Case fan directly over HSF: good or bad idea?

This is purely a research question.
I am planning the fan layout of my new pc, because of the case design I won't have any front intake, just side intake and back/top exhaust.
I will be using 92mm vantec stealth fans, I would like to place one just below the halfway point of the video card, to feed cool air to the video.
I then would like to place another near the HSF, but also near the system RAM, for cooling.
Would I be disrupting the air or causing any problems by placing an intake fan directly over the HSF?
Thanks,
-TheComputerGuy23

it should acutally lower the temperatures of the CPU. Your intake fan will be feedint nice (relative cold) room air directly to the cpu HSF, which produces the most heat in the pc. People also do ducting. Conecting the intake fan to the HSF with a pipe, so that all the cold air goes straight to the HSF.

yeh it will be good for temps.. bad for dust

I don't know where your power supply exhaust is, but if it's right next to your intake you're going to have to deal with that somehow. Use a chimney on the PS exhaust or something.

I advise keeping hot air from the PS and the heatsink out of the case by reversing your heatsink fan and ducting it out with a case exhaust fan.

If you have great case ventilation this would be redundant but if you don't, then doing this will lower your case temperatures a lot (and lower case temperatures help make lower CPU temps.)

the duct fanatic wesson

If you check out this link it will give you some different ideas on intake options.Cruise the whole thread for a different view of air cooling. THE FANMAN

PS:Theres a little H2O cooling too!

THREAD LINK

TCG,
Since you are asking a purely theoretical question allow me to avail you of my purely experimental answer.

There is no way of knowing till you try.

Over the past few months I have tested a (seemingly) infinite combination of fan combinations on my rig.
To the point where my case is beginning to resemble the victim of a driveby attack using 80mm ammo.
Intuitively logical layouts didn't work. Silly ideas did...sometimes.
Occasionally, a layout worked well/poorly at one fan speed and the result reversed with another setting.

Short of a very sophisticated/powerful modelling program, it's well nigh impossible to predict the outcome of temp variations vs. fan placement/rpm.

In this case you can't beat real life observation...

TheWesson said:

I advise keeping hot air from the PS and the heatsink out of the case by reversing your heatsink fan and ducting it out with a case exhaust fan.

Click to expand...

i dont want to just come out and say that you are wrong, thewesson; because that isnt necessarily true. but you should be careful how you give this advice, because there a LOTS of HSFs that this advice will be really bad for. so, dont go reversing air flow on your HSF and expect that it will be great. there are a lot of reasons for this, and you can probably figure them out on your own in a couple thoughtful seconds, so i wont expalin here unless it looks necessary.

your other suggestion of ducting in air from outside is certainly a good one. i'd go with that.

mod on

In welding class we have to build something, I am building a computer case..Although still in the design phase, I am planning on having a 120mm fan placement directly in front of the CPU, and then more of them going down fromthere to cool the CPU,northbridge area,a little bit of the ram maybe, and the AGP.

So I hope that it will be better!!

On another note, I had my PSU blowing air onto my P4 before (when I was in my bad case, not the huge Koolance one). It made my temps sooo nice.. then when that PSU broke, and I put another one in that did not have a 92mm fan blowing on my HSF my temps went upto like 120F idle..lol

The case I am looking at is not your traditional mid-tower, so the PS is not near the HSF.
I think I will put a 92mm hole on the side, duct it, and then stick a fan in it and see what happens.
Thanks for the replies, I wan't sure if this would disrupt the air or anything.
-TheComputerGuy23

As long as you are doing it, why not a 120mm? It will be quieter than a 92mm. I have a 120 Enermax in the side of my case, right over the HSF.

Smirabi said:

i dont want to just come out and say that you are wrong, thewesson; because that isnt necessarily true. but you should be careful how you give this advice, because there a LOTS of HSFs that this advice will be really bad for. so, dont go reversing air flow on your HSF and expect that it will be great. there are a lot of reasons for this, and you can probably figure them out on your own in a couple thoughtful seconds, so i wont expalin here unless it looks necessary.

your other suggestion of ducting in air from outside is certainly a good one. i'd go with that.

mod on

Click to expand...

