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psu at the bottom of a case?

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Last response: in Components
June 24, 2008 3:30:33 PM

Hi, I’ve seen some cases with a design that holds the psu at the bottom. What’s the idea behind this design? As far as I know the hot air goes up not down.

More about : psu bottom case

a b ) Power supply
June 24, 2008 3:41:59 PM

You can mount the PSU upside down. Or you can buy a PSU that blows the hot air out the back, like the Silencer 750W.
June 24, 2008 3:43:13 PM

Yeah. Exactly, cases like Antec Nine Hundred have a big fan at the top to take all the rising hot air away. It also keeps all the hot air away from the PSU, also I think it just makes it easier to build.
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a b ) Power supply
June 24, 2008 4:22:47 PM

hot air goes up slowly, so a slight draft generally overcomes it, separating the heat sources will help to reduce the overall heat levels, and therefore reduce noise, and stress on components.
June 24, 2008 4:36:40 PM

Thanks guys, now it makes perfect sense to me
June 24, 2008 4:59:46 PM

Just one final note - many cases that are designed for a PSU at the bottom have a grill under it. This goes with a trend in PSUs to have a 120mm fan (or just an opening) at the bottom with the air blown out the back. This means that cool air gets sucked into the PSU from under the case and blown out the back. Maximises PSU cooling while also minimising the PSUs effect on the rest of the system
June 24, 2008 5:09:37 PM

ummm another final note......if that's ok Kraynor? :pt1cable: 

If you are building a system with a PSU mount at the bottom of the case.....make sure your cables from your PSU are long enough to reach all of your parts, or you'll need extentions.
June 24, 2008 5:29:40 PM

Kraynor said:
Just one final note - many cases that are designed for a PSU at the bottom have a grill under it. This goes with a trend in PSUs to have a 120mm fan (or just an opening) at the bottom with the air blown out the back. This means that cool air gets sucked into the PSU from under the case and blown out the back. Maximises PSU cooling while also minimising the PSUs effect on the rest of the system



this can also increase the amount of dust that comes into the system.
something to be carefull for as well
June 24, 2008 5:36:20 PM

Cosmos has this.
June 24, 2008 5:57:25 PM

Is there any reason why mounting the PSU upside down on the bottom of the case would be a bad idea? The Lian-Li A70 I'm currently using allows mounting the PSU either at the top or at the bottom, I opted for the bottom, beneath my 8800GTX. I figured the PSU would help exhaust some of the GTX's heat since its fan is facing the GTX's case-side exhaust. There's 2 x 12CM fans at the top of the case.
August 15, 2010 11:42:02 PM

From all of the threads that I have read concerning the placement of the PSU, and some high school level science, I feel that I have come to a decent understanding of why someone would put the PSU at the bottom. I also want to comment about cases that come with a PSU versus not.

Working on the base concept that heat rises, one would first hesitate at the thought of having the PSU at the bottom of the case. Logically, why would you want warm air coming off of the PSU and rising across your components? As I understand it, the PSU sucks air from inside the case and blows it out of the case. This would cause the heat of the PSU to be blown out of the PSU, sucking air from the bottom of the case.

My next point is if you are concerned about the placement of your PSU, you probably are building a decent machine. Otherwise, you would have gone to DE11 and just got a desktop computer for surfing the internet.

Following that thought, of why you are even concerned about the placement of the PSU, leads me to believe that you are going to be building a more powerful machine. This being said, you will probably be buying a more powerful PSU. The point of the PSU is not to cool your system, but to rather power it. That is what the other case fans are for. Being that you are using a more powerful PSU, and it isn't it's job to cool the case, you would want the PSU to keep itself cool and not contribute to the problem. If you had the PSU at the top of the case, knowing that heat rises, the PSU would not only have to take care of itself, but all of the heat generated by the other components, which are requiring more power, thus more heat...

The next step in this logic. You are building a more powerful machine, which probably means that you are putting in larger PCIe x16 cards (one, two or even three). The sheer size of these, and the fact that they have their own fans for the GPUs tell me that these are a major factor into the heat of your system. Also, these boards are towards the bottom of your motherboard. I can logically see hot air getting trapped under the video cards, seeing as air is being blown in from the bottom front, and sucked out at the top rear. Having the PSU at the bottom would logically help pull air across the video cards, giving the hot air somewhere to escape. Now, this does place some of the heat from the video cards onto the PSU as a burden, but at least it is not all of the heat of all of the components.

Coming close to a close, those who claim that the PSU at the bottom changes how much dust gets into your computer may not have thought this through fully. Maybe the dust collects more at the bottom, rather then being spread across all of the components. Maybe this is a more efficient way of moving air in your case, causing your system to pull more air through. I really have a hard time believing that the position of your PSU in your case has anything to do with how much dust is in the air. I mean, the thought that having the PSU at the bottom of the case causes more dust follows the same thought process that maggots came from meat (High school biology lesson). Besides, you should be cleaning your case monthly seeing that static electricity is the #1 killer of PC components and dust is the #1 cause of static electricity (humidity also).

