Guide to fan noise and case thermodynamics

This thread is (I hope) going to help clear up some misinformation and (also hopefully) give the community a little more knowledge of one of the most overlooked components in a computer case.
A case fan is rapidly achieving the status of a “major component” when dealing with high-end systems especially when OC’ing.
Fans are used not only in the case itself, but added to passive heatsinks, and more than ever before we are seeing modders/OC’ers replace existing fans on HSFs, PSUs and GPUs to achieve either higher air flow, lower noise levels, or a mixture of both.

I started doing some serious research into fans when I realized that my newest case mod will have a total of 13 fans in it when it is complete. That’s right, 13 FANS !! (The case will be actually divided into sections, so fans are needed to direct the air flow from section to section. I will be posting some work logs in the near future in the Case modding/worklogs section of this forum. The case mod I am speaking of will be called “From Hell”)
Anyways, with that many fans my PC was going to sound like a Harrier Jump-Jet taking off in my room. So I decided to do some research. This thread is the fruits of my labor. I hope you can bring something away from this post.

Let’s start with a little technical info:

Fans are commonly listed with many numbers and letters giving you detailed specs on it.
Such as this: 120x25mm, 3pin, 12V DC, 1600RPM, 63.57CFM, 27dBA, 2 ball bearing

The 120x25mm is just the size of the fan. The 25mm is the thickness of the fan. This is of important consideration when working in small areas or in places where you don’t want the fan so big it takes away from the looks of the overall effect. This is also very important when replacing the stock PSU fan because some PSUs do not have room in it to mount a thicker fan. I ran into that problem with my old OCZ Modstream PSU when I switched out its stock fan for a quieter one that moved more air. Some fans are as big as 46mm thick. The two most common thicknesses are 25mm and 38mm, with 25mm being the most commonly found by far.
The 3pin is simply how it connects to power. If you are looking to plug into your MB fan headers, make sure you get the right fan or buy the 4pin-to-3pin adapter (if it doesn’t come with it).
I’m sure that the “12V DC, 1600RPM, 63.57CFM, 27dBA” stuff you are all familiar with, so I will not go over them except to say that CFM stands for Cubic Feet per Minute. And dBA stands for A-Weighted Decibels.

So in a nutshell, CFM is how much air the fan moves per minute and dBA is how loud the fan is at that given CFM. These are the two most important stats when comparing fans.

Lastly, there will normally be a stat that says one of the following:
Sleeve bearing, Ball bearing or S-DFB (Sony-Dynamic Fluid Bearing)
Instead of getting into the mechanics if each one, here is just a quick breakdown of the 3 types and the estimated lifespan of each type. (these are not my estimates. These estimates are the industry wide MTBF (Mean Time Between Failures) and all companies use them)
*Sleeve bearing – this is by far the most common type. Easy to make, cheap to produce. These fans are “normally” the loudest and have a MTBF (or estimated lifespan) of 30,000 hours.
*Ball bearing – becoming fairly common to find. Usually quieter and lasts 6x longer, but generally costs more than a sleeve bearing fan with equal specs. Has a MTBF of 180,000 hours
*S-DFB (or just DFB) – this is the newest technology in fans. Harder to find, costs the most. Doesn’t last quite as long as Ball bearing types, but capable of being the quietest of the 3 types. Has a MTBF of 150,000 hours.

<Continued below>

Well, now that that is over, let&#8217;s get to the &#8220;meat&#8221; of this thread.

When looking around and comparing fans, there is a very simple formula to come up with its CFM/dBA ratio. This will tell you which fan has the best &#8220;air flow-to-noise&#8221; ratio. Simply divide the dBA into the CFM. Thus a fan rated at 63.57CFM, 27dBA would have a ratio of 2.35 (or this fan moves 2.35CFM for every dBA). Obviously you want to have the highest possible ratio while still maintaining a comfortable noise level.

Now, this is all fine and good, for just ONE fan. But how many PC cases have you ever seen that had only one fan?? So how do you come up with the total sound (or loudness) of a full PC??
This gets a little tricky if you want to do it exactly. But don&#8217;t worry. This doesn&#8217;t need to be an exact science. We can do this in just generalized ways that will give us a good idea of our TOTAL sound.
*NOTE* For those of you who are more &#8220;scientifically minded&#8221;, you may want to check out http://www.engineeringtoolbox.com/adding-decibel-d_63.html link for a much more detailed, and exact way to figure out total sound levels.

