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

[ProtoflareX]

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.

[Cyracus]

Should be fine

[dfsdfgfkjsefoiqzemnd]

That top fan in that position will pull some of the air from the front fans out of the case before the air has a chance to cool anything, but that's not a big deal.  The CPU cooler should have no problem pulling in air of its own. 

[ProtoflareX]

1 minute ago, Captain Chaos said:

That top fan in that position will pull some of the air from the front fans out of the case before the air has a chance to cool anything, but that's not a big deal.  The CPU cooler should have no problem pulling in air of its own. 

So the top exhaust fan in that case actually has a negative effect on cooling? Even if the difference in temperatures isn't enormous, would it still be better to forgo the top exhaust fan? It would save me some money.

[Jurrunio]

Tbh I don't think it really matters? As long as there's more air going past the heatsink it will help the CPU cooler. Having a fan is better than not having one in this case.

[ProtoflareX]

1 minute ago, Jurrunio said:

Tbh I don't think it really matters? As long as there's more air going past the heatsink it will help the CPU cooler. Having a fan is better than not having one in this case.

But wouldn't there be more air going past the heatsink if the top fan wasn't there since it's taking some of the air away?

[dfsdfgfkjsefoiqzemnd]

4 minutes ago, ProtoflareX said:

So the top exhaust fan in that case actually has a negative effect on cooling?

It's kinda hard to predict.  On the one hand there is some leakage if the fan (or part thereof) is in front of the cooler (which is the case with the Amazon picture), on the other hand the fan will pull out hot air so the overall effect may very well be a lower overall temperature. 

Ideally you'd move the top fan as far as possible to the back, but I'm not sure about the mounting options of that case.

 

Give me 20 minutes, need to run to the store real quick before it closes, but I'll draw a sketch of it as soon as I get back. 

[Jurrunio]

4 minutes ago, ProtoflareX said:

But wouldn't there be more air going past the heatsink if the top fan wasn't there since it's taking some of the air away?

the fan is also speeding up air that did go past the heatsink.

[ProtoflareX]

1 minute ago, Captain Chaos said:

It's kinda hard to predict.  On the one hand there is some leakage if the fan (or part thereof) is in front of the cooler (which is the case with the Amazon picture), on the other hand the fan will pull out hot air so the overall effect may very well be a lower overall temperature. 

Ideally you'd move the top fan as far as possible to the back, but I'm not sure about the mounting options of that case.

A 200mm top exhaust fan can only be mounted in the position shown in the picture, there are no other options for it. However, I do have the option of purchasing a 140mm fan instead. The top of the case accommodates two 140mm fans. One can be mounted closer to the topmost intake fan, and a second can be mounted closer to the rear exhaust fan. Do you believe it might be a better option for me to buy the 140mm fan and mount it closer to the rear exhaust fan?

[dfsdfgfkjsefoiqzemnd]

Okay, here's a quick and dirty sketch of what your airflow will probably look like with the 200mm.

 

 

Air gets sucked in through the front 200mm fans.  Some of the air is sucked straight out by the 200mm in the top, the rest goes to the CPU cooler and GPU cooler. 

 

Once air passes through the CPU cooler, it'll go straight to the rear fan.

 

The air from the bottom 200mm intake will mostly go to the area below the GPU.  Once it passes through the GPU, it will radiate upwards and be sucked out by the 200mm in top and the 140mm in the rear.  Note that most of this hot air from the GPU goes over and under the CPU cooler, not through it.  A small amount will enter the CPU cooler indeed, but that cooler has its own airflow going on thanks to its fan, so the air from the GPU that enters the CPU cooler will be blown towards the rear straight away.  
 

 

The big question is if the pattern will be better with a 140mm at the back. 

Personally I don't think so.  In terms of position it will be better, but the 200mm simply moves a lot more air at a low RPM than a 140mm would.  That more than makes up for the bit of air (the top 2 blue lines)that gets lost.

 

All in all, my advice would be to not overthink it.  Regardless of what setup you choose, you won't run any risk of overheating things due to bad airflow. 

You can always mess around with fan curves once you have the PC up and running.  Perhaps lowering or raising the RPM of the top fan compared to the rear one can drop the temperature by a couple of degrees. 

[Tristerin]

If you have what is known as Negative air pressure (more air exiting than being allowed in the case from the intakes) than your PC, unless sealed, will drag air in from every crack.  Your fans cannot create a void of air, your not a helicopter.

 

If you have what is known as Positive air pressure (more air intaking than being allowed to exit the case from exhausts) than your PC, unless sealed, will push air out of every crack.  Your fans cannot create a positive environment enough to cause your cause to blow its side off.

 

In either of these scenarios its nearly impossible (unless you have only a minor positive or negative situation) to have a hot spot.  So it doesn't matter.  The point of Negative and Positive is its ability to eliminate hot spots.

 

Now if you have Neutral Air Pressure - you can create hotspots and fan orientation, placement matter.

[ProtoflareX]

54 minutes ago, Captain Chaos said:

Okay, here's a quick and dirty sketch of what your airflow will probably look like with the 200mm.

 

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Air gets sucked in through the front 200mm fans.  Some of the air is sucked straight out by the 200mm in the top, the rest goes to the CPU cooler and GPU cooler. 

 

Once air passes through the CPU cooler, it'll go straight to the rear fan.

 

The air from the bottom 200mm intake will mostly go to the area below the GPU.  Once it passes through the GPU, it will radiate upwards and be sucked out by the 200mm in top and the 140mm in the rear.  Note that most of this hot air from the GPU goes over and under the CPU cooler, not through it.  A small amount will enter the CPU cooler indeed, but that cooler has its own airflow going on thanks to its fan, so the air from the GPU that enters the CPU cooler will be blown towards the rear straight away.  
 