The last point concerning the PSU and heating, I have seen it argued that having the PSU at the top near the other drives keeps your heat creating components all together, and that being a problem. I don't see this as being an issue, because if you care where your PSU is, you probably have good video cards, and having it at the bottom puts it near those.

The last points concerning the PSU position that I have found substantial is that it makes the case less top heavy (which I could see as a problem in a full tower case) and it makes managing the cables much easier. (A big plus, considering having your cables managed better would help improve airflow through the case... instead of a rat's nest slowing things down).

The one significant complaint I have found about having the PSU at the bottom is the distance between the power supply and the power connector on the board. If you buy a case that requires the PSU be at the bottom, I suggest getting an extension cord for the power. I found a good one on a popular online computer parts store for $6.

Lastly, those who say that cases that don't have a PSU upon purchase are $**t haven't thought this through. Most PSUs that come with cases are generic ones that probably don't have enough power. If you care enough to worry about where the PSU should go, then you probably shouldn't be using the PSU that comes with a case. Now consider two cases that are the same price. One has a PSU the other does not. I would think that the one that does not would be a higher quality case for two reasons: The people that made the case understand that anyone serious about building their own machine will probably buy another PSU; and there was more money put into the construction of the case, rather than filling it with a generic PSU.

All this said, I found a great case. The Antec twelve hundred. It has a place for the PSU at the bottom, and at the top, where the PSU would have been, is a large fan, facing up, not back. This makes a gaping hole in the top of the case pulling the air up and out of the top, where, knowing that heat rises, the heat would naturally float to.

Hope This Helps,
NPE (Null Pointer Expert)
a b ) Power supply
August 16, 2010 12:00:02 AM

npe, are you ok in the mind...? :) 
thats one damn huge post but its (porbably) damn wort hreading... :) 

but i disagree with the psu thing. some actually quite good cases some with shitty psus. but they arent much more expensive than the same (or similar) case without a psu. why? because the psus suck and only cost a few $ to the case company.
August 16, 2010 12:47:41 AM

npe633 said:
From all of the threads that I have read concerning the placement of the PSU, and some high school level science, I feel that I have come to a decent understanding of why someone would put the PSU at the bottom. I also want to comment about cases that come with a PSU versus not.

Working on the base concept that heat rises, one would first hesitate at the thought of having the PSU at the bottom of the case. Logically, why would you want warm air coming off of the PSU and rising across your components? As I understand it, the PSU sucks air from inside the case and blows it out of the case. This would cause the heat of the PSU to be blown out of the PSU, sucking air from
the bottom of the case.

My next point is if you are concerned about the placement of your PSU, you probably are building a decent machine. Otherwise, you would have gone to DE11 and just got a desktop computer for surfing the internet.

Following that thought, of why you are even concerned about the placement of the PSU, leads me to believe that you are going to be building a more powerful machine. This being said, you will probably be buying a more powerful PSU. The point of the PSU is not to cool your system, but to rather power it. That is what the other case fans are for. Being that you are using a more powerful PSU, and it isn't it's job to cool the case, you would want the PSU to keep itself cool and not contribute to the problem. If you had the PSU at the top of the case, knowing that heat rises, the PSU would not only have to take care of itself, but all of the heat generated by the other components, which are requiring more power, thus more heat...

The next step in this logic. You are building a more powerful machine, which probably means that you are putting in larger PCIe x16 cards (one, two or even three). The sheer size of these, and the fact that they have their own fans for the GPUs tell me that these are a major factor into the heat of your system. Also, these boards are towards the bottom of your motherboard. I can logically see hot air getting trapped under the video cards, seeing as air is being blown in from the bottom front, and sucked out at the top rear. Having the PSU at the bottom would logically help pull air across the video cards, giving the hot air somewhere to escape. Now, this does place some of the heat from the video cards onto the PSU as a burden, but at least it is not all of the heat of all of the components.

Coming close to a close, those who claim that the PSU at the bottom changes how much dust gets into your computer may not have thought this through fully. Maybe the dust collects more at the bottom, rather then being spread across all of the components. Maybe this is a more efficient way of moving air in your case, causing your system to pull more air through. I really have a hard time believing that the position of your PSU in your case has anything to do with how much dust is in the air. I mean, the thought that having the PSU at the bottom of the case causes more dust follows the same thought process that maggots came from meat (High school biology lesson). Besides, you should be cleaning your case monthly seeing that static electricity is the #1 killer of PC components and dust is the #1 cause of static electricity (humidity also).

The last point concerning the PSU and heating, I have seen it argued that having the PSU at the top near the other drives keeps your heat creating components all together, and that being a problem. I don't see this as being an issue, because if you care where your PSU is, you probably have good video cards, and having it at the bottom puts it near those.