For us that are not rocket scientists, the simple formula for adding multiple fans that have the SAME dBA RATING is simply shown by the following chart:
*NOTE* This chart is for adding multiple sound sources with the EXACT SAME dBA rating ONLY !!
This chart shows the effect of something called &#8220;harmonization&#8221;.

Number of fans - Added dBA
2 - 3
3 - 4.8
4 - 6
5 - 7
6 - 7.8
7 - 8.4
8 - 9
9 - 9.5
10 - 10

As an example; we have multiple fans that are ALL rated at 30dBA. If we put two of those in a case, the total noise level would be 33dBA. If we put 7 of them in a case, the total noise level would be 38.4 (30+8.4). Get it? Good.

Now some of you may be asking, &#8220;why did I even bother to add this part??&#8221; Well, having a case that all the fans are the same is very possible. Here is just one example of how:
Say you are running a water-cooled system. You may have 2 or 3 120mm fans on your res, and you have a 120mm fan in your PSU, and maybe a couple of 120mm case fans. The other parts are all cooled by your WC setup.

<Continued below>

OK, only in specific set-ups would you have ALL your case fans rated the same. To figure out your total sound of many fans that have different ratings all you need to do is use the chart below and start adding them up.
To use the chart, first find your highest rated (or loudest) fan. (If you do not know what your fan is rated at, you can usually find out by just a little searching on the net.) Next write down the dBA ratings of ALL the fans in your case. (Don’t forget to add in the PSU and GPU fans, as well as any NB or SB chipset fans you may have)
Now that you have all your ratings on paper, all you need to do is subtract each fan’s rating from your highest rated fan to come up with a difference. Then use the chart to find how much sound you add to your total based on that difference.
Do that for each fan, writing down the difference and added sound on your piece of paper until you are done.
Now that you have all your numbers, all you have to do is add all of them up, then add it to your highest rating.

Difference between dBA - amount to add to highest dBA
0 to .9 - 3
1 to 1.9 - 2.5
2 to 3.9 - 2
4 to 4.9 - 1.5
5 to 7.9 - 1
8 to 10 - 0.5
Anything over 10 is 0 because the loudest fan is too overpoweringly loud compared to the other

As an example; let’s say we have a Zalman 9700 (CNPS9700 LED), it has a rating of 35dBA. We also have OCZ GameXStream 700W PSU with a rating of 34.3dBA, an Antec Nine Hundred PC case and an 8800GTS with a Thermaltake TMG ND5 aftermarket cooler that is rated at 16dBA.
The Antec case comes stock with three 120x25 fans rated at 79CFM @ 36dBA (high) and a single 200x30mm fan rated at 134CFM @ 32dBA (high).
The loudest part are the three 120mm case fans @ 36dBA each (damn that is loud, but it IS stock).
*NOTE* since there are 3 of them all rated the same, remember that you will be using the 1st chart to add up to a single number for all fans rated the same. This holds true even if you have multiple sets of fans that are rated the same. (ie: 4fans @ 36dBA, 2 fans @ 32dBA, and 2 fans @ 24dBA)
Anyways, 3 fans @ 36dBA gives us a total of 40.8dBA (now you can see why 3 fans only rated at 36dBA each can be so damn loud in your case)

The 200mm case fan @ 32dBA gives us a difference of 8.8dBA giving a +0.5 modifier.
The PSU @ 34.3dBA leaves us with a 6.5dBA difference which, by looking at the chart above, gives us a +1dBA modifier.
The CPU HSF @ 35dBA gives us a modifier of +1dBA
The GPU is so silent that the CPU HSF is greater than a 10dBA difference, so there is no modifier for that.
So our case total would be 43.3dBA. (40.8dBA for our “loudest part” + 0.5 for the 200mm fan + 1 for PSU + 1 for the CPU HSF)
This is actually WAYYY too loud in my opinion. So what can we do about this? Let’s find out.

