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Which is better, positive or neutral air pressure?

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Vishera

Member
Joined
Jul 7, 2013
I've ruled out negative air pressure as I don't want more dust build up than is necessary. It seems to me that neutral would be best, especially in the NZXT S340 that I'm looking at as a new case. It has four fan mounts, 2 in front, one on top, one in the back. I figure having the front be intake and the top and back be exhaust is best, since that would create a nice breeze over the components (to quote ATM, isn't the whole point to achieve airFLOW?), but I've heard having more cool air pulled into the case and having it sit there for a bit while it cools down the internals of the case, before being pulled out by the lesser amount of exhaust fans, can be a better scenario. In this scenario, I'd put the exhaust up top (heat rises, right?). Just kinda looking for the best arrangement for this case. If it matters I'm looking at the white version, and using 3 140mm Corsair AF fans, and a single AF120.
 

Coty0010

Member
Joined
Jan 21, 2013
Location
Ohio
Neutral is better but really difficult to achieve and sometimes not possible in most cases. The difference between positive and neutral is so small though that most people just settle for positive pressure, myself included.

So long as you have a large amount of air moving through the case, it doesn't matter to much, for me I'm drawing air in with 3 120mm noctua fans and exhausting air through 2 80mm fans from the rear of the CPU tower. Excess pressure is exhausted through the cases vents (and a significant amount of it to.)

Not sure who told it's best to let air sit in the case for a while... but it's wrong, more air moving through at a fast pace it better... That's basic thermal dynamics...
 
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Alaric

New Member
Joined
Dec 4, 2011
Location
Satan's Colon, US
I try to keep positive pressure coming through only filtered fans. We have stupid amounts of dust here and I try to avoid it sneaking in the cracks and crevices. I do the same thing with my home. LOL
 

Blaylock

"That Backfired" Senior Member
Joined
Jun 5, 2013
Location
Go Blue!
I agree with Coty, Alaric and ATM. For best temps maximum airflow through the case is the ultimate goal. For dust reduction your best bet is positive pressure using the front and bottom (filtered) as intakes and top and rear (unfiltered) as exhaust. I'm not sure if your ASRock Pro3 has the capability, but you can also reduce the fan speed of you top and/or rear fans to create that positive case pressure scenario. A fan controller would work too. This will of course slightly reduce your airflow.

Whoever stated that it's a good idea to let air sit in a case...I...I would not take advice from them any longer.

EDIT: Looking at that case the only bottom intake is for the PSU. So in that particular case your bast scenario is to intake from the from and exhaust from top and rear. Set your PSU up to intake from the bottom and exhaust from the rear. Since you'll be buying fans for this case you can also go by the CFM of the fans to determine positive/negative pressure. Generally 140mm fans produce higher CFM than 120mm ones of the same make.
 

Alaric

New Member
Joined
Dec 4, 2011
Location
Satan's Colon, US
I should have mentioned I run my intakes at a higher RPM than my exhausts, too. The 200 mm on the front moves a lot of air so I can run that fairly slow (quiet).
 
OP
Vishera

Vishera

Member
Joined
Jul 7, 2013
I agree with Coty, Alaric and ATM. For best temps maximum airflow through the case is the ultimate goal. For dust reduction your best bet is positive pressure using the front and bottom (filtered) as intakes and top and rear (unfiltered) as exhaust. I'm not sure if your ASRock Pro3 has the capability, but you can also reduce the fan speed of you top and/or rear fans to create that positive case pressure scenario. A fan controller would work too. This will of course slightly reduce your airflow.

Whoever stated that it's a good idea to let air sit in a case...I...I would not take advice from them any longer.

EDIT: Looking at that case the only bottom intake is for the PSU. So in that particular case your bast scenario is to intake from the from and exhaust from top and rear. Set your PSU up to intake from the bottom and exhaust from the rear. Since you'll be buying fans for this case you can also go by the CFM of the fans to determine positive/negative pressure. Generally 140mm fans produce higher CFM than 120mm ones of the same make.

Would the fact that the 120 has less CFM make a huge impact on the airflow? The 140's are all the same CFM, the only difference is the 120.
 

ehume

Member
Joined
Oct 30, 2009
I take a different tack. I cut out that perforated patch they call a "grill." That way the air flows out. You don't even need a fan. Better: if your heatsink has a pull fan it will entrain case air and improve your exhaust. Don't believe me? Check out this thread.

I used to file the cut edges. Now I line them with the cheap plastic spines for report covers (from you office supply company or Wal-Mart) suitably trimmed. Quieter and more effective.

