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Positive or Negative Air Pressure?

Last response: in Components
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Positive or Negative?

Total: 67 votes (35 blank votes)

  • Positive
  • 37 %
  • Negative
  • 40 %
  • Neutral
  • 25 %
a b ) Power supply
March 15, 2012 12:40:27 AM

READ THIS BEFORE VOTING

SilverStone Article About Air Pressure
Outdated Forum Thread that I stumbled Upon

Positive = More intake fans than exhaust
Negative = More exhaust fans than intake
Neutral = Same no. of intake fans to exhaust

There are no wrong answers since it all depends on the case.

Post which type you prefer and your case
-Optional- explain why you prefer that type of air pressure
a b ) Power supply
March 15, 2012 12:43:15 AM

Hi :) 

Cases have had filters for over 20 years.... lol

Lian Li ...(mine)

And people voted negative as its the CORRECT way to build a Pc....my shops have custom built thousands....

All the best Brett :) 
March 15, 2012 12:48:10 AM

e56imfg said:
READ THIS BEFORE VOTING

SilverStone Article About Air Pressure
Outdated Forum Thread that I stumbled Upon

Positive = More intake fans than exhaust
Negative = More exhaust fans than intake
Neutral = Same no. of intake fans to exhaust

So in the old thread, many people voted for negative than positive most likely because most cases back then didn't have any filters. Nowadays, filters come in almost every case so I wanted to start a new poll about which type of air pressure you prefer the most.

There are no wrong answers.

Post which type you prefer and your case
-Optional- explain why you prefer that type of air pressure



It has nothing to do with filters........
It has to do with the basic fact that "Heat Rises""........
You want plenty of air coming into the case, but you want it all to exhaust...negative pressure is best.
Related resources
a b ) Power supply
March 15, 2012 1:00:30 AM

I have built both, but I agree that most folks would prefer negative pressure. My new phantom build will be negative pressure. Really either way works, but equal pressure is to be avoided.

Neutral= dead air pockets. Either positive or negative pressure will increase airflow.
March 15, 2012 1:03:07 AM

I think it depends on a the case and how its setup.
Some case's are much more suited to one over the other.
Especially when you account for obstructions

If you look at servers pretty much all of them are positive air pressure based, but they are all designed around air channels and set flow paths in their cases.
March 15, 2012 1:08:20 AM

Positive pressure is the best, even if you have filters negative pressure will pull dust from other places where no filters are found. Its true that negative pressure moves more air through a case but positive will cool components better. An example are the silverstone cases, they remain clean even after a year and have some of the best cooling performances of any case around.
March 15, 2012 1:17:39 AM

terryd75 said:
It has nothing to do with filters........
It has to do with the basic fact that "Heat Rises""........
You want plenty of air coming into the case, but you want it all to exhaust...negative pressure is best.



Positive pressure is best, keeps your case tidy. When you are exhausting more air from the case than your feeding this is called negative pressue. When you have More air intake over air exhaust is called Positive pressure. Hope you know the difference now.
a b ) Power supply
March 15, 2012 1:18:29 AM

What are your guys' thoughts on the SilverStone Article?

Still prefer negative? - Because I'm really not sure.
a b ) Power supply
March 15, 2012 1:19:33 AM

Brett928S2 said:
Hi :) 

Cases have had filters for over 20 years.... lol

Lian Li ...(mine)

And people voted negative as its the CORRECT way to build a Pc....my shops have custom built thousands....

All the best Brett :) 

Fixed :??: 
March 19, 2012 4:36:49 AM

I happened to have just tested this a few minutes ago on my CoolerMaster HAF-X, reversing the top fans from exhaust to intake, and my GPU temp dropped by about 8C idle and 10C at load, so clearly the positive pressure is beneficial in my case (no pun intended). After reading the SilverStone article, this makes sense, since the hot air exhausted from the video card (and indeed the case itself) is not getting sucked back in through the vents. I would say, however, that it likely varies from case to case and both arrangement and quantity of components. The biggest takeaway for me is that testing is good.
a c 104 ) Power supply
March 19, 2012 6:40:42 AM

My old system was negged out,
but my current is definitely positive, three intakes, one exhaust
not counting the one on the Cpu block or the four on the roof as they don't affect the volume, the Cpu one affects flow though obviously,
with all the mesh on the original case though, I think In win wanted to make it hard either way to affect pressure
Moto
a b ) Power supply
March 19, 2012 5:08:00 PM

I prefer positive pressure, for the two reasons outlined in the Silverstone article. The major one, in my opinion, is that positive pressure virtually guarantees that ALL air flowing into the case has gone though a filter to remove dust. Negative pressure guarantees that some dusty air will enter the case.