First let’s handle those loud a$$ case fans. Lets replace those 79CFM/36dBA (2.19 ratio) fans with some Scythe “Minebea NMB Silent IC Series” (4710KL-04W-B29-V52) fans rated at 76CFM/31dBA (2.45 ratio) (nice move there). We will also replace the 200mm fan with a new one. This becomes a problem because of the 200mm size. You can either replace it with a smaller 120mm fan or a bigger 220mm fan by just drilling a few holes. For the purpose of this example, we will replace it with a monster 220x30mm fan by Yate Loon. The D22SL-12H is rated at an incredible 135CFM/25.5dBA (an awe-inspiring 5.29 ratio)

Now let’s replace the PSU fan with a Scythe “Slip Stream” SY1225SL12M fan rated at 68.54CFM/24dBA (2.86 ratio)

So our loudest part is still the case fans (remember “harmonization”) @ 35.8dBA. Subtracting the other parts one at a time, we come up with:
PSU @ 24dBA gives a modifier of +0
CPU HSF @ 35dBA gives us a modifier of +3
220mm case fan @ 25.5dBA gives us a +0 modifier
The GPU’s aftermarket TMG ND5 is still more than 10dBA so no modifier added for it.

So our new total is 38.8dBA. (35.8dBA for the loudest part + 3 for the combined modifiers)
Holy crap…we just shaved off almost 5dBA simply by switching some fans out. And 5dBA is a very noticeable difference in sound volume.
We could cut it down even more by replacing the CPU HSF with a different HS that we could buy our own fan for. By doing that and using another Scythe “Minebea NMB Silent IC Series” (4710KL-04W-B29-V52) fan rated at 76CFM/31dBA (2.45 ratio) as the fan for the new HS we would end up with a system that has a total sound level of only 37dBA (I’ll let you do the figuring on that one as a test ). That gives us a total of 6.3dBA difference between before and after. Not bad at all.

*NOTE* I am very aware that there are many different variables in determining a “total case” noise level. Things such as HDD noise, the differences in exhaust and intake sides of fans, the total area of the case, acoustical resonances of different case materials, ect., ect..
This is not meant to be an all inclusive guide. This is just simply a guide to try and help get the best possible air flow with the least possible noise.

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Now that you know how to pick good fans (using the CFM/dBA ratio), let’s look at what to do, and not do, with them.

Understanding case thermodynamics has become almost a necessity in the modern modding community. With PC components running hotter, more modders putting stuff in their PC cases (ie: res, rad, stuffed animals, and all manner of other stuff), and of course the noobie (who, after all, is who this is written for), understanding a few simple little “rules” will mean the difference between overheating and the best possible case/component temps.

*NOTE* If you are an experienced PC person, you can probably just stop reading right here. The following is very basic stuff on how to achieve the best PC case cooling.

First, and probably the most important, thing to understand is air flow. By keeping as much stuff out of the way of your case’s air flow as possible is a great first step. Try to keep all your cables, wires, ect out of the way (hidden) as possible. Use wire ties, zip ties, ect to move and hold everything along the edges and/or behind other parts. If you are still using the old “ribbon” type flat cables for your IDE components, you may want to change them out for newer “rounded” IDE cables. Utilize your case’s “dead space”. (ie: try running MB header wires and your case’s front USB/audio wires under the MB. Drill holes to run zip ties through to hold stuff along the sides, top and bottom of the case.) This not only helps promote good air flow by having as little air restriction as possible, but it just plain looks better.

The next thing to understand is that a case will be cooler when it has a negative air pressure.
Now I am not going to get into the physics behind this. Suffice it to say that you want more air being pulled OUT of the case than you have being pushed INTO the case. In other words, add up the total CFM of all your intake fans, then add up the total CFM of your exhaust fans (don’t forget to add in your PSU fans into your exhaust fan total). You want to have AT LEAST 25% more air going OUT than you have going in. If you want more info on the physics behind this, just do a little searching on the net.

