hand, if there is more intake air (higher CFM) and less exhaust air (lower CFM), then exhaust will always have enough air to push out, so it continues to operate at its maximum efficiency; that is, it will never have any dearth of air to push out. The intake fan continues to feed air into the system. But it can not do that indefinitely. At some point the air pressure inside the system becomes large enough that it will prevent the intake fan from pushing any more air into the case. (This happens because the air inside the case will exert itself against the intake fan and will prevent it/block it from sending any more air inside.) Thus the efficiency of the the intake fan will go down and it will stop operating at its maximum CFM. However, note that the exhaust fan has no such issue, so it continues to pump the air out at its maximum efficiency; which will eventually cause the air pressure inside the case to drop. This will again enable the intake fan to recover its efficiency and it starts sending in more air; until the whole scenario plays itself again.

Thus, this system is self-balancing. I have broken it down as happening in discrete steps, but i na real system, this effect will be continuous and the system will practically work at an equilibrium; where output fan works at maximum efficiency and the input fan works at a slightly lower CFM than what what it is capable of.


ALSO, there is one more point. When there is no air (in the case of -ve pressure), components will not be able to transfer their heat to the air. So how much pressure you want in the system depends on what pressure gives best heat transfer. (I don't know if the answer is the more the pressure, the better the heat transfer).

Hence, I think there is no reason to even consider -ve pressure. The question is what should be the ratio of intake to exhaust CFM for the most efficient cooling. Clearly if there is no exhaust, intake will stop sending in new air at some point when the pressure inside the case becomes too much +ve. Slower exhaust would mean the intake has to wait quite a bit before going back to its full efficiency. This gives lot of high pressure inside the case (which increases the heat gathering capacity), but because of low exhaust heat is not removed as quickly! Equal speeds would mean there would not be any pressure difference, hence the heat-gathering capacity of air may not be as high as in the case of a +ve pressure system. Exhaust can not be higher than intake (as we have theorized above).

So, I think, here are the parameters based on the assumption that more pressure means more capacity to gather heat.

- intake needs to be as high as it can be; more pressure means more capacity (of the air) to gather heat from the components.

- exhaust has to be slower than intake to maintain a +ve pressure inside the case which is automatically self-balancing. It has to be high enough so that intake continues to operate at near-maximum efficiency; that is; keep increasing exhaust speed untill intake efficiency starts to go down. That is the sweet spot.

Thanks for listening to this rant. Didn't mean it to go this long but one thought led to another.

 

[Alling]

I spoke to Intel representatives at the IFA 2011 two days ago, and they said they recommend negative pressure inside the case, since the most important thing is to pull the hot air out of the case.

Who knows what's best if Intel doesn't?

 

[JustinChiu24]

There should be a positive pressure inside the computer case, since there are holes in the case to let air out, if there's a negative pressure, the air will go in without a dust filter and the dust will increase in the case. In my opinion, the best way will be to have exhaust on the top, since hot air goes upward, and the intake fans will use a dust filter, this means the air that goes INTO the case will be clean, and since the pressure inside is higher than the outside, air wouldn't go in from the holes of the case, and therefor dust will be kept out of the case. Of course, that's just my theory, but I'm pretty sure it'll work.

 

[JustinChiu24]

Positive to keep the dust out

But if you are pushing more air in than out therefore positive than you are pushing in more dust as well...

But you can use dust filters on the fans, but not on the whole case. the air that goes in from the fans will be clean, and dust wont go in from the hole cause the inside of the case has higher pressure

 

[NeilR]

I personally believe in negative pressure, if you want, you can read my theory:

To understand this working you must remember that...
...pressure is proportional to heat, an increase in one will result in an increase in the other.
...heat is transferred in three ways; conduction (vibration of particles; one particle vibrates its neighbors, which vibrate their neighbors and so on), convection (a substance expands, giving it a lower density, causing it to rise, taking its heat energy with it), and radiation (heat carried as infra-red waves/radiation).
(this isn't my working, it's scientific fact)

High pressure: Air is hotter (components will lose heat to air less quickly), but it's particles are closer together (heat moves through air more quickly – better conduction).
Low pressure: Air is cooler (components will lose heat to air more quickly), but it's particles are further apart (heat moves through air less quickly – poorer conduction).
High pressure will slowly move heat from components away from components.
Low pressure will quickly move heat from components to air around components.
Air is a poor conductor at the best of times, most heat transfer in air and other gases happens through convection.
High pressure: Heat is slowly drawn from components and whisked away by convection.
Low pressure: Heat is quickly drawn from components and whisked away by convection.
Therefore, low pressure (negative pressure) is, in my opinion, better, as it draws heat from components more quickly than high pressure. However, this theory could be affected if fans were placed in such a way that they reduced convection, such as blowing down from the top of the case. Another consideration with positioning fans is size. A bigger fan can generally run more slowly to produce the same CFM as a similar smaller fan running at a higher speed, and so larger fans are quieter. Because of this, I would personally place a large exhaust fan (as large as I could buy and fit) on my side panel (rather than the top of my case, the obvious choice to assist convection), as this would not affect convection, would produce the negative pressure, and would be able to run slowly and quietly.

