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So I've got a straightforward build: two fans drawing in air from the front, two fans exhausting air through a radiator on the top, and a spare fan I'm considering mounting in the rear of the unit.

All 4 fans report mostly the same RPM and they're all 120mm, so I'm reasonably confident that they're all moving about the same amount of air (the intakes have an air filter to get through and the exhausts have a radiator to push air through).

Cables are mostly out of the way, and I don't have any whacky components clogging up the main compartment -- just a modest, horizontally mounted video card and the water cooler's supply and return cables to/from the cpu. So there are no "gotchyas" as far as I can tell with regards to airflow.

As far as temperatures, the CPU is completely happy with its water cooler, and the mobo temperature sensors are generally fine (one peaked at 76 degrees, but I unfortunately don't know where the sensors are located on the board), but since I have that extra fan and some space in the rear of the chassis to mount it, I figured I'd look into mounting it.

Am I correct in my assessment that deciding whether to mount the fan and in which direction basically comes down to the "negative pressure versus positive pressure versus neutral pressure" argument? If not, which direction should I mount that rear fan, if at all?

bvoyelr
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  • Some folks may suggest this belongs in http://hardwarerecs.stackexchange.com . Me, I suggest you experiment. Data beats theory every time. Try inwards, outwards, and unmounted, and see which keeps your system coolest. Also see https://superuser.com/questions/61441/optimal-fan-placement-and-direction-for-air-cooling-a-computer?rq=1 – K7AAY Oct 23 '19 at 20:25
  • More fans will not help. PC cases in general have ventilation design that is essentially an afterthought. Primary issue is poor air circulation and the creation of dead zones of stagnant air that allow heat buildup. – sawdust Oct 23 '19 at 20:55

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The general idea is always to push the hot air out of the box, not the other way around. If, for example, you only had one fan, that fan should blow the hot air outside of the box. In theory, this should always yield better results. And if you're a nitpick, remember to install the fan as high as possible, as hot air rises up.

In the case with multiple fans, if the computer case ends up having negative pressure, air will get sucked in - and dust can become a problem. You can mitigate this by attaching a few filters to all the openings you can - OR - you can change the fan RPMs so that air intake will be greater than the air outtake.

In your case, adding another fan could help (and it should blow the hot air out), but in order to have a positive pressure it must be running at a lower RPM (and I'd advise you to lower the RPM of the other 2 outtake fans as well). I suggest installing it and then TESTING the results (with the extra fan ON and OFF) as there is no guarantee it will actually help.

GChuf
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  • Some misleading info posted. *"The general idea is always to push the hot air out of the box..."* -- This bad idea found in a lot of PC cases is refuted by industrial equipment that almost always blow filtered air *into* the enclosure. FYI your primary advice for one fan also creates *"negative pressure"* just like *"multiple fans"*. *"Negative pressure"* alone is not the cause of dust accumulation. If there are no intake air filters, then there will always be dust. You conflate RPM with airflow, or CFM, but there is not always a direct correlation, esp. different fans. – sawdust Oct 24 '19 at 00:20
  • When talking about pushing the hot air out, I'm thinking about a single fan at the (top) rear side of the case. I imagine having an intake fan with an air filter blowing over the hot parts of the computer could be better. As always, the answer is "it depends". As for RPM/CFM, how are they not directly related? Different fans produce different CFM at different RPMs, but higher RPM always results in higher CFM, does it not? – GChuf Oct 25 '19 at 12:13
  • *"As for RPM/CFM, how are they not directly related?"* -- Sure, in the simple case of free airspace where there is no back pressure. But impede the airflow and/or install the fan in an enclosure, then you find that the fan characteristics are not linear. Ever look at [fan curves](https://www.pittsburghairsystems.com/PDF/Fan%20Applications/The%20Basics%20of%20Fan%20Performance%20Tables,%20Fan%20Curves......pdf)? – sawdust Oct 27 '19 at 23:09