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Case fan below PSU - intake or exhaust?

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August 11, 2004 4:28:34 PM

Just wondering what everyone's thoughts and opinions are regarding this. I have a standard-design mid-tower case, 2 case fans and the PSU fan.
A fan in front near the bottom blows outside air across the HDD, the PSU Fan exhausts out back at the top. The second case fan is in the standard position right below the PSU.

Should that fan be an exhaust or an intake? Does it make a difference? Would being an exhaust serve better because it pulls the hot air away from the CPU's HSF, or would being in intake be better because it blows cooler air in?

Thx for your thoughts.

Mike.
a b K Overclocking
August 12, 2004 5:40:50 AM

Normally it will keep the rest of the system cooler as an exhaust fan.

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August 12, 2004 1:05:35 PM

Thanks. That's how I have it set up. Lucky guess I guess. :) 

Mike.
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August 12, 2004 9:55:08 PM

its optimal to have the rear fan as an exhaust for multiple reasons. If it were an intake fan, the only way for hot air to escape would be your psu fan creating a high pressure environment that relies on air to naturally flow into hot spots which isnt very good. also if ur psu is pushing hot air out, the rear fan would be sucking that hot air right back into your case. By having it an exhaust you improve air circulation/flow, removing the hot air rising up. By having an 2 exhausts and 1 intake you create a low pressure environment which creates a vacuum that helps suck in cool air to the hot spots. Some people find that low pressure vacuum will cause dust problems but regular cleaning is recommended anyways.
August 13, 2004 3:21:36 PM

WHOA NOW! first off you shouldn't want a neg. or "low preasure" case. It makes the CPU fan work harder and it makes exhuast fans work harder to move less air. also the cracks and open spots in the case would suck in air and dust. yes you could clean it out but i think we would all prefer to clean less then more.
if you want to have the least amount of dust you want it to be SLIGHTLY positive. Like 1-5cfm. Nothing serious.

This is how Highrise buildings are run, slightly positive. It helps with so many aspects of the building operation. Including optimizing the speed and stregth of the fans.

What setup do you have that a back exhaust of the PSU would make hot air FLOW DOWN to where the fan is at the back. *IF* he used the back as in intake, WHICH HE SHOULD NOT DO, it wouldn't suck in hot air from the PSU.....first off its blown outside the range that the bottom fan can suck in, secondly HOT AIR RISES AND DOES NOT FLOW DOWN.

The ideal situation would be the volume of air comming in is equal to the volume of air going out....this can't be done, because its impossible to be exact. bwecause you must be one way or the other....make it EVER SO SLIGHTLY positive.

one more thing.
low preasure means that the preasure is low which is what you want because of the strain on parts and the cleaning of extra dust, negative preasure means a vacuum and stuff is being sucked in.

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August 15, 2004 4:29:02 PM

You're missing one. Add a side fan to draw air in and blow directly down on the MB and you will be faily well balanced. The front fans have a hard time drawing air in because of all the crap in the way.

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January 21, 2005 3:56:53 PM

i have a question as well..at the moment i have a 80mm at front as intake..80mm exhaust at rear below psu and the 120mm on the psu as an exhaust..i can't add a side fan as the case is next to monitor (only place it can go so no room for it) so i was looking at adding a top mounted fan but i'm unsure as to have this as intake or exhaust..(i do have a drive bay cooler/temp gauge which has 2x 40mm fans which are not used a lot)..So is there anyone that can advise me on what to do..Thx
January 21, 2005 5:23:26 PM

You will probably want to make that an intake fan to provide more cool directly to the cpu and psu. Put a filter on it though bec it will suck in a lot of dust :) 

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January 21, 2005 6:22:45 PM

If you have a fan on the top of the case, you want it to be an exhaust fan because hot air naturally moves up. Plus it pulls hot air off your mobo, etc.
January 22, 2005 2:21:32 AM

Thats true... Then you would probably want to reverse the rear fan to sort of balance things out and give a little more cool air to the system.

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January 22, 2005 4:14:34 AM

cOUPLE OF THINGS YOU CAN TRY. Reverse the rear fan so that it pulls in cool air and revverse the front fan so that it exhaust the hot air. It may work better it depndes on your case and components. The idea here is that if your front fan is pulling the cool air in first it is ususally smomewhat restricted by the case door and other things. Second by the time it gets to the components that need the cool air it has already began to heat up. Especially if you have one of the harddrive racks that sits directly in front of the intake fan. If you reverse the two fans and use the front as the exhaust then you are pulling fresh cool air in through the rear and it is available to the components that need it the most. Then it exhasut out of the front which even if somewhat restircted still gets the hot air out. This worked well on one of my systems but didnt do any good on the other. You just have to try it to see what effect it will have. Its easy to do so why not?
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