It is common knowledge that heat rises, correct? We all know this. But if that is the case, why do so many people who mod their case’s neglect to put an exhaust fan in the top of their case? If you are going to mod your case, think about adding a top exhaust fan. You will be glad that you did. I know that for some case mods, due to the theme, or other reasons, this is not practical. That’s cool. But if you can, do it.
Also, speaking of “heat rises”, proper air flow direction is very important as well. I have seen more than a few cases where the owner put the exhaust fan in the front of the case and had the intake fans in the rear. (No, I am not kidding, I have seen this many times with customer PCs)The idea behind doing this is simply that the CPU HSF is much closer to the rear fans, so blowing air right on it should help keep it cooler (Hey, you have got to give them credit for trying). Doing this takes the cool air and brings it in high in your case and tries to blow it out low in the case. The problem with that should be obvious once you just simply remember that “heat RISES”. Not to mention that in doing it this way the PSU is immediately pulling a lot of this cool air right back out again.
Another thing to keep in mind when designing your case’s air flow is that we are trying for a smooth flow of air through the case. Not a turbulent one.
I say that to say this; I have seen many people add fans in their case that blow right on the GPU(s) or chipsets, or whatever. This disrupts the smooth flow of air from the bottom of your case to the top. You may get a couple of degrees lower temps on that single component, but the trade-off is that by disrupting the flow and creating a turbulence you now actually GAIN those few degrees back in your overall case temp.
To effectively do this, those cases, or mods, that have a side panel fan are great. But only if it is set up as an exhaust fan, NOT an intake fan, and ONLY if the fan is positioned high enough on the side panel to exhaust hot air from the CPU. Those cases with a low mounted side panel fan should be avoided if possible as using that as an intake will disrupt your air flow, and using it as an exhaust fan will be almost useless because all you really will be doing is exhausting some of the cool air coming in from the front of your case. If you have to use a case that has a low mounted side panel fan, just don’t put a fan there. Leaving it blank is a better option than using it as either an intake or exhaust fan.

Another overlooked thing is the direction of air flow of the CPU’s HSF. It is more efficient (cools better) to pull hot air AWAY from the CPU as opposed to blowing air AT it. If you buy an aftermarket HSF, be sure that you mount the fan blowing AWAY from the CPU. With some coolers this is not possible. But if you can, make sure of the direction of air flow.
Also, while on the subject of HSFs, some coolers will allow you to mount them where the fan can blow straight at the back of the case. This obviously would be the best solution considering that the case’s rear exhaust fan(s) can immediately take the hot air and expel it from the case. So when adding a new HSF, first see if it can mount in an “East-West” orientation, as opposed to the traditional “North-South” orientation that you may automatically want to use because it has become habit.

One last thing, your optical drive; you know that heat rises, and you also know that your optical drive produces heat. With those two things in mind, try not to mount your optical drive in the top most spot in your case. Instead use the second spot, or even lower. This will allow two things to happen. One, it opens up a “dead air” space that the heat of your case can go to until it can be exhausted from the case. Secondly it helps your optical stay a little cooler running because it is no longer in the “hot air zone”, while also allowing the heat that it produces to rise up and away from it.
Anyone who questions the amount of heat an optical drive can produce need do nothing more than play a music CD for a few songs or put in a movie DVD and watch it for about 15 minutes, then immediately eject the CD or DVD and feel how hot it is.

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I know that a lot of this may seem very small and/or petty. But how many of us have done crazy amounts of work just to get that extra 10-20 points on our benches, or drop and extra degree or two in heat. How many of you have voided the warranty on your high dollar CPU by lapping it? Voided the warranty on your high priced GPU by putting on an aftermarket cooler? Voided the warranty on your PSU by changing out a fan? Cut up you HDD to add a window to it just for looks?
See what I mean? So while some of this may seem small or petty to you, some of us will do anything and everything to gain just that little extra performance, a little lower temps, a little less noise.
So all I ask is that if you didn’t find this post helpful, please do not post negative stuff. Someone else may actually learn something (I hope).

One final thought for those of you who want to get the best case temp/noise ratio. If you will simply pull off BOTH side panels of your case, you can then disconnect ALL of your case fans. The only fans you need running in that case is the CPU, PSU and GPU (if it has one). This will give you ambient room temps inside your case, which is the absolute best temp you can get on air cooling alone.
Now this obviously isn’t something that everyone can, or will do. Some people have put A LOT of time, effort and money into custom modding and/or paint to NOT show it off. That piece of advice is for those who do not care that their PC is sitting there wide open.

