I am planning to scratch build an HTPC to replace my ASUS Pundit.

My initial thinking was to have the motherboard mounted to the top of the case, have the CPU heatsink pulling air through a large open grill at the bottom of the case and exhaust out a grill at the rear.

I was thinking of using a Scythe Shuriken 2.

I realized quickly that this will put the heatpipes in the wrong vertical orientation. I have looked around to find some hard data to judge how much of a disadvantage this is, and whether I should change the layout.

So far, I've read some old threads, including this one, but I've yet to get a clear idea. It seems to be somewhere in the range of "not much difference" to "will cook your CPU".

Does anybody have experience with a blow down heatsink that is mounted upside down and how much difference that makes?

Well most GPU's with heatpipes are upside down, AFAIK all heatpipes work on a wicking principal so the cool liquid soaks some kind of wick. Much like if you put a piece of string over the lip of a glass and leave it you will find after a while a puddle next to the glass, where the water has soaked up the string.

Inside the heatpipe this wick is heated and turns the liquid into a vapor and this in turn condenses at the cool side to then be picked back up by the wick.

So AFAIK there should be no problem operating heatpipes upside down, but someone may correct me if my understand of them is wrong.

EDIT: Found a picture to explain it a little better (i hope)



Also found this
http://www.silentpcreview.com/forums/viewtopic.php?t=40410

No real difference. Gravity is a pretty weak thing compared to the electromagnetic force.

Whether a heatpipe works upside down depends on the type of wick. There is data about this available out there. So the question then becomes: what sort of wick do typical CPU heatsink heatpipes use?

About half of all heatpipes are working upside down right now. They "wick" the liquid back to the base using the walls of the tube.

Thanks for the great article!

It seems that the performance is highly dependent on the type of wick used.

But even for the metal powder type, which is best able to work against gravity, the difference is pretty massive, especially with increasing heat loads. (Figure 10 in the article.)

For the groove and also probably mesh types, running a heatpipe upside down is probably out of the question.

For the heatpipe itself, the data is pretty clear and clearly against running it upside down. I am still curious about the overall performance as a heatsink, where the effect would probably not be so drastic as just the heatpipe itself.

Nevertheless, it seems that my plan to have the motherboard upside down will have to be revised.

Well, computing heat pipes rely on mesh type wicks. It is quite an effective wick, but cannot do miracles... It is quite effective when put in horizontal position for lengths up to half a meter. The usual configuration foreseeing the heat source at the lower end and the heat sink at the upper end (or all along the pipe length) is actually the most effective, simply because you are not using the heat pipe wick capillary effect to pump the liquid back from the sink to the source: you are just using the gravity. Technically speaking, that is not an heatpipe: it is called a thermosyphon, and it is relying on the bouyancy effect, which is quite effective. Putting it upside down means the hot fluid tends to go towards (or remain next to) the heat source while the cold fluid tends to go towards (or remain next to) the heat sink, and this effect is to be counteracted just by the capillary effect of the wick (no electromagnetic force involved at all). The latter is quite limited and thus working "against gravity" makes more difficult for the heat pipe to transfer the heat towards the sink, for high heat loads basically result into the "dry out" of the heat source end (the so called "evaporator") which makes the heat transfer collapse.
That said, putting the heat pipes into the configuration you are talking about would result into situation going from a less efficient heat transfer to a dry out condition in the worst case (i.e. high ambient temperature, high power load). To assess the power load limit is not so difficult, but requires proper devices to test the heat sink. Since it is the worst working condition you can put the heat sink on, I guess you will not found any data regarding it... However, you can test your setup simply by trying it before mounting it into a case. You can put your motherboard upside down onto two supports and increase the power load of the CPU while monitoring the temperature and see what happens. I am pretty sure that this will not result into the chip instantly frying, mainly because the temperature would not go up so dramatically fast to prevent you putting it on idle state again (moreover, nowadays there is also the BIOS cutting the power down if temperature goes beyond the safety limit).

P.S. The groove type wick is just used where there is not gravity at all. In fact it is commonly used into spacecraft heat pipes. For that is part of my job, I can confirm that it is nearly impossible for them to work upside down on Earth. They can cope with just few millimiters of elevation of the heat source above the heat sink, whatever the length of the pipe is. Providing a more steep slope results into a sudden drop into the heat transfer capability of the pipe, for the groove wick is almost totally unable to counteract the gravity force making the liquid flowing down like it would do into a gutter.

From reading the PDF and all the info in this thread (very interesting btw)
Am i right in assuming CPU heatpipes (Mesh?) are different from GPU heatpipes (powder?)
As a lot of GPU's have the heat sources at the top of the heatpipe.

Or am i having a thick'o moment

I doubt if there's any difference between CPU and GPU heat pipes. All the heat sink manufacturers outsource their heat pipes; they "just" build the bases and fins, and do the assembly. Although I don't know for sure, it would surprise me if there are more than a few heat pipe manufacturers who supply the enthusiast computer heat sink manufacturers.

As has been written above, almost all heat pipe based GPU heat sinks have the fins (condensers) below the bases (evaporators), and they work just fine. By comparison, nearly all tower-type CPU heat sinks are mounted horizontally, and work exceptionally well.