I do not buy the "hot air rises" arguments. Sure it does, IF there is no other activity forcing air to move. But the whole point of case fans is to force LOTS of air movement, thus completely overpowering the effects of air density and gravity. The things to pay attention to are the locations of the fans, the pathways for air flow, and removal of obstructions to air flow. It is FLOW of air over hot components that does two things: (a) remove heat from the hot components; and (b) move the heated air out of the case.

One argument I've seen that I find ridiculous is the idea that, in a positive-pressure situation, the air inside is denser due to that pressure, and hence it is more efficient at removing heat from hot surfaces. The pressure we're talking about makes such a microscopic difference in air density that this effect is virtually nothing!

I also remind people that you can NOT have more air flowing into a case than flowing out (or the other way around). Flow in MUST EXACTLY equal flow out! Otherwise the case would continually build up more and more trapped air until it exploded!

Now, a couple comments on how you make sure you have what you expected. (This relates to several comments I've seen about experimenting to see how it works.) Firstly, you can plan what you should get from fan counts and air flow ratings of the fans. BUT two things really foul up those plans. The first is that putting filters on fan intakes (as you ALWAYS should) significantly reduces the air flow, and this effect gets even worse as the filter gets clogged, so remember to clean them! Thus, in general, Intake fans with proper air filters bring in LESS air than the specs say, but Exhaust fans with no filters do not have that problem. The second factor which can affect both types of fans, is that obstructions in the air flow pattern near the fan can always change their performance in unpredictable ways. (I once had an odd poor-cooling effect, and traced it to too may cables collected around the exhaust ports on the OUTSIDE of the back of my case.) So what to do? After you get your system set up, you need to verify by measurement whether your case has positive or negative interior pressure. One good suggestion I read in a forum is to use incense sticks! Light one and it emits a bit of smoke as well as scent. Move it slowly everywhere around you case while it's operating, and observe the smoke flow pattern. In or out? Fast or slow? My own preference is slow outward flow, indicating a nearly-balanced intake vs. exhaust, with a net positive pressure inside so clean air leaks out.

My other comment is about estimating the air flow pattern inside the case, and this has nothing to do with positive or negative net pressure. Moreover, I don't know a good way to measure any of this, so I just make a good guess. You want to be sure that the air can flow freely everywhere (one good reason for cable management). And then you want to ensure that, for sure, there is lots of air flow near the components that generate the most heat. What are those? Well, for a good clue, all the ones with heatsinks, and especially the ones with fans. That puts the CPU and the graphics card at the top of the list, followed by the RAM and the North- and Southbridge chips. HDD's generate some heat, but not generally as much as these other devices. As an example of this thinking, I built a relatively simple machine a couple years ago, and realized that, in addition to the exhaust fan on the PSU near the top rear, the case had one intake fan at the bottom front blowing over the HDD's and into the case, and one exhaust fan at the bottom rear. But the case bottom had very little there, so the air flow pattern inside was a clear path from front to back across the bottom of the case, nowhere near the heat generators. I created a baffle mounted on the case's bottom to force the incoming air flow up towards the RAM and CPU, before it could come back down over the rest of the mobo and out the bottom rear exhaust fan.
a b ) Power supply
March 19, 2012 9:31:30 PM

Good topic btw op . I'm torn on this one as well . I guess the way I see it is that whether you go with positive or negative pressure The difference is going to find voids in the frame and skin of your case to exploit . So to me its a matter of whether you want excess hot air finding a place to exhaust or unfiltered air finding a way in . I can say for certain that there are many open spots on my Lian li for both . Hope I articulated this well .
a c 139 ) Power supply
March 19, 2012 9:50:21 PM

It's not always a 'one-size-fits-all' proposition.
Differences in case / fan / component geometry and thermal profile can vary a lot.
What works best for one system might not work as well for the next.
Getting too aggressive with airflow can cause a 'curtain effect' trapping hot air.
Anyone experience an air curtain while entering or exiting buildings?

This is one premise that can be tested by an individual on their own system.
March 19, 2012 10:48:15 PM

So I don't know how many people have considered the ramifications of Bernoulli's principle, but in this case I think it's highly significant.

The principle is simple enough, and to sum it up the faster moving an air current is the lower it's pressure. Now let this idea sink in to what you've learned about positive pressure, negative pressure, neutral pressure etc.

