[SOLVED] Can top exhaust fans "steal" air from heatsink fans and rear exhaust fans?

ProtoflareX

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The case setup that I'm planning to have is essentially the exact same as the setup portrayed in the rightmost picture of this Amazon review (you can click the picture to enlarge it). However, instead of liquid cooling, I intend to use a Thermalright Le Grand Macho RT CPU cooler, which blows heated air directly at the rear exhaust fan. The fan on the Le Grand Macho RT's heatsink would be located directly to the left of the leftmost RAM stick in the picture. I would like to know if the 200mm fan at the top of the case would "steal" air from the Le Grand Macho RT's heatsink fan as well as from the case's rear exhaust fan. If it's relevant, the case portrayed in the picture is a Cooler Master H500.
 

tennis2

Respectable
Correct, since the case has positive air pressure (more air being pushed in than sucked out), each of your exhaust fans has more air waiting for them than what they can suck out.

Regardless, if you can, mount the top fan as far back as possible and keep it at a low speed.
 
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ProtoflareX

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Correct, since the case has positive air pressure (more air being pushed in than sucked out), each of your exhaust fans has more air waiting for them than what they can suck out.

Regardless, if you can, mount the top fan as far back as possible and keep it at a low speed.
You mentioning mounting the fan as far back as possible led me to another question. Inside the Cooler Master H500 case, a 200mm top exhaust fan can only be mounted in the position shown in the picture, there are no other options. However, I have the option of choosing a single 140mm fan instead. As a top exhaust, there are two slots for me to mount it in. The first slot is closer to the case's topmost front intake fan (which would put the fan close to the area that the heatsink fan draws in air from). The second slot is closer to the case's rear exhaust fan (this position would put the fan behind the heatsink's fan). Do you believe it might be better for me to forgo the 200mm fan in favor of a 140mm fan mounted closer to the case's rear exhaust fan? I've heard that doing this can cause hot air from the GPU to be drawn into the heatsink fan.
 

Karadjgne

Titan
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Heat rises. When left alone. It's the entire thought behind rear exhaust fans, they are a dinosaur idea leftover from the old AT towers and pre-built pc's that had a solid top. Heat goes up and collects at the top. When a fan blade moves, it displaces air creating a vacuum. Nature abhors a vacuum, so fills the void with the nearest local air, which at the top of those cases was kinda warm. The result was the next blade creating a vacuum again, the byproduct being the original warm block of air being pushed out the back. Rinse and repeat. The only coolers that now benefit well from rear exhaust are towers where the cpu exhaust is aimed directly at the rear exhaust fan. For any other air cooling, majority of that heat is still going up, most rising up and out the top vents. The addition of another 200mm Fan up top at slow rpm will just add to the upwards movement, hastening that heated air out.

You don't need gale force winds in the case, only need enough to keep the airflow moving up and out. In cases where a rear exhaust is used, having another fan nearby doesn't 'steal air' it just adds to the low pressure area, the vacuum, so you end up with a stronger flow from low front to high rear corners. With a 120mm rear and 140mm top rear, you basically duplicate what a single top 200mm is doing.

Intakes aren't for pushing air in, intakes are there to enable the replenishment of the exhausts vacuum with cooler air at higher efficiency. Relying on natural process takes too long, the air has too much chance to heat up and isn't exhausted fast enough, so case temps rise, lowering the efficiency of any heatsink, which allows its heat to rise. Intakes speed up the process, so the generated heat is moved out faster, replaced faster and case temps are lower.
 
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ProtoflareX

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If I understood your post correctly, the heat exhaustion performance of a 120mm rear exhaust fan and 140mm top-rear exhaust fan is equivalent to the performance of a single top 200mm fan? As a result of that, I would be better off going with the 200mm top exhaust, correct?
 

Karadjgne

Titan
Herald
I would. That 200mm will output roughly the same cfm, at lower rpm as the 120mm + 140mm combined, if not more.

