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Heat rises, so why not put an exhaust fan on top?

Last response: in Overclocking
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February 7, 2006 11:52:31 PM

I'm building my own case currently, and obviosly, cooling is a key issue.
Instead of having many various fans everywhere in random places, here's my idea:
I will have two PSU's, one for the mobo and drives.
Second will be for floppy drive, neons (two) and a 120mm fan.
The fans on the psu's will do their usual, and the 120 will blow out the top. Over the top, will actually be an "umbrella" made from plexiglass, set about 1/4" above the actual top of the case. This will draw all the hot air out of the top, and out of the case, while drawing cool air from underneath the motherboard, which has the typical holes from the case that i've destroyed for my project. Otherwise, everything will be sealed up pretty tightly, hopefully creating a vaccuum around the motherboard.
OH, also, the cpu will have a heatsink type cooler, with the 120 centered directly above it, hopefully drawing cool air through it.
I was a little worried about making a channel from the 120 to the cooler thinking that the air would get heated up by everything else in the case and then go through the cooler and then out, so i'm thinking of just leaving it open.

So what do ya'll think?
February 8, 2006 1:31:23 PM

Or like I did, place the fan on top outside of the case worked for me, wusy, sounds like some topics mentioned last year dude.
February 8, 2006 2:07:45 PM

That makes a lot of sense but I guess the manufacturer are also designing the cases with security/safety in mind. Be honest, how many of us that plays pc and drink dew or red bull, or worst get drunk and place bottles and cans on top of the case. One spill, liquid goes through the blow hole and into the mobo and graphics card, ending up frying the system.

Thats basically my concern, Im not against the idea. I have a Thermaltake Kandalf with a huge opening on top and equip with a 80mm fan. The blow hole is above the psu and mobo and it helps cool down the cpu no doubt. I have plenty of light inside the mobo and it looks cool coming out of the blow hole. But sometimes my buddies put shit on top of my case, driving me crazy.

It already has good vent, but I went ahead and punch two more holes in the side with 80mm fan to direct air to the cpu hsf and the dimm. And I don't have to worry about overheating it.
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February 8, 2006 5:34:48 PM

yeah spills and other 'stuff' that gets placed on top of a case are a good reason not to put blowholes in the top.

Now if we know ahead of time whats comming, the case can be positioned such that its not convient to place your can of whatever on top and go from there. But that still leaves an opening for stuff to fall in while the PC is off, or for little hands to put stuff in the grate.


On the other hand - has anyone with a dual cpu rig had success with such mods? Channeling air for a single cpu is all fine and good, but when you have 2.. most of the research doesnt apply.
February 8, 2006 10:08:24 PM

BlowHoles are one of the most effective and easiest case mods.

Dell had a great solution for Dual CPU's on their Workstation 550 series (P3's). They had a custom plastic shroud that enveloped the tow processors and blew cool the air into the case and over the processors, there was another inlet and another exhaust.
February 9, 2006 1:10:29 AM

Alternative to top blowholes is to mod the case with cardboard/foamboard/acrylic glass by creating separate chambers for objects that generate heat.. ie CPU and Graphics Card, then exhaust it out, pulling air into the areas will not be a concern for all the heated air will be secluded from the rest of the areas in the case.
February 9, 2006 1:16:35 AM

Quote:
Alternative to top blowholes is to mod the case with cardboard/foamboard/acrylic glass by creating separate chambers for objects that generate heat.. ie CPU and Graphics Card, then exhaust it out, pulling air into the areas will not be a concern for all the heated air will be secluded from the rest of the areas in the case.

So you think that it will be better to suck air out from the components rather than blow through them? I definatly think i'm going to do that though... kinda FORCE the windtunnel idea...
Thanks!
February 9, 2006 11:54:54 AM

Its very effective, My A64-3000 NewCastle idled around 45C and 55C 100% load, after the mod, it idles around 25-30C depending on ambient and 35C on 100% load.
February 9, 2006 2:09:17 PM

If ur afraid that something might fall thru the blowhole, find something to build a tunnel (with 90° bend) with. It should look like the arrow on the Enter key turned by 180° (the air leaves the case horizontally and the blowhole is safe from stuff from above^^).
February 9, 2006 2:53:48 PM

Hi,

I am worried about one statement...

"Vaccum around the motherboard" THIS IS A BAD THING.

If anything you want higher pressure airflow through the case.
The lower the pressure and closer to vaccum you get the air density drops. When this happens the thermal conductivity of the air lowers. (less air molecules to come in contact with the heatsink)

I'm not sure if you meant just extremely high airflow through the case or -'ve pressure in the case. The second scenario is very very bad!

My 2 cents.
February 10, 2006 1:48:16 AM

I also have a hole on the top of the case of one of my computers w/ 80mm exhaust fan.

