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I thought the whole point of putting a PSU at the top of the case was so the heat would rise out of the case without warming the rest of the case. Why are modern PSUs now placed at the bottom of the case?

The better question is, "Why do modern cases put the power supply at the bottom?" And I think it's part of an effort to ensure an ideal distribution of heat, where the warm air generated by the CPU and GPU aren't taken up by the power supply, reducing its efficiency and potentially reducing its lifespan. That's just a guess though.

Concupiscence wrote:

it's part of an effort to ensure an ideal distribution of heat, where the warm air generated by the CPU and GPU aren't taken up by the power supply, reducing its efficiency and potentially reducing its lifespan.

Agreed. Also, most cases have a rear exhaust fan (typically 120mm on modern cases) above the IO port cluster. That can still grab the rising warm air from the GPU and CPU and push it out the case.

Pretty much the answer... PSU's needed more cooling.

If you do a quick web search I found a quote from Intel on it.

It allows the PSU to have its own air intake on the underside of the case so it is not sucking air that has already been heated up by other system components. The alternative of leaving it at the top of the case, flipping it over, and putting the PSU air intake on the top of the case is not a good idea, because the hot exhaust from the back of the case tends to rise; the PSU would be sucking some of that hot air back in.

That said, I've got mixed feelings regarding this type of case design. Although the PSU theoretically gets cooler intake air, it will also be dustier, especially if the system is sitting on the floor. This will lead to more dust bunnies in the PSU or a need to clean the intake filter (if there is one) more frequently to prevent clogging.

The last time I used a case with a bottom PSU mount I intentionally mounted the PSU upside-down, so that it still draws air from inside the case. The environment was dusty enough that I decided the dust concerns overrode any potential improvement in cooling.

Potential sucking up more dust aside, I have found an unexpected benefit with a bottom-mounted PSU setup: weight distribution. I think at around the introduction of 450W+ PSUs, I was feeling that the typical ATX Tower setup became a bit top heavy. WIth increased wattage and the emphasis of quality components, PSUs have become heavier than the units of old. Add to the oversized+overweight tower-style CPU HSFs with the GPU(s) perhaps lying towards the middle/upper-middle part of the chassis, the weight seems to be more and more concentrated towards the top. Moving the PSU to the bottom helps balance things out IMO.

In the earliest days of the computer, the PSU fan was the only exhaust fan in the entire system, and sometimes it was the only cooling fan as well. Placing it at the top of the case allowed it to optimally ventilate the case.

In modern times, there isn't any particular reason to have it there.

just brew it! wrote:

.. leaving [the PSU] at the top of the case, flipping it over, and putting the PSU air intake on the top of the case is not a good idea...

shoot...I bought the wrong case then..

just brew it! wrote:

Although the PSU theoretically gets cooler intake air [when mounted at the bottom of the case], it will also be dustier, especially if the system is sitting on the floor.

Most DECENT cases with the PSU intake under the case will put a filter there. If not, I'd highly recommend fashioning your own (or buy one) for the reason listed by JBI.

Ah, that is interesting to know. I'm using an older case, so I um, had to cut a hole in the top and mount my PSU upside down to give it a clear output area. The fan doesn't spin up much though, so I don't think it is getting too hot.

Thanks for all the information.

Flying Fox wrote:

Potential sucking up more dust aside, I have found an unexpected benefit with a bottom-mounted PSU setup: weight distribution. I think at around the introduction of 450W+ PSUs, I was feeling that the typical ATX Tower setup became a bit top heavy. WIth increased wattage and the emphasis of quality components, PSUs have become heavier than the units of old. Add to the oversized+overweight tower-style CPU HSFs with the GPU(s) perhaps lying towards the middle/upper-middle part of the chassis, the weight seems to be more and more concentrated towards the top. Moving the PSU to the bottom helps balance things out IMO.

I don't see why center of gravity is important here. We're talking computers, not cars where in weight distribution plays a significant role in handling and safety. I think the reason why PSUs sit at the bottom of modern enclosures has to do with air circulation and ventilation. And as someone has said here, PSUs have become increasingly packed with components so cooling it has become more critical. Before, the issue was moving air in through the front of the case, up the PSU, and out the PSU's exhaust. It was an ideal solution because unlike today, components back then were relatively cooler and pushing air out using the PSU seemed like a cost-effective approach. Today, components have become so much hotter and more than ever hot air needs to be blown out of the enclosure, but so has cooling the PSU become more critical than ever. Placing it at the bottom meant it could suck cool air from outside for itself and out through the PSU's rear vents. Some heat from the PSU would still find its way into the system but now putting several fans specifically for exhaust has become common, filling in for the role the PSU had previously.

Youv'e never picked up your PC or moved it or bumped it with your leg? Weight distribution is always important. Especially when it sits on the floor. Having weight at the lower end of the case helps keep it upright.

derFunkenstein wrote:

Youv'e never picked up your PC or moved it or bumped it with your leg? Weight distribution is always important. Especially when it sits on the floor. Having weight at the lower end of the case helps keep it upright.