Anyways, that’s it. I hope that at least someone can get something out of this. If just ONE person can bring something good away from this post, then it has all been worthwhile.

Great post. I was just looking at maybe switching out some radiator fans (Yate Loon lows) for sleeve Scythe KAZE fans (1200rpm, 68.54 CFM, 24 dBA, 2.85 ratio) or then moving the Yates to case fan status to get rid of some bad case fans.

Do you know anything about the effect of rubber grommets during mounting?

A few points to remember:

- There is a fourth fan bearing technology - maglev. Panaflo fans, among others, use this. Being a non-contact method, it can make for a quiet fan.

- I have found that some fans have blades that are not moulded cleanly. When manufactured, there is some flashing left on the blades where mould halves come together. Carefully cutting off and smoothing these bits on the leading and trailing edges of fan blades can have a small effect on reducing the fan noise.

- Not all fan ratings are consistent. Manufacturers are not obliged to use a specific standard for measuring the airflow, noise, etc. Check reviews on the fans you're interested in to see whether the ratings that look good correlate with comments in reviews. Don't be surprised if you see completely different comments on the same fan.

- Negative vs positive air pressure - this is an overrated issue. I recently changed my fan arrangements to go from slight negative to slight positive pressure. Most would claim that the temperature would increase; in fact, the temp decreased by 2C. One advantage of positive air pressure is reduced accumulation of dust inside the case. Dust is an insulator and will reduce the effectiveness of heatsinks, etc.

- Heat doesn't rise; heat goes from where it is hot to where it is cold. In a static environment, hot air will tend to rise. In a case with air pushed around by fans, thermal convection will be insignificant compared to the overall air flow. The reason for making sure you have a fan on the top of the case is that there is otherwise a dead air space between the PSU and the device slots in the front of the case.

astyler said:

Do you know anything about the effect of rubber grommets during mounting?

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I have not seen any definite testing on this to say for 100% certainty, but the concept would seem valid only in specific cases. Such as if you have a small, high RPM fan, it would vibrate, thus leading to the belief that grommets would quieten it down. But on bigger, low RPM fans, they do not vibrate much (if any), thus the grommets would do no good.

Again, to my knowledge there have been no real tests on this, so this is just all a guess using a little logic.

Alright, thanks. You may want to mention static air pressure of fans, because some fans (like the Scythe Slip stream series) have good ratios but terrible pressure, so are not suited to be mounted on a radiator or heatsink (as I've found out, looking for a good alternative atm.)

Michael Daly said:

A few points to remember:

- There is a fourth fan bearing technology - maglev. Panaflo fans, among others, use this. Being a non-contact method, it can make for a quiet fan.

Click to expand...

Actually there is a fifth one as well. Sleeved shaft (or something like that). I didn't mention these because as of now they are still not very common to find. Although since its introduction, 4-bearing is becoming more readily available.

Michael Daly said:

- I have found that some fans have blades that are not moulded cleanly. When manufactured, there is some flashing left on the blades where mould halves come together. Carefully cutting off and smoothing these bits on the leading and trailing edges of fan blades can have a small effect on reducing the fan noise.

Click to expand...

Very true. Thank you for bringing up this point. I can not believe I forgot to add that into my article. Great catch man.

Michael Daly said:

- Not all fan ratings are consistent. Manufacturers are not obliged to use a specific standard for measuring the airflow, noise, etc. Check reviews on the fans you're interested in to see whether the ratings that look good correlate with comments in reviews. Don't be surprised if you see completely different comments on the same fan.

Click to expand...

Amen to that. I have already been jumped on by a poster on another forum where I posted this. He actually has the Antec 900 that I used in my example, and was mad that I said the dBA of his fans were higher than they are rated on the Antec website. I then directed him to two different websites that tested those fans and showed that the manufacturer given specs were not accurate.

Michael Daly said:

- Negative vs positive air pressure - this is an overrated issue. I recently changed my fan arrangements to go from slight negative to slight positive pressure. Most would claim that the temperature would increase; in fact, the temp decreased by 2C. One advantage of positive air pressure is reduced accumulation of dust inside the case. Dust is an insulator and will reduce the effectiveness of heatsinks, etc.

Click to expand...