Ever been to a restaurant that has a "Big Ass" brand fan? They are like 12ft across. Those blades are so huge they move a ton of air, and yet barely move at all, like less than 10rpm. Dead silent. Compare that to a regular ceiling fan on high speed, noisy and doesn't move nearly as much air volume even though it has a stronger 'breeze'. It's the volume of air moved that counts.

Edit: the area of that 200mm is @ 125,600 sq.mm and the area of a 120mm + 140mm is @ 106,800 sq.mm, you'll get better flow across a broader area of space with the single large fan than 2x smaller fans at half or less rpm. The only real reason to keep the 120mm at rear exhaust would be if you have a tower type cpu cooler. Other than for looks of course.
 
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ProtoflareX

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I would. That 200mm will output roughly the same cfm, at lower rpm as the 120mm + 140mm combined, if not more.

Ever been to a restaurant that has a "Big Ass" brand fan? They are like 12ft across. Those blades are so huge they move a ton of air, and yet barely move at all, like less than 10rpm. Dead silent. Compare that to a regular ceiling fan on high speed, noisy and doesn't move nearly as much air volume even though it has a stronger 'breeze'. It's the volume of air moved that counts.

Edit: the area of that 200mm is @ 125,600 sq.mm and the area of a 120mm + 140mm is @ 106,800 sq.mm, you'll get better flow across a broader area of space with the single large fan than 2x smaller fans at half or less rpm. The only real reason to keep the 120mm at rear exhaust would be if you have a tower type cpu cooler. Other than for looks of course.
This is a little late, but I have an additional question for you, if you don't mind. If my CPU cooler has a large heatsink that is very close to the backplate of the GPU (essentially almost touching), will the heat being dissipated by the heatsink increase the temperatures of the GPU? Or perhaps the opposite would occur, and heated air from the GPU would be drawn into the heatsink via its fan, causing it to increase in temperature. Are either of those two scenarios plausible?
 

Karadjgne

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Nope lol. The front fan will blow air through the heatsink. With most aircoolers you get what's called bleeding. This is where a portion of the air escapes out the sides of the heatsink. That air is blowing directly on the backplate, very gently like a slight breeze. Net affect is that the breeze treats the backplate like a giant heatsink and helps moderate temps when the gpu runs hot. The backplate itself is usually attached to the gpus ground plane, and many small components use the ground plane as a heatsink.

It's one of the ways aircoolers have the advantage over AIO's.
 
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ProtoflareX

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Nope lol. The front fan will blow air through the heatsink. With most aircoolers you get what's called bleeding. This is where a portion of the air escapes out the sides of the heatsink. That air is blowing directly on the backplate, very gently like a slight breeze. Net affect is that the breeze treats the backplate like a giant heatsink and helps moderate temps when the gpu runs hot. The backplate itself is usually attached to the gpus ground plane, and many small components use the ground plane as a heatsink.

It's one of the ways aircoolers have the advantage over AIO's.
Oh... Welp, looks like I was worried for nothing.
 

Serinox

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This is a little late, but I have an additional question for you, if you don't mind. If my CPU cooler has a large heatsink that is very close to the backplate of the GPU (essentially almost touching), will the heat being dissipated by the heatsink increase the temperatures of the GPU? Or perhaps the opposite would occur, and heated air from the GPU would be drawn into the heatsink via its fan, causing it to increase in temperature. Are either of those two scenarios plausible?
Since i did recently quite some testing on improving my cooling i can give you some advice.

I am using an overclocked 3930k (~260w power draw on prime) with a 1080TI in a dark base 900 pro case with 5 silent wings 140mm fans. My air cooler is also the LGMRT.

The perfect setup for my build is 3 intake fans front, 1 intake top right above the cpu cooler and 1 back exhaust. Psu is also exhaust on the bottom of the case. Gpu in Slot 4.
Top exhaust fan vs intake was like 2-3 degrees worse. Also gpu in slot 1 resulted in a higher temperature for gpu and cpu alike.
 

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