cooling: blow more in or suck more out?

[randomperson21]

i got a quick question for ya all.

currently, my case cooling setup is as follows.
2x80mm rear blowing in
2x80mm front blowing out
1x120mm side in

in your opinion, is it better to be blowing more air in than sucking air out, or vice versa?

i'm planning on changing this setup, adding another 120mm fan blowing out on the back, changing the 2x80mm rear to blow out, the 2x80mm front blowing in, and perhaps a pci slot fan thingy blowing out in the back.

 

[cjoyce1980]

more out is better, as these fans will pull more air thorugh. mine is 4 in and 2 out and my athlon xp sits at 45c idle, before this it was 53c

 

[pt]

Does your case is ATX/BTX?

 

[randomperson21]

atx.

so higher flow out is better. hmm.
perhaps a 80mm-120mm fan adapter is in order.

btw, i have a raidmax scorpio case (don't remember the exact model #)

 

[GLD]

I would offer my 2cents and say you should switch the direction of your 80mm fans. Hot air rises? Yes? OK, so you want your back fans exhausting, just like your power supply does, and haveing the front fans intaking air in to your case. The side fan is OK intaking air. This is how I set up all my builds. Intake front and side, exhaust back.

 

[randomperson21]

yeah, i usually do that too.

after i finished ripping apart my computer, i had to remount the case fans. but i screwed up and mounted them backwards. and since the fans are a pain to get on and off, i felt too lazy to fix them.

 

[bbriand]

I have often been wondering this as well. I think it would depend on your case design. I would be worried that with too many fans blowing out you might create "dead spots" in your case where air doesn't circulate to.

For just straight fans I have 4 intakes (3x80mm and 1x120mm) and 1 exhaust (120mm). This doesn't count my PSU and I have an AC Silencer which blows air out. I have contemplated switching my top mounted fan to exhaust. But in the end I figured more cool air in would be better and I have an AC Freezer on my CPU that helps direct air at my exhaust fan.

My rationale was this. On a regular heatsink would you place the fan blowing onto the fins or trying to suck air through them? So the way I figured it my intake fans are all pretty much blowing directly at something to cool it down.

I think it is case dependant to a degree though.

Bill

 

[randomperson21]

in my case at least, i'm noticing quite a few "dead spots" where hot air will just sit and collect.

the front half of my case is staying pretty cool, the intake fans are doing their jobs.

but a ton of heat is being generated by my cpu and 7900gt, and especially with the gfx card, the hot air is just sitting there. same goes for the cpu to some extent. so, since i figure i'm already pushing air thru the heatsinks, i might as well suck out all that stagnant hot air.

does anyone know of any good pci-mounted fans that suck out?

 

[bbriand]

I know what you're saying about the CPU and GPU pumping out a lot of heat. It was my driving force in purchasing my two coolers. The GPU heat gets directly blown out the back and the CPU heat gets blown directly into my 120mm exhaust fan.

I should have paid closer attention to my case temps before I installed those 2 coolers but I was so excited to get them in I never thought to check any other temps then the CPU and GPU directly.

I need a pair of infrared goggles so I can see the hotspots in my case

Bill

 

[randomperson21]

ha! ir goggles would be cool.

you have an ati silencer rev2, right? those things blow directly out the back of the case.

problem is, i have a zalman vf700alcu, and hot air sorta blows out laterally all over.
this is compunded by the fact that its already really really hot in the room where it is (think, 90-95F), so i get really high idle temps. 50C for the GPU on a bad day, 45C for the cpu.

but on cooler days, its totally different. 39C for the cpu, 43C gpu.
perhaps i can get variable speed fans i can tone down for the winter.

 

[error_f0rce]

Yes, the golden rule is: Front = IN, Back = OUT, SIDE = IN, TOP = OUT.

You typically want negative pressure in your case, this will eliminate the so-called "dead spots", which means you need more air exausting than intaking.
So do some simple math to calculate your air CFM (cubic feet per minute) ratio. Go to the manufac website if you don't know what CFM your fans are rated at, it can differ greatly even between the same size fans.

Example:
IN: 2x 80mm [front] @ 34 CFM + 120mm [side] @ 50 CFM = 118 CFM IN
OUT: 2x 80mm [rear] @ 34 CFM + 80mm [top] @ 34 CFM = 102 CFM OUT
118/102 = positive pressure inside the case.

So in the example you would want to increase the exhaust or reduce some of the intake. This is made much easier if you can adjust your fan speeds (i.e. reduce your x2 80mm intakes from 34 CFM to 20 CFM, to achieve a negative pressure of 90/102). The ideal scenario would be to increase the exhaust, therefor not compromising the rate of flow.