Please keep in mind that not all cases are going to respond equally. Variables such as the case design as well as type, size and positioning of components, the type, number and layout of case fans, and MANY other things can (and will) give different levels of performance gains or losses.
I was only speaking in a general sense. If you switched from negative to positive case pressure on 1,000 different computers, the majority (by far) would notice a temp increase. You just cannot fight the laws of physics. Simply put, “Energy as heat is directly proportional to the pressure exerted in any given unit of measure. As pressure increases, heat increases. As pressure decreases, heat decreases.”
But there will still be those that drop temps due to variables in their case that throw it off the "norm".
Although this phenomenon is rare.

Positive case pressure can SUPPOSEDLY help reduce dust in a case (I have yet to see a test on this that fully convinces me though). So it is up to you to decide what is more important to you, a little better temps, or a little less dust. Since I clean my PC once a month (at least) anyways, better cooling is what I shoot for. But this is your choice. It is just like fans, you have a choice, do you want quiet or super high CFM? I prefer the best balance I can get, but I have met people that don't care how loud it is, they just want the highest CFM they can get.
I know that people say they have less dust build-up by using positive case pressure, but by how much? Does positive case pressure eliminate ALL dust build-up? What about people who live with a cleaning fanatic for a Wife and no "furry" pets?
See what I am getting at? While yes, you do have a valid point; it is not going to be the case for ALL people. Again with those damn variables. All I am doing here is covering info that will be valid for the MAJORITY of readers. It would be impossible to cover all the different variables.

Michael Daly said:

- Heat doesn't rise; heat goes from where it is hot to where it is cold. In a static environment, hot air will tend to rise. In a case with air pushed around by fans, thermal convection will be insignificant compared to the overall air flow. The reason for making sure you have a fan on the top of the case is that there is otherwise a dead air space between the PSU and the device slots in the front of the case.

Click to expand...

I am going to try to answer this without getting to “scientific” sounding.
First, Yes, heat does rise. At least in the common usage of the word “rise.” Heat rises because it is less dense than cooler air. Instead of getting into the physical laws (ie: Boyle’s Law, Charles Law, Ideal Gas Law, ect..) behind this. Let’s look at it in a very simplified way:
“Many of us experience the effects of falling cold air and rising warm air on a regular basis. It is happening all the time in the air above and around us and is one of the components in our weather systems. You may notice that when there is no heating or air conditioning operating in your house, the rooms upstairs are slightly warmer than those downstairs. Another example is the hot air balloon that works precisely to this principle. By heating the air inside the balloon, the craft will be lighter than the surrounding air and will rise. This is actually the result of cold air gushing downwards around the balloon at the same time as it is rising.
So why does cold air fall? That is simple: it is heavier than warm air. And why is it heavier? That is slightly less simple, but only slightly. As with any gas, the air (a generic term for the mixture of the gasses in our atmosphere), contains molecules that move (or agitate). This movement (or agitation) is greater as the temperature rises. The molecules move in ever greater orbits, taking up more space. This causes the mass of the air to expand. Although the total mass of a lump of air has not changed, the mass is more spread out and so any given cubic area of it will be lighter.
“
That is a direct quote from here:
http://ezinearticles.com/?Why-Does-Cold-Air-Fall-and-Warm-Air-Rise?&id=302338

Next is your comment, “heat goes from where it is hot to where it is cold.” That is rather an odd statement when looking at heat convection, isn’t it? Basically you would be agreeing with me if you are talking about convection theory.
Once again, you can disagree with me all you wish. But you can not argue with the Laws of physics.
Now you may be referring to conduction theory. This is the physical law that is shown by a heatsink. Heat travels away from the source through touch. But since we are talking about case air flow and fans, I doubt this is what you meant it to mean.
That being the case, and there only being 3 heat transfer mediums that we would have to deal with in a PC case..(Convection, Conduction, Radiation) I can only assume that you mistyped what you were trying to say.

In any case, the point remains that heat DOES rise and cold does fall.

