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Positive or Negative Pressure Air Flow

Which Air flow setup do you use?

  • Positive Air flow pressure

  • Negative Air flow pressure

  • Neutral Air flow pressure

Results are only viewable after voting.
C

combatflexo

Junior Member
I'm getting ready to replace all my 120mm case fans and now debating on positive or negative pressure for my case. Around the other forums, it seems to be a toss up between positive and negative air pressure. I'm creating a poll to see which is the preferred/popular method. This is not taking in consideration of case cracks or the psu (depending on how it's mounted shouldn't have to much of an affect on the pressure).

Positive
If you have more CFM coming IN, then OUT you'll have positive air flow. Benefits include less dust in your case.

Negative
If you have more CFM coming OUT, then IN you'll have negative air flow.
Benefits include pulling more hot air out of the case keeping it cool.

Some fan recommendations I have come across from various threads and forums for a 120mm seem to be. Has anyone tried or recommend the XIGMATEK XLF-F1254 120mm?

1. Scythe Gentle Typhoon AP15
2. Noctua NF-P12
3. Scythe KarmaFlow 2
 
For some reason negative in my mind seems like it would result in greater airflow than positive and therefore lower temps, but also more dust. Positive would lead to higher temps and less dust. Neutral is my choice.
 
Nizzzlle said:
Why would having a positive type airflow result in less dust? More air coming in equals more dust, no?
Click to expand...
Positive airflow must be filtered, else it would be just as dusty as a negative pressure setup. Mine has 6 intake fans with filters and 1 exhaust at the back, very little to no dust at all as the bigger particulate will be filtered, tiny ones still gets through but not much of an issue. Plus it is easier for me to clean the magnetized filter, I could rinse it with water or just brush off the dust with an old toothbrush.
 
My old Antec case has two exhaust, and three intake fan mounts. I removed all the intake fans three years ago and now only use the exhaust fans, with a filter in the empty front fan bays. Temperatures stayed exactly the same as when I had all 5 fans running, but it's much quieter.

So I would vote negative pressure, after all, you can't push more air in, than you let out.

Experiment and see what results you get.
 
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WilliamM2 said:
So I would vote negative pressure, after all, you can't push more air in, than you let out.
Click to expand...

In the realm of the ideal system (the real world rarely is, of course) pressure is pressure. The above argument doesn't hold. Any case, regardless of where or how many vents there, still has holes. At any location there is not an active fan, air will either be pulled in or pulled out by the total system pressure, regardless of direction. Even with a negative pressure system, total airflow is limited by the amount of air that can get IN to the system through openings that aren't modulated by a fan (vents, empty card slots, gaps, etc.). In a totally sealed box except for several negative pressure pushing fans, airflow will be zero because any air the fan pushes out will be negated by back-pressure behind the fan.

I run a single positive pressure fan at the bottom of my well-ventilated Fractal Design Arc Midi. This case has large dust-covered vent areas at the top and bottom, so I let convection current carry heat out of the top, and the single fan provides just enough positive pressure to keep dust from getting inside the case. My temps stay nice and cool, even when my CPU is at full load transcoding movies. It should be noted, though, that I chose all my components for efficiency and low heat (3570K using iGPU, platinum rated PSU, etc), so I really don't need too much cooling.
 
I'm not sure how that changes my statement. You can't put more air in than you exhaust out, or as you said, exhaust more than you take in.

I did make an error above, my case has two exhasut and three intake (Antec SX-840). I previously had positive pressure by running all 5 fans. After removing the three front intake fans, the temps stayed exactly the same, now with negative pressure.
 
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Axial fans are more efficient creating positive pressure than negative pressure, and like LoveMachine said, it's the pressure differential that counts, not the location of the fans. Whichever setup creates more pressure differential will be more effective. That said, there are other advantages to possitive pressure cases as well as the better fan performance.

1. the fans can direct airflow to certain parts of the case such as the video card area

2. Dust is more controlable because air only enters from the fans which can be filtered.

3. Axial fans are more efficient when producing positive pressure. This makes sense when you consider that the blades can grab more air from a higher pressure environment, so creating negative pressure inside the case reduces the amount of air to pull.

But, the pressure differential inside a computer case is relatively low so the efficiency doesn't really come into play that much. Putting dust filters in front of the fan will increase the resistance to flow and will ofset or make possitive flow cases worse than a negative pressure case that gets its air from a lot more sources such as open holes in the case.

Negative pressure cases are also typically easier to design considering the ATX guidelines.
 
WilliamM2 said:
I'm not sure how that changes my statement. You can't put more air in than you exhaust out, or as you said, exhaust more than you take in.

I did make an error above, my case has two exhasut and three intake (Antec SX-840). I previously had positive pressure by running all 5 fans. After removing the three front intake fans, the temps stayed exactly the same, now with negative pressure.
Click to expand...

Yeah you can actually =/
But you're pressure/temperature will be variable.

PV=nRT
 
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