Why is airflow always front in, back out?

Question

The motherboard on my gaming computer recently died, leaving me with a lot of functional components that just need a new mobo/cpu, so I'm taking this opportunity to 'build'(/rebuild) my first PC. As the old motherboard was microATX and the new one is ATX, I've had to also buy a new case: the one I went for was the BitFenix Enso, and I have rather belatedly discovered it has a bit of a reputation for poor airflow and overheating, so I'm looking at ways of fixing it.

Which brings me to my question: why does everything I read insist that airflow in a case must run front-to-back? I understand bottom-to-top: hot air rises. But I don't see the difference between front-to-back and back-to-front. To me it seems that the simplest solution with this particular case would be to just swap the directions of the front and back fans, so that the intake fan has access to the freely-circulating air at the back of the case and the front fan, which is hidden behind the front panel, becomes the exhaust and pushes the hot air out of the front vent.

Is there something I'm missing? Everything I've read says this is a rule, but nothing explains why or what the consequences would be for ignoring it. Explanations for the newbie, please?

Solution

There's a few reasons:

• CPU Fans generally aren't revesable on their mounts on the heatsink and a lot now work in a 90 degree orinitation to the CPU itself, meaning they'd either blow 'front to back' or 'back to front' (or up and down in a tower case but I've never seen one which did this). Because it's always been the case of CPU cases (even before towers) blowing front to back, CPU Fans have been designed to blow front to back too.

• Both Desktop and Tower cases were first envisioned as being sat on peoples desks, sitting at a desk with a PC blowing hot air into your face constantly would not be nice. Even when a lot of PC cases are now kept on the floor rather than on a desk, it'd still be unpleasant to have it blowing onto your legs.

• Dust and other detritus may gather behind a PC, sucking this in is surely going to cause a bad time for a fan, or at least block up a radiator faster than the generally 'clear' air from the front of a case.

• Noise concerns as the direction of air travel will somewhat effect the sound levels reaching the user who's going to be at the 'front' of the machine.

• In a Data Center environment, it's standard to have hot and cold isles, which means all the heat from the servers in a rack is output to the same side of the rack. Having a agreed on standard means you don't need to install any servers 'back to front' to make sure they are exhausting air to the right direction.

However, even with these points in mind, it is possible (and fully valid) to build a case which did blow back to front if you really wanted to, it'd just take bit/a lot more planning as you're going against the generally accepted conventions.

Something to note is that having it blowing back to front, isn't going to make the cooling any better really, as you'll still be moving the same amount of air through the case just in the other direction.

Comments

[Solar Mike]

The components inside may have been mounted to benefit from the front to back arrangement...

[davidgo]

Another 2 reasons - noise (thats why first class is in the front of a plane), and - not relevant here but worth mensioning - in rack environments it allows for more efficient air conditioning (warm/cold isles)

[djsmiley2kStaysInside]

Both good reasons, I'll add them @davidgo

[Dan Sp.]

A note on why then the standard is fixated on front-in back-out. The PSU in every case I have built is on the back and has its own exhaust. Reversing case flow will bring in exhaust heat from the PSU into the case.