I think the only thing that can be said "positively" about positive pressure is that is keeps dust out, but that's about where the list of pros ends. Cons: positive pressure will create "dead spots" where air is swirling, but unable to escape because the intake/exhaust ratio is tipped in favor of the intake. Too much air in, not enough air out = dead spots. Now picture "negative" pressure as a vacuum cleaner, there goes all the hot air right out the case... however, you want to maintain decent intake because if the ratio is tipped to far in favor of the exhaust, you will again lower you rate of airflow through the case.

Although truly you should calculate your PSU fan(s) into these equations, I've never seen it done. This may be due to the fact that the PSU fan(s) spins at different speeds depending on load, ambient temp etc.

Hope you made it all the way through this and hope it helps

EDIT: High pressure, or low pressure... it's all about the CFM's.

 

[randomperson21]

higher pressure as in blowing more in?

lower pressure is blowing more out, correct?

the primary reason i'm thinking of blowing more out is because my case doesn't really have the capacity to blow more in.
it only has 2x80mm slots on the front, and a door and plastic crap covering everything else.

but on the back side, i can add a 120mm and some other stuff.

 

[randomperson21]

thanks for the great post, error_f0rce.

i think i have too much positive pressure in my case right now. 2x80 rear in is like 50cfm combined, and my 120mm side is like 70cfm in.
my 2x80mm front is maybe 50cfm out.

so that would explain the dead spots.

i'm at work right now, but i'll try to make some changes tonight and see if that helps temps, etc.

and thanks for all the help everybody! tpu rocks!

 

[error_f0rce]

thanks for the great post, error_f0rce.

i think i have too much positive pressure in my case right now. 2x80 rear in is like 50cfm combined, and my 120mm side is like 70cfm in.
my 2x80mm front is maybe 50cfm out.

so that would explain the dead spots.

i'm at work right now, but i'll try to make some changes tonight and see if that helps temps, etc.

and thanks for all the help everybody! tpu rocks!

Sounds good, post and let us know how it goes!

 

[bbriand]

Very nice post error_f0rce! My only comment would be that if the case was air tight this would be bang on. But most cases aren't even close. I don't know how this would affect the equations.

I'm thinking if you had a bucket and two hoses. One blowing 35CFM and one sucking 35CFM. What would cool off a hot plate of chili on the bottom faster?
1. The hose thats sucking air placed in the center of the bucket, or
2. the hose thats blowing air placed in the center of the bucket blowing directly on the chili.

Logic tells me number 2 would be faster but I have never attempted this experiment.

The only thing I can to do is if its hot blast it with air from outside the case. Then have a fan to remove the hot air as well as the vent holes.

Bill

 

[randomperson21]

bbriand, i see your point.

i think my primary problem is stagnant hot air. i have these dead spots in my case (under the gpu, between the cpu and ram) that get really hot. not the physical components themselves as much as the hot air let off just sits there.
so if i could figure out a way to suck all that hot air out, temps should lower.

 

[bbriand]

Maybe if you took a 120mm fan and placed it at the bottom of your case and put just enough power to it to get the air circulating (maybe even a variable speed control).

The fan wouldn't be directly attached/mounted to the case in the traditional sense but just sitting in there to circulate air. Possibly even angle it a bit so the air moves towards the exhaust.

Something like this:

Its a similar design for say my AC Freezer Pro. Take the hot air and direct it towards the exhaust.

I don't know if this would work or not but don't we all have at least one spare fan kicknig around?

I would say this might take all of 5 mins to setup - tops! I'm thinking more like 45 seconds. 15 to get the side off, 15 to plug in and prop up the fan and 15 to get the side back on.

Anyhow good luck

Bill

 

[randomperson21]

hmm that sounds like a good idea.

i was thinking something similar, except w/ a pci mounted fan thingie. i was at frys a few days ago and saw one. they mount in your pci slot, and exhaust hot air out the back. i was thinking of putting one next to the card.
but this'd work just as well i think. and then take off some pci covers on teh back to let it vent maybe????

problem is, i used my last spare fan (that i know of) for my pong clock.
so its back into the dark closet of RIP computers to go fan hunting!

 

[bbriand]

Radio Shack has a fan assembly (with 3 fans) that mounts in a 5 1/4" bay. I think I saw them there for $10. Pretty good buy.

As for fans I swear I collect them whenever I see them on sale. I have a box of odd sized fans/blowers (some with heatsinks) that I'll probably never use and never throw away. My recent fan purchase was to see if I could make a homemade lens defogger for my paintball mask.

The PCI slot exhaust fans are pretty cool. I think I have one in that box I just mentioned. (No free PCI slots on my case).

Bill

 

[randomperson21]

thats cool.
problem is, i have a door over my front panel.

stupid raidmax cases.

i have a whole collection of p3 computers. dells, mostly. so theres gotta be something in there.