Quite so ...

Probably reversing your heatsink fan makes it somewhat less efficient. That has been my experience. The air doesn't blow right through the center of the heatsink, perhaps. I have seen an apparent difference of 2-4C doing this.

*however* you can also gain from the physical neighborhood of the CPU being cooler (the MB and socket don't have warm air blowing on them.)

So what I have found from my *personal* experience is that it's somewhat a wash (in CPU temp) between ducting air *in* and ducting air *out*.

what I have found is that ducting *in* seems better for the first few minutes, but you get a gradual temperature rise over many minutes making it equal (CPU-temp-wise) to ducting *out*.

But, ducting air out from the CPU has the advantage of
- not drawing air in from the PS exhaust by accident
- keeping your entire case nice'n'cool w/o a great deal of fannage.

now of course I agree HS design will make a big difference here. I am guessing heatsinks with huge fins will accept out-cooling or even passive duct cooling much better, for example.

And if your case is *very well ventilated* (naturally 22-25C) then ducting is a little silly and you should definitely have your HS fan blowing the direction the factory wanted it to.

As ever, one should experiment and monitor the results. But I do believe that reversing HS fan is unlikely to produce tragic results, with the damage (temp rise) most likely being limited to 5C or less.

thanks
the wesson

Another interesting effect is that (in my case) the duct works *better* if it is not airtight around the CPU HS+fan - leaving 1/8" gap all around the HS+fan.

I don't know why exactly - I speculate that the case exhaust fan can't deal at all with the extra impedance caused by being airtight, so it doesn't help pull HS air through, and all in all you're better off with a little extra case airflow around the heatsink generated by the loosely fitting duct.

Perhaps the 6cm-8cm 'fan adapter' I bought doesn't create an airtight seal for the same reason - the airflow would simply be wasted if it were sealed.

the wesson

I'm looking to try a ducting mod to channel hot air Right to the exhaust because my case temps are in teh 30s range. How do you attach the duct to the CPU fan and the exhaust fan??

I was thinking of getting those ducts they use in the exhaust fans in your kitchen except a much smaller diameter and probably 10 cm or less of it...... Or do you make the entire thing out of duck tape? or is it possible to make it out of clear plastic?

Snipester said:

I'm looking to try a ducting mod to channel hot air Right to the exhaust because my case temps are in teh 30s range. How do you attach the duct to the CPU fan and the exhaust fan??

I was thinking of getting those ducts they use in the exhaust fans in your kitchen except a much smaller diameter and probably 10 cm or less of it...... Or do you make the entire thing out of duck tape? or is it possible to make it out of clear plastic?

Click to expand...

Right now I'm using a 4" (~10 cm) diameter aluminum laundry duct. I improvised a fitting onto the 8cm square exhaust fan. It is fitted loosely to the CPU+heatsink (not airtight.) Since it is aluminum, any place where it could contact the mobo or anything electrical, is wrapped in transparent duct tape.

My main word here would be: don't plan on using a small duct, the case fan just can't pull much air through a small (e.g. 4 cm) duct.

Another approach (in cardboard or plastic sheets) would be:

Make a box for the case fan. Make a box for the CPU/HS. Cut out one of the boxes where it intersects the other, and attach them. Then you have a square duct.

Probably two sections of 8+cm plastic pipe with a single elbow joint would work fine too. In my case, the distance to be covered is small enough that practically just the elbow joint would be enough.

I would not try to make the duct airtight at both ends. These fans are not any good at forcing air, which is what you're trying to do by making the duct airtight.

the wesson

my CPU fan is LITERALLY 2.5 cm away from the exhaust fans

the CPU fan is perpendicular to the fans sucking air out and is positioned EXCATLY between the 2 exhaust fan height wise......

the PSU is about 5cm above the CPU fan and there is an exhaust there... its an enermax that makes a lot of squeky noises when I move the mouse.. kinda ****es me off.. any ideas of how to get rid of it??

The dimensions are the absolute edge of the fan.

Anyways as you can see my CPU fan is ridiculously close to the exhaust fans....... My door has no exhause.

