Top Mounted Case Fans as Exhaust or Intake

I've built a lot of computers. But a lot that was either replacing parts in OEM computers or starting with a case which already had fans that I didn't need to touch.

And then recently, a PC I built from scratch with one fan which I was using as a server. It was growing increasingly louder over time. So, suddenly I find myself needing to learn a bit about proper installation of case fans so that I could increase the air flow in this case. 

In the process of adding fans, I also ended up with an extra one. Basically, I had bought a fan controller and fans which plugged into that plugged into it with a non-standard connector. This left the original fan without a home. Until I realized I had extra space on the top of my desktop which had just received an upgraded CPU in the whole exchange as well and could always benefit from more cooling.

I installed it with the fan venting outward. This makes a lot of sense. Hot air rises, why fight it? But, I decided that since it is easily accessible anyway, I might as well do a bit of research. And then I came across a unique spin on it; for a top mounted liquid cooler which way should it vent?

Firstly, either way is unlikely to make a huge difference in respect to what most people are looking for. Intake on a top facing fan likely won't change the temps in the case. Some argue that if you intake from the top that the hot air will push back and negate some of the effects. But, in truth, there isn't much of a struggle in terms of temperatures unless the case is totally sealed and there is a build of pressure. Otherwise, you're mostly achieving your goal of increasing airflow.

It IS less efficient. But, this is most in terms of the life of the fan. If the fan is blowing in the direction that the air is naturally flowing it will require less energy to maintain the RPMs it is trying to reach. And, this is a stretch. I doubt there is any measurable difference in fan life. But, if there were to be a loss somewhere, this is where I would expect it.

Back to the liquid cooler. Presumably, only a gamer would bother having a liquid cooler to have this problem in the first place which does add another major heat source to the equation; a GPU. But, the simplest answer is; no change.

And the reason for that is; with a liquid cooler you're moving the heat you want to cool from the processor to the radiator. This is already a huge improvement because these radiators are generally a larger surface area. If you didn't have the processor attached to a liquid cooler, then that heat would be more in the center of the computer anyway. That CPU isn't generating any more or less heat than before simply because you liquid cooled it (unless you also overclocked it more). And now, much of that heat is being sent to the radiator.

Now, here is the first clue. If you hadn't used that liquid cooling system, that heat would be generated somewhere nearer the center of your build. And the obvious answer then is to move it away from there and out of the case. Why would you change tactics, simply because you switched to a liquid cooling solution? The amount of heat isn't inherently changed. And you've manage to move the fans and the heat dissipation closer to where you wanted it in the first place. Why stop there?

Blowing heated air back into the case likely won't make much of a difference, but, if it does, that difference is unlikely to be positive. 

In an ideal situation, airflow is fine and convective currents, fans and other natural processes will mean you likely won't notice a difference regardless of configuration. 

But, if airflow is insufficient then you are creating a positive feedback loop for the heat. That heated air will recirculate back to the CPU and GPU and will (because hot air rises) recirculate back to the radiator as well. You will hit a stable temperature, but it will be higher than it would have been if you had just vented out. This is because even though the airflow may be inadequate, when you blow out the hot air, convective currents will pull in cooler air to replace it and those same convective currents should work less effectively when you're sucking hot air in.

The other factor is that presumably this is a gaming rig with a dedicated GPU. And the last thing you want is to push hot air at the GPU or do anything to directly increase the ambient air around the it. GPUs are bit more thermally sensitive than CPUs.

The GPU will be sucking air up from the base of the case and pushing it out the back.

So, given the natural flow upward of hot air and the backward flow of air from the GPU, you will generally want to either assist the flow of air in one of these directions by blowing our out one of those ways. Alternatively, you want to suck in from one of the opposing directions, so suck in from the bottom or the front to encourage cooler air to come in and naturally back out. If you don't have additional fans, then you want to seal off the back and top as much as possible aside from any active ventilation and then open up the other sides of the case.

What people often fail to acknowledge is that a fan does not blow OR suck. It always does both. Your fan blowing through the radiator is SUCKING air from behind it to do so. So, you always want to prioritize forcing hot air OUT. And then you want that air replaced by the coolest air possible. This is why I said that you want to seal the back and top aside from active ventilation. Those fans blowing hot air out will suck air back in from the easiest place possible. You want that to be somewhere as far as possible from the (presumably hotter) air you just blew out. 

Another thing to consider (and again, this is probably negligible) is that the air at the top of the case is warmer than air at the bottom. By sucking in from the top you are getting the air from the warmest possible place outside of the case and pushing it out into the coolest areas and warming that up directly. By blowing out the top, you will draw more air into the case from the bottom and sides which will naturally be cooler air.

The conclusion here is... while it is interesting to theorize, most likely, you won't notice a difference. But, it is probably best to vent out the top. 

What is perhaps more interesting is what to do with a FRONT mounted CPU liquid cooler. And on that one, I don't really know what makes the most sense. In that case you likely have a case fan on the back venting out and the GPU venting out as well. Sucking in the air would go with that flow, but would feed in warmer air. But I don't know if it would be better to blow out the front as well. But, that case is probably even more dependent on how much circulation there is elsewhere in the case.