Question Second guessing my fan setup

PanicPaladins

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Sep 28, 2016
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So I am close to having all the parts for my new PC build and I'm wondering about how I should have my fans setup. The case I'm using is a Fractal Design Meshify C. I am using a liquid cooling AIO for my CPU that I definitely want to use as exhaust at the top of the case. The AIO is a 240mm and I was going to have two 140mm fans as intakes in the front. Now I am wondering if it would be better to have 3 120mms at the front and an extra 120mm (3 intake, 3 exhaust) on the back or keep the two 140mm fans at the front and still tack on the extra 120mm on the back exhaust (2 intake, 3 exhuast). Just trying to figure out the most optimal airflow for my setup. Thanks for any help and suggestions!
 

Paperdoc

Glorious
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The aim here is twofold: good total airflow, and a balance of intake and exhaust flow rates so that you have a small net excess intake capacity and the case has a small positive pressure inside. At least, that's my opinion. That way any air leakage (and there ARE leak points) will have air leaking from the inside to outside, preventing intake of air at leaks. Then you must ensure that the real intakes at the front have dust filters to ensure the air inside the case is clean.

I can assure you that the only info you have on this - the specs for the max airflow of each fan - can NOT tell you the full answer to this. However, they can guide you to a good starting point. To begin, the SIZE of the fans is not the info to examine, although it is related. You need to look up the specs of each fan involved and find the max Air Flow rating for each. Add up the intakes. Add up the exhausts. Ideally, intake total should slightly exceed exhaust total. As a TREND, air flow from a 140 mm fan will be a lot more than from a 120. So two 140's at front intake will certainly exceed the two 120's as exhaust on the rad at the top, and may well exceed the total exhaust even when you add the rear 120.

Next step is to recognize that those totals are wrong. The specs are all given for absolutely no restriction on air flow. But the dust filters on the two 140's at front do reduce air flow though that pair by a bit. (Remember to clean those filters from time to time.) Then the radiator fins are a significant restriction on air flow being exhausted by the 120s on the rad. Those two don't quite balance each other, but the small reduction of flow on the front fans that deliver more air anyway means there is surely excess air intake between those two groups. Now, add in the unrestricted flow of the rear exhaust fan, and you might be very close to ideal balance.

This will give you good air flow and close to what I call the ideal balance. IF you want to make any fine adjustments, you need to verify which way the air leakage is. For that you need a smoke tracer source. I have used a smouldering cigarette or a smouldering incense stick. With the system running and maybe doing some normal workload (or maybe just at idle), go around the outside of the case at small openings like cracks or seams. When you get the smoke source near that, does the smoke flow away from the case or into it? You may even get a "feel" for whether the flow of smoky air is moderate or really fast. That will tell you roughly what the air flow balance is.
 

Karadjgne

Titan
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What many ppl confuse is Air and Airflow, thinking they are synonymous. Very far from the truth. Ppl will believe that filling a case to capacity with fans and more cfm is the way to go. Not so.

What any case needs is decent air Flow. That's not just Air shoved in, it's a created and maintained channel of air that picks up/absorbs case heat and then exits the case.

Thinking about flow itself, you have to envision what each fan is doing, what is the direction of fan exhaust affecting. That includes not having a tower aircooler. You have an AIO and that changes airflow patterns.

Warm air rises until its obstructed. You have no tower in the middle to grab the rising air, shove it 90° sideways to the rear of the case, making the rear exhaust very important. Instead you opted (sounds like) to put the AIO on top. Warm air rises. So that's exactly where you want it to go, right into the arms of those fans. Straight up and out. No rear exhaust necessary, or wanted.

Now all you need is a couple of fans to supply those AIO fans. For this, you'll not want high cfm/airflow fans. You'll want Balanced or Pressure fans. Not only do they work better with dust filters, but there's more air supplied that actually reaches the gpu, the hotter of the 2 heat sources in a gaming pc.

Shove air at the gpu, which in turn pushes warm air up the side of the case, which is exhausted out the top by the AIO fans.

That's airflow. 2x 140mm in front, 2x 120mm on top is plenty to direct a channel of air in a constant stream. Additional fans, intakes set high, etc would only confuse that airflow of in/out and you end up with a circulatory pattern of warm air that just gets warmer, and ends up affecting not just case temps, but cpu/gpu temps too as a result.
 

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