As for the comment, “In a case with air pushed around by fans, thermal convection will be insignificant compared to the overall air flow“, let’s try a little experiment, shall we?
Reach over and put your hand on the bottom of your case. Now put your hand on the top of your case.
Unless you have really good air flow OUT of your case, which by your own admission you run a POSITIVE pressure set-up, you should feel a temperature difference. I say “should” because you may have a really good PC case or many other factors that could cause you to not “feel” a difference.
Let’s take it one step further. If you have a digital laser thermometer that most OC’ers nowdays have, just point it at the bottom center of your case side, the check the top center of the same side. See the difference? I just did it to mine and my wife’s PCs. Her PC had a 2c difference, mine had a 1c difference.
If you do not have a laser thermometer, you can use regular glass thermometers (those flat, stick-on types used in aquariums work great also). Simply take one thermometer and lay it in the bottom of your case, as close to the center as you can get it. Then take a reading from it after 10 minutes running your PC under load. Now take that thermometer and tape it to the inside, top of your case (again, try to keep it as close to center as possible). Once again take a reading after 10 minutes of running your PC under load.

These simple tests will show you that even though there is an air flow that “stirs up” the air inside your case, the heat, or energy (which is actually what heat is) will always be transferred to its surroundings, and will ALWAYS “RISE”. The amount of air flow you would have to have inside your case to completely negate this effect would be much, MUCH more than anyone would ever truly need, or want to deal with the noise for that matter.

Please do not take this reply to be confrontational. That is not the way I am meaning it to come across. I am simply trying to give you some information that (hopefully) will help you see the flaw in your thinking. Thus, allowing you to gain additional knowledge that will allow you to work towards getting that optimal case cooling lay-out. Which, after all, is what this whole thread is about.

astyler said:

Alright, thanks. You may want to mention static air pressure of fans, because some fans (like the Scythe Slip stream series) have good ratios but terrible pressure, so are not suited to be mounted on a radiator or heatsink (as I've found out, looking for a good alternative atm.)

Click to expand...

Another very good point. My original draft of this article did actually include some info on this topic. But I realized I was getting too deep into stuff for this to be targeted at those either new to case thermodynamics or noobies in general.
I was just trying to keep this as basic and "generalized" as possible without overloading the reader with every possible detail.

I have actually had someone else say the same thing to me. So I may do yet ANOTHER revision that will get into this a little.

Again, thanks for the advice.

cloudkat1970 said:

Heat rises because it is less dense

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Heat is energy, not matter. It does not rise or fall as it is not affected by gravity. Energy will move from a location where energy is high to where energy is low - from a source to a sink. If you put a hot object on top of a cool object, the heat will flow down until the lower object and the upper object come to thermal equilibrium.

Hot air will rise above cooler air, as I stated, in a relatively static environment. In a case with fans, the issue is moot - the moving air can wipe out any tendency for the hot air to rise. If you have an intake fan on top and an exhaust fan on the bottom, the hot air will flow out the bottom.

On the issue of Case Pressure:

In cases where the intake fan has a filter having +ve pressure does keep out dust. If you have -ve pressure due to more exhaust then what the front fan can provide, some 'unfiltered air' will leak in from gaps in the front panel bringing in dust.

Another issue with -ve pressure is the placement of the case. If the back of the case is placed at a location (corner of a room under a table next to a cabinate) where the hot exhaust cannot readily dissipate and have cases where the back is full with holes. The that environment the hot air may get recycled back into the case.

cloudkat1970 said:

As pressure increases, heat increases. As pressure decreases, heat decreases.”

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In a closed environment, pressure changes cause neither an increase nor a decrease in heat. They cause an increase or decrease in temperature, but the amount of heat is constant.

In a good case and fan configuration, the amount of air flow will more than make up for any small increase in pressure. If it doesn't, you've goofed. You don't need a high pressure differential to keep dust out.

Please do not take this reply to be confrontational.

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I will reply to make the information correct. If you're going to title the thread as containing "thermodynamic" information, it should at least use correct terminology and information.

cloudkat1970 said:

You just cannot fight the laws of physics. Simply put, “Energy as heat is directly proportional to the pressure exerted in any given unit of measure. As pressure increases, heat increases. As pressure decreases, heat decreases.”

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?? this is why you recommend negative pressure? I don't understand how this works as I've read other arguments that say having positive air, and thus denser air in your case allows heat to transfer from your computer components to air quicker. What your saying is in a vacuum there is no heat?