 

[bbriand]

Yeah I have a door as well but I am reluctant to close it. I don't want to get in a habit of closing it as I burn a lot of CDs/DVDs and they autoeject and I don't want the door latched when they do.

Sounds kinda silly I know but its one less thing to have to worry about when I tear my butt from my computer chair.

Bill

 

[error_f0rce]

Very nice post error_f0rce! My only comment would be that if the case was air tight this would be bang on. But most cases aren't even close. I don't know how this would affect the equations.

I'm thinking if you had a bucket and two hoses. One blowing 35CFM and one sucking 35CFM. What would cool off a hot plate of chili on the bottom faster?
1. The hose thats sucking air placed in the center of the bucket, or
2. the hose thats blowing air placed in the center of the bucket blowing directly on the chili.

Logic tells me number 2 would be faster but I have never attempted this experiment.

The only thing I can to do is if its hot blast it with air from outside the case. Then have a fan to remove the hot air as well as the vent holes.

Bill

Thanks Bill,

I agree with your theory! I also like the metaphor, although the chili doesn't constantly produce heat, therefor it doesn't require constant venting.
I would liken a properly place side intake fan (above the CPU) to your hose idea blowing directly on the chili. Now in a bucket, there is obviously no lid, so the hot air escapes effortlessly. Unfortunately with most standard computer case, it needs a little "encouragement" , ergo the air flowing through the case.

I think it also worth mentioning that if your CPU's HS/HSF is doing it job, there should be no problem with the temperature of the CPU; the heat wants to escape on its own, we just have to provide it a means, i.e. exhaust. That is why I don't worry as much about "blowing" heat off the CPU with a fan, as much as exhausting the existing heat that is trapped in the case. However, IMO it's most efficient to use both methods simultaneously. The efficacy of this method can also be enhanced by installing a "windtunnel" (aka ducting) going from the side intake to your CPU, channeling the air directly to it. These come with some cases, but can also be purchased and bolted to the back of your side panel: http://www.xoxide.com/ovcokit.html (check out the heat graphs).

Whew, I'm all tuckered out for today I think...

 

[randomperson21]

well, i got home and started fiddling with my fans, and i discovered something horrendous.

the total air output of my fans was being reduced by about 50% by the fan grills on the case!

now if i had my digital cam, i'd post pictures, but since i don't, i'll do my best to discribe.

so there are 4x80mm slots for fans on the case. but instead of being just holes for fans, they are just peforated sections of sheet metal. problem, there is too much sheet metal and not enough holes.
so when the fan is blowing by itself, it moves 40cfm. that number is cut in half as soon as its mounted.
actually, its sooo bad that air is bounced back into the case by an exhaust fan. when i mount the fan, and then put my had over the intake end, i shouldn't feel air blowing back at my hand. but i do. and its because 50% of the air is bouncing off the grill and back at my hand.

so, i'm going to do some mods to the case:

Mods to do:
1. Cut out 2x80mm back grills for 100% airflow
2. Cut 80mm (120mm??) blowhole on top. not sure which one will fit yet.
3. cut out 2x80mm front (90% sure of this)
4. mount 1x120 mm back thru 80mm-120mm adapter
5. fan grille 80/120mm for new openings in back and top


Total CFM

IN: 2x80mm front in @ 80cfm + 120mm in side @ 70cfm == 150cfm - (impediance from
front grill)

OUT: 1x80mm back out @ 40cfm + 1x120mm out @ 70cfm + 1x80mm top out @ 40cfm +
120mm psu out @ ???cfm = greater than 150CFM

perhaps add a pci exhaust fan under gfx card to hit that dead spot.

so i'll be pulling more out than pushing in, if just by a little bit. If i mount a 120mm on the top instead of a 80mm, total out would be more like 180CFM. Also, all my 120mm fans can be set anywhere between 30-70cfm, so come winter, i can switch them all on low and still get pretty much the same ratios.

also, someone just posted that tigerdirect is selling an ultra $40 fan controller for $5-something, so might pick that up. too good a deal to miss out on.

comments?

 

[randomperson21]

one more thing:

about the ultra fan controller: it takes 7 3pin fans, up to 12v.

4 of the fans in my case have 4 pin molex connectors.

soooo

would it be possible just to splice the +/- on a 4 pin to the +/- on a 3 pin? and just ignore the rpm sense cable?

 

[error_f0rce]

one more thing:

about the ultra fan controller: it takes 7 3pin fans, up to 12v.

4 of the fans in my case have 4 pin molex connectors.

soooo

would it be possible just to splice the +/- on a 4 pin to the +/- on a 3 pin? and just ignore the rpm sense cable?

Right... just keep in mind that the yellow molex wire is the 12v and the red wire is the 5v. Probably don't want to mix those up Yeah, ignore the rpm sense cable on the 3-pin (yellow?)