If you have neutral airflow, let us say three yate loon 120mm high speed for intake, and three more for exhaust. This setup is going to take advantage of the completely ignored "wind tunnel effect" that you can find some people reference. Bernoulli's principle effectively says that if you have a high speed air current moving through your case, this will create a low pressure condition(in the regions of highest airflow) drawing surrounding stagnant air in towards the more quickly moving current. SO you will not suffer from stagnant air pockets of any kind if you are moving air through the case at a significant rate of speed. This is the principle that allows airplanes to fly, and it has been quite thoroughly tested and scrutinized and is quite sound.

This is a strong argument for neutral or near neutral airflow, because even if you had PERFECTLY even fan arrangement(identical intake/exhaust) the Bernoulli's principle would cause a negative pressure condition. The more airflow you have inside your case the more strongly negative the pressure in the case will be in comparison to outside the case, sucking in stagnant room air even with a perfectly balanced fan setup.

This also explains why many people prefer substantially positive pressure, because even with very similar intake/exhaust setup (slightly pos. or slightly neg.) this principle of fluid dynamics is going to create an overall negative pressure condition, drawing air(dust) in through all the small cracks.
a b ) Power supply
March 20, 2012 10:09:53 AM

Wow an aviation buff ... Of course high air flow causing a venturi effect with a cpu at idle could cause carburator icing and your pc could stall and fall out of the sky .. Not to mention that Bernoulli was a plumber by trade and would have preferred water cooling anyways so now I dont know what to think :) 
a c 104 ) Power supply
March 20, 2012 11:58:06 AM

Watercooling doesn't mean no fans, in fact it often needs more :-)
Moto
March 20, 2012 12:00:16 PM

Me? I chose Positive. Why? I currently have a full water cooled case, so I just need the fans for the HDD and SDD, the Motherboard, Chipset and PSU. Positive pressure does reduce cooling, but it keeps the dust out. Its useful because I don't use dust filters.

Dust filers, blocking 90% of your airflow since 2003!
a b ) Power supply
March 20, 2012 1:22:53 PM

Motopsychojdn said:
Watercooling doesn't mean no fans, in fact it often needs more :-)
Moto

Master of the Obvious ! lol
March 20, 2012 1:30:40 PM

How so, more?
August 25, 2012 12:22:52 AM

A few months late to the party, but I've just re-evaluated the cooling on my desktop and having spotted this thread whilst doing a search on the subject I thought I'd add a little to it, so please excuse the bump.

I'm using the best of both worlds, positive AND negative pressure. My system is a bit dated, although the only limitation I'm having is the older slower HDDs, but as a poor student I can't afford better.

I've a 120mm Arctic F12 3 pin on the front and an 80mm F8 3 pin on the lower rear corner on one side. Both are connected to the Motherboard (ASUS P5Q-E) on separate channels, and are set to "Turbo" in Q-fan. They're completely silent, or at least I can't hear any difference whatsoever regardless of what setting I have them on, so I've put them on the highest,

Then I've got an F8 Pro PWM exhaust on the rear which shares the 4 pin PWR header with the CPU cooler (stock Intel which is enough to keep my C2D E4500 cool at 3.0GHz, but there's an Arctic Freezer 7 Pro Rev. 2 in the post so I can put the fsb up to 400 and have a 1:1 dram ratio), and finally the PSU is an OCZ StealthXstream2 600W which exhausts out the rear at the top of the case (no top facing exhaust port in this one).

Because the exhaust fan shares with PWM header, at low load & idle (i.e. when browsing or writing assignments) the flow is reduced and the intake flow is greater than the exhaust flow, so positive pressure. This means most of the time I'm keeping the dust at bay when I don't need the best cooling power my setup can provide. Conversely when I'm doing CAD work (Engineering student) the CPU and exhaust fan speed up together and the exhaust flow is more than the intake, so I get the better cooling effect of negative pressure, only when it's needed.

It's been running like this for a couple of days now and using some thin paper to test it, I've visually seen the difference between negative and positive case pressure. By holding the paper over some of the 'fan-free' vents that air can be drawn from and seeing whether it's attracted to or pushed away from them - there definitely is a switch from negative to positive under load. Just to be safe though I've covered all purposely designed vents on the case with this stuff (with the exception of the exhausts, naturally) to minimise dust intake. That said even using that stuff there was still some dust ingress using my previous negative-pressure-only setup of a single 12mm exhaust fan + PSU fan and no intake fans, which is what encouraged me to make the switch.
a c 110 ) Power supply
August 25, 2012 12:38:22 AM

NO^^
August 25, 2012 1:01:13 AM

Care to elaborate?
a c 110 ) Power supply
August 25, 2012 3:42:52 AM

Read the thread date.
August 25, 2012 4:02:19 AM

Read the first sentence I wrote... ;) 
April 18, 2014 3:56:33 AM

nice article but you're a year late
!