Are you serious? Or do you just need to win the discussion? Kicking your PC is out of the question in the first place. I thought it was common sense to put it where you aren't likely to kick it. Besides, even with the PSU on top it's not like the whole thing will roll over if you happen to hit it with a foot. That aside, I still don't believe weight distribution is the reason why case manufacturers moved the PSU down below. It helps weight distribution, but it's just a fortunate bonus.

ronch wrote:

derFunkenstein wrote:

Youv'e never picked up your PC or moved it or bumped it with your leg? Weight distribution is always important. Especially when it sits on the floor. Having weight at the lower end of the case helps keep it upright.

Are you serious? Or do you just need to win the discussion? Kicking your PC is out of the question in the first place. I thought it was common sense to put it where you aren't likely to kick it. Besides, even with the PSU on top it's not like the whole thing will roll over if you happen to hit it with a foot. That aside, I still don't believe weight distribution is the reason why case manufacturers moved the PSU down below. It helps weight distribution, but it's just a fortunate bonus.

No one is saying that weight distribution is the only and primary reason why the modern layout puts the PSU on the bottom. Read the whole thread and get the full context please. And yes, ideally you want to put your chassis so you cannot kick it. But ever heard of accidents? By definition it is something you do not expect. Or you put the tower on your desk and someone just happened to be in your room doing big arm swings and stuff. Freak accidents do happen, as long as the probability is not zero. I did say "unexpected benefit", which you have now confirmed with "fortunate bonus". Also, when you need to move the chassis for whatever reason you will have to pick up the computer, and that is the time you will feel the weight distribution too.

For me it's weight distribution + longevity of the power cable not being bent downwards.

Yeah, I get what you mean. Just making a beeline for the main reason. Any extras, well, they're just that.

I don't like it. You have to clean out the psu more than you would otherwise when it's at the top. Heat? I only buy gpu's with blowers, and my cpu fan blows sideways towards the rear exhaust. Heat isn't an issue. Weight? Big GPU's and hard drives offset the weight, which isn't that much. I'd say my biggest concern overall is dust collection, which means I'll be avoiding cases that mount the psu at the bottom. Mounting the PSU at the bottom means you're using a case and components designed to-exhaust heat internally for the case to handle, which usually means cheaper components. The PSU is your PC's worst failure point, so I'd rather avoid cheaper parts at the expense of dust collection. Dust doesn't make a good heat conductor or fan lubricant, so bottom mounting doesn't offer any real benefits other than allowing for cheaper components, which get outdated and fail more often requiring frequent upgrades. That's why in a nutshell, IMO.

steelcity_ballin wrote:

For me it's weight distribution + longevity of the power cable not being bent downwards.

Cable bending upwards is good for longevity because...?

Flying Fox wrote:

steelcity_ballin wrote:

For me it's weight distribution + longevity of the power cable not being bent downwards.

Cable bending upwards is good for longevity because...?

I think he meant the external power cable that plugs into the PSU. No less trivial though.

Flying Fox wrote:

No one is saying that weight distribution is the only and primary reason why the modern layout puts the PSU on the bottom. Read the whole thread and get the full context please. And yes, ideally you want to put your chassis so you cannot kick it. But ever heard of accidents? By definition it is something you do not expect. Or you put the tower on your desk and someone just happened to be in your room doing big arm swings and stuff. Freak accidents do happen, as long as the probability is not zero. I did say "unexpected benefit", which you have now confirmed with "fortunate bonus". Also, when you need to move the chassis for whatever reason you will have to pick up the computer, and that is the time you will feel the weight distribution too.

This. I have cats (against my will, admittedly) and cannot control where they go or what they do. Totally fortunate bonus.

derFunkenstein wrote:

Flying Fox wrote:

No one is saying that weight distribution is the only and primary reason why the modern layout puts the PSU on the bottom. Read the whole thread and get the full context please. And yes, ideally you want to put your chassis so you cannot kick it. But ever heard of accidents? By definition it is something you do not expect. Or you put the tower on your desk and someone just happened to be in your room doing big arm swings and stuff. Freak accidents do happen, as long as the probability is not zero. I did say "unexpected benefit", which you have now confirmed with "fortunate bonus". Also, when you need to move the chassis for whatever reason you will have to pick up the computer, and that is the time you will feel the weight distribution too.

This. I have cats (against my will, admittedly) and cannot control where they go or what they do. Totally fortunate bonus.

With cat hairs flying around I suppose you have to really watch the dust+hair accumulation by a bottom-mounted PSU then?

Primary is better airflow and thermal management in the case.

Secondary is cable management, (graphics cards are often closer to the bottom of the case than the top...ditto with multiple HDD) and weight distribution.

The old large tower cases were indeed prone to tipping if bumped on the edge of the table.

All in all, bottom is really a better solution.