If you don't quite follow my working, or see a flaw, please reply and let me know.

I take NO RESPONSIBILITY for any damage, loss, or injury caused as a direct or indirect result of following any advice I place on any forum.

 

[JustinChiu24]

Positive to keep the dust out

But if you are pushing more air in than out therefore positive than you are pushing in more dust as well...

Something called air filters

 

[edwarde54]

In any given environment it is simple logic that the quicker hot air is expelled the quicker the surroundings cools down, right! It is as simple as that and would you agree, applicable to any computer be it desktop or laptop. We want hot air out fast to let cool air in to prevent from frying. We have unlimited supply of fans and strategic places to fit them inside the housing for optimal air circulation. We have the option to apply the "negative or positive air flow configuration" (hot air out fast or cool air in fast), our choice. In regards to dust, an unavoidable circumstance. We don't like it but we have it. No matter where we direct our fans, whatever filters we use, dust will eventually slip in no matter what & the only sensible thing to do is to just brush it off. And that is that, we have only ourselves to choose what we think is best.

Now, most of the setups mentioned here regarding air flow only refers to intake & exhaust fans, the number of fans used or to use, etc. Question. What if an aftermarket heatsink/fan's installed (like for example, a Coolermaster V8 with 2 fans (in & out), how do we configure the optimal air flow with considerations to negative and positive air pressure? Do we consider the air flow generated from the heatsink's fans in regards to CFM's? The air drawn into the heatsink from the attached fan & expelled from the other end by the 2nd fan. Where does the cfm fit in? We have the intake & exhaust fans plus the vga fan(s) & the PSU fan considered. Just an example of a situation that needs mentioning.

 

[NeilR]

Something called air filters

Air filters work either way, whether you're positive, or negative, or neutral. My main concern with air filters is their affect on air flow. Cleaning isn't an issue, as they are easier to clean than the whole inside of the computer.

 

[edwarde54]

NEGATIVE AIR-FLOW WORKS BEST!

The subject on negative or positive air flow has been interesting and highly informative in regards for better cooling environment for optimal computer use. Out of curiousity I had to find out first hand what would be the best air-flow configuration and so I did a little experiment on my desktop mainly for the purpose of improving CPU temperature: (all stock) Intel QuadCore i5-760@2.80GHz; Asus P7P55D-E LX Mboard; Win7Ultimate-SP1_x86; Palit GTX460-768Mb video card; 3.50 GB RAM; Gigabyte Superb 720W PSU and Coolermaster CM90 Nvidia Edition (6 fans) mid-tower alum case. These are the layout of my (6) 120mm Coolermaster fans: I have a Sickleflow (69.69cfm) positioned at the upper rear end of the case; 2 more exhaust (Standard) fans on top; 1 side, 1 front (filtered), 1 bottom (filtered) as intake (also Standard) fans [all 5 Standard fans are 44.03 cfm's each]. Using Asus ProbeII Monitoring Device, CPU temps on this setup read 38-42 degrees C on low workload considering normal room temperature ranging to 30-34 degrees C. I interchanged the 2 fan layouts only between the front Standard fan & the rear Sickleflow fan to allow more air in hoping to improve air-flow thus lowering CPU temperature and ran the computer at normal operations. In less than an hour I was surprised of what I learned, CPU temp kicked in at 54-55 degrees C at 2-18% workload. The dramatic increase of the CPU temp drove me immediately to replace my fans back to their intended positions leaving me no choice but to discontinue further intended testings. CPU reading's back to normal at 40'sh. In this little experiment I conclude that "negative air-flow" works better at least for my setup, that is. So I'd go for MORE HOT AIR OUT.

 

[tiler]

I read or skimmed every post of the thread. Congrats. It's a meaningful thread and I've enjoyed it.

I'm in the camp that believes you're not going to make an appreciable difference in pressure between the inside and outside of the case. With that in mind, not very many of us are going to be able to split this hair scientifically; there's no hard and fast rule given the wide array of configurations. I believe my super inexpensive case was designed to have some differential between intake and exhaust because it has a mesh grate directly adjacent the PCI slots... free flow. (Coolermaster Elite 310)

I further believe if you're heavy on the intake and light on the exhaust, you've got hot air waiting in line to get out with intake pushing them. Alternatively, more exhaust means you've got cold air waiting in line to get in the case. I desire a surplus of cold air rather than hot. For purposes of the poll, I'm a negative.

I think if you're worried about it or if you have a heat problem, you need to test a couple of configurations and see if it'll improve. Nothing inside there is going to cool below the ambient room temp, so whether you're pointing them in or out, your best bet is to cycle the air inside the case as often as possible. As I said, with all the different layouts and variables, it may take some testing to determine what's best in your setup. I thought I had it nailed down for mine until I read this thread and now I'm going to go rearrange some fans and put theory to practice.

 

[A Bad Day]

Since no computer cases have the same design, my suggestion is to simply experiment with the placement of the fans and see what you get.