The last points concerning the PSU position that I have found substantial is that it makes the case less top heavy (which I could see as a problem in a full tower case) and it makes managing the cables much easier. (A big plus, considering having your cables managed better would help improve airflow through the case... instead of a rat's nest slowing things down).

The one significant complaint I have found about having the PSU at the bottom is the distance between the power supply and the power connector on the board. If you buy a case that requires the PSU be at the bottom, I suggest getting an extension cord for the power. I found a good one on a popular online computer parts store for $6.

Lastly, those who say that cases that don't have a PSU upon purchase are $**t haven't thought this through. Most PSUs that come with cases are generic ones that probably don't have enough power. If you care enough to worry about where the PSU should go, then you probably shouldn't be using the PSU that comes with a case. Now consider two cases that are the same price. One has a PSU the other does not. I would think that the one that does not would be a higher quality case for two reasons: The people that made the case understand that anyone serious about building their own machine will probably buy another PSU; and there was more money put into the construction of the case, rather than filling it with a generic PSU.

All this said, I found a great case. The Antec twelve hundred. It has a place for the PSU at the bottom, and at the top, where the PSU would have been, is a large fan, facing up, not back. This makes a gaping hole in the top of the case pulling the air up and out of the top, where, knowing that heat rises, the heat would naturally float to.

Hope This Helps,
NPE (Null Pointer Expert)


Wow, I guess a late reply (2 years???????) is better than no reply at all.
August 16, 2010 3:34:34 PM

No - I am not crazy. I just understand that doing research on the internet can be daunting to say the least. I figured if I had the knowledge, I might as well share it and save someone some time (Ironic considering the size of the post). I would have liked to come across a thorough post like this one in my research...

Plus - I am a professional software developer, I am used to typing a lot. :) 

As per your question about the PSU being mounted upside down, I haven't seen anything like this. Seeing that the only mechanical function inside most PSU is the fan, and turning it upside down doesn't change how the fan is spinning, I don't see it as being an issue. From my understanding, electricity doesn't care about gravity, and most of the components in the PSU are electronic.

Many people will run their PCs on the side or standing up. I haven't seen any warnings to running a PC on it's side, which leads me to believe that upside down shouldn't be an issue. However, seeing that this reply came two years later, you probably found the answer to your question ;) 

Lastly, in response to shovenose: I am sorry for the mis-communication. I wasn't trying to say that cases that come with a PSU are bad. What I was trying to say is that between two cases that are the same price, the one without the PSU is probably better quality (Now, there are also a lot of variables that go into this). I don't think the quality of the PSU has anything to do with the quality to the case. My comment about cases that don't come with a PSU and the manufacturers knowing that you will probably buy another means that the manufacturers probably have gamers in mind, not novice system builders. This leads me to believe that they might have put more thought into the design of the case. However, I also see the point of the company making a few extra bucks.
a b ) Power supply
August 16, 2010 5:07:37 PM

i get what ur saying but i still think you missed the point of what i was saying or i wasnt clear. two very similarly priced cases, one with a psu and one without, i would say that the build quality of the case wont be very different, because the mostly junky psus in there only cost like $1 to the case company :) 
August 16, 2010 9:36:19 PM

Okay - I get what you are saying now. Thank you for the clarification.
January 7, 2013 2:20:06 PM

My 2 cents on this... A bit late but here goes...

PSU on the bottom is a bad idea IMHO. My reasons for this are as follows..

1. AT the bottom is where the dust and debris gathers.

2. One dropped screw into the PSU is a dead PSU. If its on its more than likely instant death to the PSU and perhaps sensitive parts on the mainboard, usually something like one of the secondary SATA connections, or the system fan connectors will go first with a power spike or short.

3. Whether at the top or bottom the PSU still does the same job and that is to suck air in and out the back. Effects of gravity on air in a closed space powered by a fan is almost imperceptible, making the efficiency argument null. All the other garbage they sell you in a case such as; a filter and air intake under the PSU is just marketing nonsense. What good does that do? the air comes in from the opposite side of the PSU anyway. Air filters at top or the upper back or even the sides makes much more sense.

4. You need A PSU with long enough mainboard power connector wires which can be a pain to find as well as be more expensive. And the usually longer 4 and 6 pin auxiliary GPU power connectors now will be much closer making the extra wire a nuisance and makes even more clutter at the bottom, where the dust gathers.

I do not know what gave a designer the impression this is a more efficient design. I would suspect the idea came from selling people special products to fit the new need would be part of it. If you are a case maker who happens to sell PSU's (most do or have deals with certain manufacturers) its a win, win. Case at bottom = more dust and debris into the PSU which = a shorter life span of the PSU = buyers purchasing more PSU's over a lifetime. OR you can make a killing selling special PSU's with longer cables to the mainboard and shorter ones to the GPU. And if that wasn't enough you can sell adapters to make the cables long enough. Either way its very lucrative over time.

But then thats my cynical side coming out..