Does a computer case need exhaust fans?

[pete_101]

Hi, this may be a stupid question, but there goes. My computer case as lots of holes for fans (both 120mm and 140mm). If I only use inlet fans, surely the hot air has to be pushed out the other holes which have no fans, right?
The case only had one 120mm fan (an inlet fan on the lower front) and I have added two more 120mm inlet fans on the side panel as they are blowing onto the motherboard, cpu, gpu etc. The psu is at the lower back and sucks filtered air from underneath and exhausts it out the back, so doesn't act as an exhaust for the case.
The top panel has room for two fans and the top back can take another, the holes for these fans are quite large as I have removed the filters, so the way I see it the hot air will rise and go out them due to the positive pressure from the three inlet fans. Am I missing something here? Why do most people say you need as many exhaust fans as inlet? To me that's a waste of the fan's capacity. If they all act as inlets and the air can get out through the holes at the top, surely there will be a greater airflow, right?

 

[USAFRet]

Yes, you need exhaust fan(s) as well.
You want to move air through the case, not just blow it around inside.

In the front/bottom, out the back/top

 

[pete_101]

Yes, you need exhaust fan(s) as well.
You want to move air through the case, not just blow it around inside.

In the front/bottom, out the back/top

Yes that's what everyone says. But if there is a positive pressure of air coming in, doesn't that mean air will also be going out and not just blowing around inside?

 

[USAFRet]

Yes, you need exhaust fan(s) as well.
You want to move air through the case, not just blow it around inside.

In the front/bottom, out the back/top

Yes that's what everyone says. But if there is a positive pressure of air coming in, doesn't that mean air will also be going out and not just blowing around inside?

A little bit, yes. No case is really sealed. But the amount of air blowing out the little cracks and seams of a case is far, far less than an actual fan sucking the air out on purpose.

 

[Paperdoc]

I get what you're saying, OP, and will agree with partially. What goes in MUST come out, no way around that. I think it important, though, to make sure there are large enough exit pathways. In other words, if your case has many fan mounting spots with openings to the outside, do not fill more than half of them with intake fans. Leave the rest open to allow air to exit. Do not depend on cracks and slots in the case for major air exit.

Don't forget, a 4x4 (16 sq in) hole in the case will not have as much air flow through it unaided as it would with an exhaust fan mounted in it. Adding an exhaust fan will increase the air flow out at that point, and thereby increase the total air flow through the case. So, to maximize the air flow through the case for a fixed number and size of fans, your best strategy is to have en equal number of fans (more precisely, an equal air flow rate) as intake and exhaust units.

My own preference, though, is to seek such a balance almost - I like to have slightly more intake capacity than exhaust, so that air leaks at case cracks, etc. are blowing OUT, and hence not drawing in dust. Then I ensure that the intake fans all have dust filters. However, that means that the actual air flow through the intake fans is LESS than their specs because the filters slow down air flow. So you can't actually calculate the balance. You have to install fans and filters and then measure (in a simple manner) the air flow at the cracks to see which way it is moving.

 

[pete_101]

I get what you're saying, OP, and will agree with partially. What goes in MUST come out, no way around that. I think it important, though, to make sure there are large enough exit pathways. In other words, if your case has many fan mounting spots with openings to the outside, do not fill more than half of them with intake fans. Leave the rest open to allow air to exit. Do not depend on cracks and slots in the case for major air exit.

Don't forget, a 4x4 (16 sq in) hole in the case will not have as much air flow through it unaided as it would with an exhaust fan mounted in it. Adding an exhaust fan will increase the air flow out at that point, and thereby increase the total air flow through the case. So, to maximize the air flow through the case for a fixed number and size of fans, your best strategy is to have en equal number of fans (more precisely, an equal air flow rate) as intake and exhaust units.

My own preference, though, is to seek such a balance almost - I like to have slightly more intake capacity than exhaust, so that air leaks at case cracks, etc. are blowing OUT, and hence not drawing in dust. Then I ensure that the intake fans all have dust filters. However, that means that the actual air flow through the intake fans is LESS than their specs because the filters slow down air flow. So you can't actually calculate the balance. You have to install fans and filters and then measure (in a simple manner) the air flow at the cracks to see which way it is moving.

I choose this as best solution as you actually read what I wrote and gave a reasonable answer.
I was curious as to whether there is some unforeseen issue of airflow if there is no exhaust fan, such as hot air getting trapped and cool air being vented instead. It seems to me most people use inlet and exhaust fans as it seems to make sense and that's what everyone else does. I'm happy to only use inlet fans as the case has a large area of opening along the top and back for the air to vent out of.....if air goes in it has to come out again. Also I just read an old post where a user said his coldest case was inlet only.