In recent systems, the GPU heat load is significantly higher than the CPU heat load (often more than twice as high), and in addition the GPU heat sink air flow is significantly poorer than the CPU's. So you really shouldn't be concerned about an "upside down" CPU heat sink; it will work just fine.

That's right, but they can rely on an higher temperature drop between the heat source and the heat sink, for GPUs are usually able to run hotter than CPUs nowadays. I.e. if the sink shall dissipate the same heat load and can rely onto a higher limit in terms of heat source temperature, a lower overall conductance is needed.
As said above, heat pipes with mesh wick are able to transfer heat with ease when horizontal; they are designed to work against gravity, but it is quite obvious that the maximum heat transfer efficiency is guaranteed when you use them as thermosyphons. As far as an heat pipe is working as an heat pipe (so as far as it is not put into a condition which goes beyond its heat transfer capability) you would not encounter any drop in efficiency and you can consider it as a nearly isothermal device, nevertheless the thermosyphon effect enhances its heat transfer capability (think about it as a boosting effect).
The main concern is to design your system relying on performance achieved into the worst condition case, thus it is not a priori true that your expectations would be met if you think the CPU temperature will be the same in upside down configuration. Moreover, the heat source shall not go beyond the heat pipe maximum heat transfer capability, which depends on its orientation. Summarizing, you should expect a drop into the efficiency and a drop into the maximum heat load manageable.

@Spoon Boy
Several years ago I tested some twicked heat pipes, based on a design driven by computing industry. At that time all the computing heat pipes were of mesh type. I did some research and I see that sintered powder wicks have been introduced into the computing market since that time. The problem with those high density wicks is that the increasing in heat transfer capability is achieved by decreasing the maximum length at which the efficiency is acceptable. That is because the capillary pump effect is progressively counteracted by drag forces affecting the fluid soaking the wick. The more the wick is dense, the higher is the maximum heat load, the higher are the drag forces, the lower is the maximum length. The production cost is also pretty higher with respect to the mesh. I guess that these points make the powder wick heat pipes acceptable on high end graphic cards, which demand high dissipation capability and are not cheap at all!

Lots of fine points above, but to simplify: any recent CPU dissipates 100W or less, even when overclocked. Any recent GPU dissipates more. GPU heat sinks work just fine upside down, even with crappy air flow. Any decent CPU cooler will work just fine in any orientation.

[The notion that GPUs can run hotter than CPUs is bunk. They are all made from silicon, which degrades extremely rapidly above about 115C, and very rapidly above about 100C. The real difference is that CPUs are intended to fail very rarely before 10 years of use, while GPUs are expected to be replaced every year...]

Call me cheap but i normally expect to get more than just a year out of my GPU.

Do you mind giving me your 1 year old GPUs every year? I haven't bought a video card since the days when a 7800GT was a $300 card (think 2005/2006).

Still have the 7800GT in one system that sees regular use and in my primary PC I am using a newer HD 5570 I got from the SPCR stash a few years ago.

I really could use soemthing faster if you realy do buy a new video card every year.

Well I didn't say *I* replaced my GPU every year, but that's certainly what the makers and the reviewers expect. Running a GPU at 100C is dumb, but that's what you'll see on many review sites.

My last GPU was a 9800 overclocked (with an Accelero HS), now sitting around because I switched to i7 internal graphics. If you pay the shipping, I'll be happy to send it to you...

Well that's because they're aimed at 'enthusiasts'. Who on earth enthuses about their computer? Any kind of enthusiast community will favour expenditure on said hobby for the sake of it. You certainly don't have to but just expect to be chastised by them with childish, poorly punctuated taunts because they are quiet blatantly bigger men than you after they spent all of the money they earn on their paper round on a new graphics card for their 'comp'. They then plan to sell the old one on eBay as ever thinking that it somehow justifies buying a new one.


My personal best is 117C but that was with a heatspreader coming loose so unintentional. Still the card survived and runs very well to this day at 65C. The limits on graphics cards are not as tight as on CPUs. The materials are all the same but this doesn't make a difference as there are Via C3 CPUs rated at 150C and they're still made of silicon right?

What the difference is is that the graphics card has the GPU, memory, board, VRMs etc. all rated to work together specificially. They can therefore run at tighter limits. With a CPU it has to work with a motherboard, RAM and other components from different vendors and therefore tighter controls tend to be put in place, like a lower thermal shutdown threshold than the 127C or whatever a graphics card is set to. There are also physical packaging reasons which make it less favourable to have such high temperatures. That materials used in a CPU vs a GPU are not relevant to it.

The discussion has got me even more curious to find out how much the difference is in real life.

So, I will be testing that as soon as my case is done and my hardware is in. (Which will be a couple of weeks, I think...)

I will post the results as soon as I have something.

...and that's the way to do it! I will look forward to the results, because they can say something about the efficiency of the heat pipes used nowadays into CPU coolers.

I totally agree with edh about the GPU vs CPU maximum acceptable temperature. It is not a matter of base materials, it is about the way they are designed, both from architecture and engineering margins standpoints. So, no: the notion that GPUs can run hotter than CPUs is NOT bunk. Not with the current technology, at least.