Your air intakes are filtered. There is no way you air pressure builds up so you get a flow-through case. I'm on my third such case as I write this. My daughter has a Lian Li case, so the opening is already there. She, too, has a flow-through case.
 

trents

Senior Member
Joined
Dec 27, 2008
Don't forget to factor into the equation the effect of the PSU fan.
 

LutaWicasa

, Immutable, Administrator
Joined
Dec 22, 2000
Location
Huntsville, AL
The 3 140's: 2 front intake and 1 rear exhaust
The 120: top exhaust

This should give good lateral flow while clearing the risen warm air.
 
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EarthDog

Gulper Nozzle Co-Owner
Joined
Dec 15, 2008
Location
Buckeyes!
Everyone is going to have a different opinion on that answer. It's been asked time and time again with no real answer..ever. first, the term pressure feels misused honestly. Tough to have pressure changes in an unsealed case in the first place. ;)

My personal preference is slight negative pressure/more cfm exhausting. I could care less about the TINY amount of dust that gets in through the cracks of the case because of it. The key is AIRFLOW. Typically front/sides = intake while top/rear = exhaust.

Also, i wouldnt take I to account psu fan either as typically their CFM isn't much in the first place, and because of their placement, either top or bottom, it's doing what it needs to do in that location anyway.
 
OP
Vishera

Vishera

Member
Joined
Jul 7, 2013
Everyone is going to have a different opinion on that answer. It's been asked time and time again with no real answer..ever. first, the term pressure feels misused honestly. Tough to have pressure changes in an unsealed case in the first place. ;)

My personal preference is slight negative pressure/more cfm exhausting. I could care less about the TINY amount of dust that gets in through the cracks of the case because of it. The key is AIRFLOW. Typically front/sides = intake while top/rear = exhaust.

Also, i wouldnt take I to account psu fan either as typically their CFM isn't much in the first place, and because of their placement, either top or bottom, it's doing what it needs to do in that location anyway.

I knew it was a difficult question to get a definitive answer on, but I figured by asking between neutral or positive it would be lless of a craps shoot. Oops. But this actually makes the most sense, thanks ED! Now if only I could get this Haswell computer sold to start buying my new parts...
 

Alaric

New Member
Joined
Dec 4, 2011
Location
Satan's Colon, US
I vacuum every couple of days. I dust every day. When we get a dust storm (The above picture is actually Phoenix) the computer gets turned off. I have a swamp cooler on the roof, and normally a window has to be left open for the airflow. When a dust storm hits I close all the windows and create a positive pressure in the house. It does a pretty fair job keeping the dust to a minimum. :)
 

Mr.Scott

Beamed Me Up!
Joined
Jun 9, 2013
I vacuum every couple of days. I dust every day. When we get a dust storm (The above picture is actually Phoenix) the computer gets turned off. I have a swamp cooler on the roof, and normally a window has to be left open for the airflow. When a dust storm hits I close all the windows and create a positive pressure in the house. It does a pretty fair job keeping the dust to a minimum. :)
I thought you were just being a wise a$$. I feel kinda dumb now. :-/
 

Alaric

New Member
Joined
Dec 4, 2011
Location
Satan's Colon, US
I thought you were just being a wise a$$. I feel kinda dumb now. :-/

Not at all. Most places don't have dust storms the size of a city. We have some special dust issues here. :) The positive pressure deal is a response to uncommon circumstances for me. If I don't dust for a week I can see the layer of dust on everything. For cooling a PC case I wouldn't think it makes much difference as long as there is sufficient air flow to carry out the heat.

At least we don't have the White Death in the winter. :rofl:

Dust storm microburst. I thought somebody nuked us for a second. These will pack dust in places you really don't want dust.
dust-storm-microbust-jerry-ferguson-arizona-2.jpg
 
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Alaric

New Member
Joined
Dec 4, 2011
Location
Satan's Colon, US
And be glad you don't have wet death in the Summer either like we do .....

I grew up in south FL. Rain is a way of life 9 months a year.

I really have no empirical evidence that my theory actually helps keep dust out of my computer, just that it helps with the house. From that I'm just guessing about its effectiveness on a small scale. My last build I had the radiator mounted on the side cover and flipped the fans to pull air through from outside the case. The radiator was nasty, but the inside of the very crowded case was cleaner than I expected. Before I got a filtered case I used pieces of pantyhose stretched over the intake fans as a filter, which probably did a lot more than any fiddling with pressure, IMO.