Things just evolve over time. You have to remember that when the PSU was the top back exhaust, it was often the only exhaust fan...and the only intakes were in the front of the case (before sides, and well before top and bottom intakes)

double post

I think it is misguided. A good PSU can handle more heat than your CPU. Placing it on the top is not going to kill it. If it is too hot for the PSU then it is too hot for the CPU that is right next to it. When the PSU is at the bottom either the fan faces up and stuff can fall into it, or it faces down and now you need a space under your PC for airflow.

As for tipping over, with the PSU at the top you do not need any clearance under the case and can bolt it to the floor or your desk to keep it from tipping over when you hit it with your knee or shoulder, and as an added bonus, thieves will be befuddled when trying to steal it.

Flying Fox wrote:

No one is saying that weight distribution is the only and primary reason why the modern layout puts the PSU on the bottom. Read the whole thread and get the full context please. And yes, ideally you want to put your chassis so you cannot kick it. But ever heard of accidents? By definition it is something you do not expect. Or you put the tower on your desk and someone just happened to be in your room doing big arm swings and stuff. Freak accidents do happen, as long as the probability is not zero. I did say "unexpected benefit", which you have now confirmed with "fortunate bonus". Also, when you need to move the chassis for whatever reason you will have to pick up the computer, and that is the time you will feel the weight distribution too.

A co-worker got his foot tangled in the keyboard cord. Tipped the whole case over on its side, and destroyed the hard drive in the process. A lower weight distribution might've saved it by keeping it from tipping.

steelcity_ballin wrote:

For me it's weight distribution + longevity of the power cable not being bent downwards.

OK, now it is my turn to say: are you serious? It isn't bending that kills wires, it is repeated flexing back and forth. Furthermore, power cords are made with stranded (as opposed to solid) wire, which is much more tolerant of flexing than solid wire. In over 3 decades of working with PCs and related tech, I can count the number of intermittent power cords I've encountered on the fingers of one hand. And IIRC all of those failures were at the wall plug end (no doubt due to people yanking on the cord instead of the plug body to unplug it), not at the equipment end.

Flying Fox wrote:

derFunkenstein wrote:

This. I have cats (against my will, admittedly) and cannot control where they go or what they do. Totally fortunate bonus.

With cat hairs flying around I suppose you have to really watch the dust+hair accumulation by a bottom-mounted PSU then?

The case is a Fractal Design Define R4, which has filters on the front and on the bottom. I'm cleaning them out every 3-4 weeks, which it's not like they're all gunked up, but I want to make sure they stay as unobstructed as possible. So weight distribution also comes into play.

I think they put the PSU on the bottom of the case to make everyone argue about why.

DPete27 wrote:

I think they put the PSU on the bottom of the case to make everyone argue about why.

It became very popular with the Antec P18x series. These are designed to separate the PSU and the HDD cage from the rest of the case using a plastic slider to choke off as much of the cable passage as possible. At the time the idea was to remove the PSU and its heat from the CPU/GPU combo which, at the time the P180 originally debuted (CES 2005), were P4s and Dustbusters. It was designed in conjunction with the guy who runs Silent PC Review and the goal of the dual chamber approach was to manage heat in each chamber to allow optimal cooling with low 'n' quiet fan speeds.

DPete27 wrote:

I think they put the PSU on the bottom of the case to make everyone argue about why.

I think they do it so when your water cooler fails, they get to sell you another power supply (planned failure mode).

Seriously though, the location is to improve cooling of the power supply.

Intel's BTX form factor "experiment", where they were trying to improve cooling air flow for Pentiums 4's and their chipset, left the power supply in the top of the case. Since only the northbridge and processor were in the ducted airflow path, power supply still got warm(-er) because it had to exhaust heat from everything else in the case. Cool components are happy components, which makes for longer component life.

Historically, why was the power supply in the top of the case? It wasn't. the original cases (and I'm talking PC, XT, AT and clones) were desktop format. The motherboard and I/O cards filled 2/3 of the case. The floppy drives were in front of the power supply and the power supply switch was a big red switch on the right side of the case. When you turned the case on its side and set the case on the floor, you wanted the floppy drives on top where they were accessible; and, you clearly wanted the power switch on top, not facing the floor. Thus... a new standard was born.

A good PSU can handle more heat than your CPU.

What gives you that idea?

Very, very high quality PSU can run at 50C...very good at 40c...average quality...well, do your best to keep it under 35C.

I think you know that A CPU can take more than 50C.

mdk77777 wrote:

A good PSU can handle more heat than your CPU.

What gives you that idea?

Very, very high quality PSU can run at 50C...very good at 40c...average quality...well, do your best to keep it under 35C.

I think you know that A CPU can take more than 50C.

Are you talking ambient air temperature or internal component temperature? Because unless you mean the internal component temperature for both the PSU and CPU you're comparing apples and oranges here.

I can guarantee you that there are components inside pretty much every PC PSU which are getting well above 35C when the system is under load. I can also guarantee you that most performance CPUs (not to mention the VRMs on the motherboard) would be very unhappy if the ambient air temperature was 50C.