Only thing is, all your air coming in the front past the hard drives, gets sucked out the top, the CPU and the rest of anything else such as the memory isn't getting any. Think about this, the air intake is at the bottom front of your case and the exhaust is at the top rear of the case (including the power supply unit). Now think of a diagram with arrows indicating the airflow straight through the bottom front of the case>straight past the motherboard>straight out through the 80mm exhaust fans on the back and the PSU.
This is purposefully designed so that air will pass the hard drives, then pass the MEMORY and MOTHERBOARD and CPU. If you have a top blowhole it just makes the cooling setup much less efficient because by the time the air gets halfway through to the middle of the case it hasn't cooled anything yet besides the hard drives - so sucking it out the top is just pointless IMO. When ideally the air should keep going horizontally thru the case, past the motherboard first and straight out the rear for max airflow throughput -none of this random air circulation where the air is just moving around in circles and hopefully being sucked out top. That is all.
February 10, 2006 2:12:17 AM

should be the intake fan blows stronger (faster) than the exhaust fan (sucking out) or exhaust fans that blows air out should be stronger than the intake fan. or all fan at Equal speed.. which one will give me perfect ventilation?


Quote:
Damn 5000RPM fans sound like vaccum cleaner
February 10, 2006 2:49:20 AM

Quote:
should be the intake fan blows stronger (faster) than the exhaust fan (sucking out) or exhaust fans that blows air out should be stronger than the intake fan. or all fan at Equal speed.. which one will give me perfect ventilation?


well think about it. if you have more air going in than out, where is all that air going?
February 10, 2006 11:20:50 AM

As said above, create separate chambers to contain the heat, and sucking it out instead of blowing air directly on the CPU/Video Card which would eventually get hot anyways...It will keep the ambient temp lower, and at the same time you will need less fans operating = less noise :D 
February 10, 2006 7:22:42 PM

wouldn't both work best? i stick with the quiet 120mm fan on front intake, and another on back, how else are you going to cool the hard drives and get more air into your case quicker?
February 11, 2006 2:09:19 AM

Both will work of course, I'm just saying that you won't need as many case fans running, since you would be effectively positioning the airflow for the fans.
February 13, 2006 1:09:57 AM

oh right. so you're talking about setting up your case with just exhaust fans and no intakes?
February 13, 2006 3:44:24 PM

wusy, yes and no

the problem becomes if the air seeping in can't keep up with the ammount you're sucking out.

you create a vacuum condition with lower air pressure which doesnt cool as well as said once in the thread.

the hole thing is you need as much air going out as in. the presure ballance should be equal.

so if you're goin to suck only, make sure you have enough "holes" around the case to facilitate the necesssary intake of air without creating such a vacuum. also it's important to get a good flow through the hole case

a lot of people will put a big suction fan on the back and think thats enough. well on some cases all the fan does is create a minivacume. the suction sucks in from available openings, often the biggest and closest to those are open PCI / AGP expansion slots at the back of the case. all this effectively does is suck air in and straight out without a good flow in the case.

this is why a front fan sucking air into the case is effective to ensure good flow from front to back. (reversable from back to front but you get the idea)
February 13, 2006 8:41:16 PM

Yes, I can prove that lower pressure = worse cooling.

If your are in Toronto, come to the UofT engineering building.

Anyways, as I said before less pressure = less molecules to cause heat transfer.

So causing a -'ve pressure situation (using only exhaust fans to suck out air and leeting air seep back in through the cracks) would be a very bad idea

The best way of Air cooling a system has already been mentioned by ducting air over certain components, which cuases high airflow with less fans. but the airflow is still at relative pressure.

The main thing to keep in mind with any air cooling solution is high airflow.

The more air you can keep flowing through the case, the cooler it will run.
February 13, 2006 8:47:33 PM

All the ideas are good. I think that ducting the air from the area of the processor, via conduit directly out the back. You still have to make sure that enough air gets in no matter what you put in for cooling. Setting the case up off the floor is a good idea as well, you don't want to be sucking in a bunch of lint from the floor.
One of my clients has a cabinet shop and dust is a real problem, so we built a wooden cabinet that has house sizse air filters on the intake (front) and changes them regularly too, they get dirty about once a week if they are doing a lot of finish sanding. The box is totally open in the front when you open the filter door, making for easy access to the ROM drive for installing software. The back of the case is the back of the box and the PS fan vents directly into the open air, the case fans, 2 80mm, draw air one from the CPU and the other from the general case. With the ductwork in place over the CPU, it runs cool and clean all the time.
We had to blow the case out completely once a month before the box was built and now we only have to change the filter.
The box also acts as a sound deadening device, so the whole unit is very quiet, of course it is hard to hear anything when the power tools are running!
February 14, 2006 4:01:40 AM

Improbable and impractical, but not impossible...
February 14, 2006 6:17:24 AM

Quote:
Why do they call it a 'blow job' when it's actually 'suck job'??? :roll:


dunno, cos they're a bit screwed in the head maybe
!