 

The big question is if the pattern will be better with a 140mm at the back. 

Personally I don't think so.  In terms of position it will be better, but the 200mm simply moves a lot more air at a low RPM than a 140mm would.  That more than makes up for the bit of air (the top 2 blue lines)that gets lost.

 

All in all, my advice would be to not overthink it.  Regardless of what setup you choose, you won't run any risk of overheating things due to bad airflow. 

You can always mess around with fan curves once you have the PC up and running.  Perhaps lowering or raising the RPM of the top fan copared to the rear one can drop the temperature by a couple of degrees. 

That image is indescribably helpful, thank you. I thought the "stealing" effect of the top exhaust fan was much more severe than it actually is. Now, you just said not to overthink this, but I must admit, I have a slight obsession with making the airflow as efficient as possible, as that was idea that led to the creation of my planned build. If I understood what you said correctly, the only advantage of the 200mm would be its ability to move more air at a lower RPM, which translates to moving more air at a lower volume. However, the 140mm fan in the back would have the benefit of not stealing any air from the heatsink fan with the only con being noise. Is that correct?

[dfsdfgfkjsefoiqzemnd]

Just now, ProtoflareX said:

If I understood what you said correctly, the only advantage of the 200mm would be its ability to move more air at a lower RPM, which translates to moving more air at a lower volume. However, the 140mm fan in the back would have the benefit of not stealing any air from the heatsink fan with the only con being noise. Is that correct?

The 140, when mounted near the back of the top would "steal" less air indeed.  So you'll have cooler air going over the VRMs and potentially a higher air pressure in front of the CPU cooler which translates to slightly better cooling.

However it is possible that all that benefit is offset because the 140mm will move less air out of the case and because there will be more air from the GPU making its way to the front of the CPU cooler, meaning that the cooler will suck in hotter air. 

 

Unfortunately there's only one way to really find out what the optimal setup for your case is, and that is to try it with a 140mm as well as a 200mm.  But is it worth it to buy the extra hardware and spend all that time building and testing only to get what will most likely be a 1-3°C difference?

[ProtoflareX]

6 minutes ago, Captain Chaos said:

The 140, when mounted near the back of the top would "steal" less air indeed.  So you'll have cooler air going over the VRMs and potentially a higher air pressure in front of the CPU cooler which translates to slightly better cooling.

However it is possible that all that benefit is offset because the 140mm will move less air out of the case and because there will be more air from the GPU making its way to the front of the CPU cooler, meaning that the cooler will suck in hotter air. 

 

Unfortunately there's only one way to really find out what the optimal setup for your case is, and that is to try it with a 140mm as well as a 200mm.  But is it worth it to buy the extra hardware and spend all that time building and testing only to get what will most likely be a 1-3°C difference?

Interesting, there are just too many variables for the answer to be cut and dried, huh? Based on one of your earlier responses, if you were in my situation, I'm guess you would opt for the 200mm over the rear mounted 140mm? If that's the case, I'm just going to save myself the trouble of testing and do the same. By the way, I must reiterate how helpful that diagram is.

[dfsdfgfkjsefoiqzemnd]

20 minutes ago, ProtoflareX said:

Interesting, there are just too many variables for the answer to be cut and dried, huh?

Indeed. 

 

Personally I'd go for the 200mm, use the PC for a week or so and then see if lowering the RPM of the top fan helps.  Or maybe I'd even put some duct tape over the section that sticks out in front of the CPU cooler and see if that helps.  If it works, I'd make a block-off plate that I could screw onto the fan. 

 

Kinda like this (albeit in plastic or alu instead of paper like my mockup).

 

 

[_Hustler_One_]

I personally don't want to have any top exhaust fan for El Macho right above its fan, but maybe right above the heatsink as far to the rear as possible instead. The El Macho's fan max speed is only 1300 rpm, so if there is other fan right above the Macho's fan to fight for air it might lose, while it has to cool the crazy vast surface area all the way through to the rear end of the heatsink fin stack.

 

I would just use a single rear exhaust fan to help the main el macho fan to deliver the air all the way through to the rear-end of the heatsink easier.

Otherwise just simply test yourself for what you are considering and see the result with and without the top exhaust fan you are visualizing..

[ProtoflareX]

On 3/20/2019 at 5:10 PM, Captain Chaos said:

Indeed. 

 

Personally I'd go for the 200mm, use the PC for a week or so and then see if lowering the RPM of the top fan helps.  Or maybe I'd even put some duct tape over the section that sticks out in front of the CPU cooler and see if that helps.  If it works, I'd make a block-off plate that I could screw onto the fan. 

 

Kinda like this (albeit in plastic or alu instead of paper like my mockup).

  Reveal hidden contents

 

This is a little late, but I have an additional question for you, if you don't mind. The Le Grand Macho RT's heatsink is very large, and I suspect that after installation, it will be very close to the backplate of the GPU (essentially almost touching it). Will the heat being dissipated by the heatsink increase the temperature of the GPU?

[dfsdfgfkjsefoiqzemnd]

Nah, it will all be dissipated towards the back. 

My Dark Rock Pro 3 is about 4mm from my 1080Ti's backplate and even under full stress I'm not noticing any temperature difference when I move the GPU to the next x16 slot. 

[ProtoflareX]

46 minutes ago, Captain Chaos said:

Nah, it will all be dissipated towards the back. 

My Dark Rock Pro 3 is about 4mm from my 1080Ti's backplate and even under full stress I'm not noticing any temperature difference when I move the GPU to the next x16 slot. 

Glad to hear that, looks like I was concerned for no reason.