Originally Posted by ozlay
I have 3 in 3 out so more out then probably
It's not so much the amount of fans, but the amount of air actually going through the case. For instance, I have three Scythe GT AP-15s in the front of my case which pull about 60CFM into the case. I have 3 stock 140MM fans in the back and top pulling air out.
Sounds like a negative pressure case, right? Well, not exactly. I have a Radiator on the top with the two of the three 140MM fans, so they aren't pulling out as much air as they could. The back one is unblocked, so it is pulling out what it is rated at. (These are stock case fans, they don't do much to begin with)
For sake of arguement let's say that they pull out 70CFM. That's more than the Scythe's right? Well, yes, but there is a radiator on two of the fans. They probably pull out less than half of that 70, and then there is still one that is unblocked. So, total, there is 35+35+70 pulling out of the case which is 140 versus the 60+60+60 pulling into the case. which is 180. So, positive pressure.
If you know a little about air and how it works, it wants to normalize pressure. Air expands to its container. Well, with more air coming in, than out, that air still has to go somewhere, right? It wants to equalize. There are a ton of vents on my case. The unused pci slots, above the 140MM exhaust fan, etc. That's where the air is escaping out, and not in.
With the air pushing out, dust can't get in and settle through those areas.
With a negative pressure case, however. The air still wants to equalize, so it is pulling air in through the same vents. Thus pulling in more dusty air.
When you install intake fans, always put a dust cover to keep your case clean, and make it easier to clean.
Positive pressure, negative pressure, no matter what you have, You'll have a dusty case if you have a dusty room. Keep that in mind also.