Originally Posted by psyclum
the importance and relevance of convection cooling is often OUTSIDE the box.... to discount convection entirely is uneducated at best... yes inside a box, any kind of fan would overpower the action of convection, however, this does not negate for physics behind heat rising to the top. with a top side intake, you are often recirculating hot exhaust from PSU/GPU/CPU right back into the case and causing further inefficiency in your overall cooling solution. this problem is especially acute if the computer is in a semi enclosed area such as under a desk where hot exhaust is trapped by the desktop and then sucked back into the case from the top side intake.
recirculation of hot exhaust (thermal short circuit) creates inefficiency and as a result higher decibel levels then necessary to achieve the same goal. overall cooling performance may not be very noticeable however the real discussion in PC cooling really lies in long term decibel level. anyone can cool anything if they used enough 5000rpm delta fans, however, to keep it under acceptable decibel levels, one must search for efficiency and this is where consideration in convection comes into play
a bottom intake and top exhaust ensures maximum separation between cool intake air and hot exhaust air because OUTSIDE the box, hot air WILL rise and convection DOES work...
Originally Posted by Snowmen
I know the thread is very old but I just wanted to answer your comment. The air outside the case doesn't matter either for these reasons:
1) If you have your case in an open space, the amount of air is so large that the probability that exactly the same air is pulled back into the case is minimal at best. Anyway, because of the laws of thermodynamics, the heat would have a tendency to dissipate itself into the rest of the air to maintain a certain balance.
2) If your have your case in a small space like the little compartments that they put in many desks, air will be recycled anyway and you should just get it out of there if you want better temps.
I would tend to agree with Psyclum, and the problem is much worse with top-exhausting cases, e.g., http://www.hardocp.com/article/2010/11/09/nvidia_geforce_gtx_580_video_card_review/8
Different circumstances will result in different risks, but many of us have tower cases underneath desks/tables that aren't very well ventilated, so hot air will tend to collect underneath the desk/table and have a chance to go back inside if there is a top intake fan.
I'm currently re-designing my airflow and plan to yank out my top 200mm exhaust fan and seal the entire top in order to complete my top "wind tunnel." I'll have an intake 120mm fan in my top three 5.25" bays throwing air at my Hyper 212 Evo with push-pull fans directing air out the back (no exhaust fan, just a gaping hole courtesy of some nibbling). Down below there will be a 200mm intake fan, a 140mm bottom intake feeding a 7970 with dual blower fans, and all the rest of my PCI covers taken off to encourage a second "wind tunnel."
There will be no need for top exhaust OR intake, and sealing them will ensure no dust falls through the top when the computer is off.
Wild card: my 200mm side intake fan. I don't think it's as problematic as a top fan intake when it comes to external air temperatures, so I intend to continue using it as an intake fan, but it merits experimentation with my infrared thermometer.
Edit: Btw, to tie this back to OP, the OP is correct. At short distances fans will overpower gravity and convection. That doesn't mean the Stack Effect doesn't exist with computer cases, but it does mean that if you were to build a case that takes advantage of hot air rising, it would have only a very modest advantage over traditional front-to-back designs, assuming all cases were using fans (which overpower convection). If fans are banned, a stack effect cooler would have a greater advantage: http://www.youtube.com/watch?v=MXKe7bdBiGc
But stack effect cases have other problems, such as a greater risk of higher perceived noise thanks to the exhaust being placed closer to the user (as opposed to facing the wall away from the user).Edited by unifiedshader - 6/11/12 at 4:44am