I'm planning on shoving in a 38mm high vantec Tornado which is higher than the 25mm fan i'm using now. That means the fan will be blowing air into the side wall about < 8 cm away ... 119cfm...

SO ........... Do you recommend ducting it to the top exhaust?

when you duct..... does your duct cover the HSF assembly all the way down to the motherboard almost?

There isn't much more room in my case as it is.

Snipester said:

my CPU fan is LITERALLY 2.5 cm away from the exhaust fans

the CPU fan is perpendicular to the fans sucking air out and is positioned EXCATLY between the 2 exhaust fan height wise......

the PSU is about 5cm above the CPU fan and there is an exhaust there... its an enermax that makes a lot of squeky noises when I move the mouse.. kinda ****es me off.. any ideas of how to get rid of it??

The dimensions are the absolute edge of the fan.

Anyways as you can see my CPU fan is ridiculously close to the exhaust fans....... My door has no exhause.

I'm planning on shoving in a 38mm high vantec Tornado which is higher than the 25mm fan i'm using now. That means the fan will be blowing air into the side wall about < 8 cm away ... 119cfm...

SO ........... Do you recommend ducting it to the top exhaust?

when you duct..... does your duct cover the HSF assembly all the way down to the motherboard almost?

There isn't much more room in my case as it is.

Click to expand...

Hmm. Not a lot of space to work with, it sounds like.

This is the picture I get ("side" view if your mobo is lying flat)


..F............
<=F.......||...
..F.......vv...
..|......FFFF..
..F....<=HHHH=>
<=F....<=HHHH=>
..F....<=HHHH=>
..+------------


F = fan, H = heatsink, arrows = air direction, . = space

So it would seem like a nice idea to build sort of a fan cowl that can help remove air from right around the base of the heatsink.

Unfortunately motherboard stuff (power supply, capacitors) tends to get in the way. Which is why I haven't done it, and instead have gone after the top of the heatsink by reversing the HS fan.


..+------------+
..F......<--...|
<=F.......\\...|
..F.......||...|
..|----..FFFF..|
..F....=>HHHH<=
<=F....=>HHHH<=
..F....=>HHHH<=
..+------------


So you could build a box for the top fan (with a hole for the HS+fan on one side) and use it to help exhaust warm air. Don't make it airtight around the HS fan and you don't have to cowl down near the base of the heatsink either (though if it were pretty loose it wouldn't hurt anything and might help a little.)

Putting an angle in there will help the air move the way it's supposed to:


..+----------\..
..F......<--..\.
<=F.......\\...|
..F.......||...|
..|----..FFFF..|
..F....=>HHHH<=
<=F....=>HHHH<=
..F....=>HHHH<=
..+------------


As someone has already mentioned, there may be a significant downside to reversing the HS fan (up to 4C, which would trash your CPU temp gains from making the case cooler.) So you have to experiment. But if your case is cooler and the CPU is the same temp, that's an improvement.

The HS fan on my Aeroflow was noisy after being reversed, until I cut out a cardboard gasket to lift the fan a few mm off the heatsink. So be careful the fan still turns and sounds OK after being reversed ...

Now I wonder about strapping a fan onto the side of a classic rectangular heatsink to help push air ... If you have a blower you could probably get away with a narrow duct (1" tube) to the HS base ... hmm ....

I can offer no help on your power-supply whining when you mouse ... that's weird ...

the wesson

Now I also wonder if you could do something like the following and still get good airflow... You definitely don't want to impede the (limited) ability of the HS fan to push air through the HS. And of course you have to make sure the bottom edges of the box allow room (or cutouts) for MB components.


..F............
<=F.......||...
..F.......vv...
..|-----.FFFF..\
..F....<=HHHH=>.|
<=F....<=HHHH=>.|
..F....<=HHHH=>.|
..+------------


Or even possibly like so:

..|--
..F..\.........
<=F...\...||...
..F....\..vv...
..|......FFFF..\
..F....<=HHHH=>.|
<=F....<=HHHH=>.|
..F....<=HHHH=>.|
..+------------


especially if your HS fan is pushing more air than one case fan can remove.

the wesson