[Solved] Very high CPU temp and throttling in custom loop. GPU temps are great.

[inoine]

Hi. I just finished building a full custom loop watercooling using Alphacool parts:

• Eisblock XPX CPU Block
• Eisblock Aurora Acryl GPX-N RTX 3090/3080 (for Reference 3080ti)
• NexXxoS HPE-30 Full Copper 240mm Radiator
• Alphacool DC-LT 2 - 2600rpm Pump
• Eisstation 80 DC-LT Pump Top + Reservoir

This is in a mini-ITX case. The above pump + reservoir are the only ones that can fit in my case. It was quite a challenge to build. I can confirm no kinks in any of the tubing and the water is flowing, though not very fast, because the pump is not very strong. I don't have a flow indicator/sensor, but the radiator gets warm when I load the GPU, so the water is circulating.

My GPU (3080ti) is cooling great! If I run FurMark stress test, the temps level off at around 64°C with 50% fan speed. This further confirms that everything is fine with my loop and the pump is adequate. I am very happy with it. 340W GPU power, at such temps, amazing!

My CPU (AMD 5800X3D) ... however ... does not seem to be doing well at all.

Idle temps are quite high, around 60°C. As soon as I start any demanding CPU workload (for example, a Blender CPU render), the temps immediately jump to 90°C and the CPU throttles. If I touch the radiator and the tubing, they do not feel warm at all.

Based on these symptoms, and the fact that my loop is otherwise working fine (given my GPU temps are great), I suspect the issue is with the CPU heat transfer into the waterblock.

I tried everything I could think of. I reinstalled the CPU waterblock several times. Less thermal paste, more thermal paste, less mounting pressure (not tightening the screws to the end), more mounting pressure (tightening them to the max), ... I tried liquid metal (Thermal Grizzly Conductonaut), which I have successfully used on other systems and like very much ... made no difference. I tried different thermal pastes. I even tried lapping the CPU IHS (1000 and 2500 grit sandpaper). Tried changing the orientation of the waterblock. Nothing makes any difference.

The CPU spikes to 90°C and throttles under multicore load, no matter what I do. It behaved better with an air cooler. I used an air cooler on this system before. The temps used to be 80-something °C at max fan speed and no throttling. I thought the watercooling would be an upgrade. But it's not working it seems.

Any ideas? Maybe something is wrong with the waterblock itself?

EDIT: also tried a different pump: an old Laing DDC pump I had laying around (bought from EKWB many years ago). Much more powerful than my new Alphacool DC-LT 2, much more flow, but still no improvement to CPU temps.

 

[Eddy]

On the XPX cooler there are nuts on the bracket with which you can fix the mounting screws. Leave them off completely during assembly and screw the whole thing on so that the springs of the bracket are not completely compressed.
You can also post a picture of the heat conducting paste impression here (file max. 2MB in size). I may be able to recognise a possible problem from this.

 

[inoine]

Well, thank you for the response. I actually found my problem and it is not from the thermal interface, but rather that I was running my fans too slowly (because I really wanted my PC to be silent) and the water temperature was getting very warm.

I did a lot of research over the past few days and learned a few new things that helped me figure it out. It seems like I was fundamentally misunderstanding how CPU coolers work and was expecting something that is actually physically unrealistic.

I am going to explain what I learned here, so that if anyone else struggles with similar problems and comes across this forum thread, they can learn from my findings.

So... I was not aware that CPUs require such a huge temperature delta in order to transfer heat! I thought that if my loop can handle 340W from the GPU, it should be able to easily handle 120W from the CPU ... but that is not actually true!

This is an amazing video I found:

. It explains why CPU heatsinks (when air cooling) or water temperature (when water cooling) need to be much colder, compared to GPU coolers, in order to extract the same Watts of heat energy from the chip. It is because of die area! GPU dies are huge, so the heat comes from a bigger area / the heat density (W/mm2) is much lower. CPUs produce very concentrated heat, because the die size is small, and the cores themselves are an even smaller section of the silicon.

This is why CPUs tend to run around 50-60 degrees hotter than the water (or the air cooler heatsink), while GPUs only around 20-30 degrees. In the video, there was a thermal camera shot showing how a graphics card heatsink is at around 40-50°C, but a CPU heatsink (or waterblock) needs to be around 30°C to keep the chip at reasonable temps. It needs that temperature delta to transfer heat!

If my water was over 40°C (which it probably was, after running FurMark with low fan RPM), of course my CPU will jump to 90°C and throttle instantly under load! That's how I was performing my tests! I was expecting my loop to easily handle the CPU if it could handle the GPU (which is not true), so I ran GPU stress tests first, to make sure my loop was working, kept the fan speeds low (because I thought they didn't need to be high, if the GPU was fine), and then I ran CPU tests ... and got shocked at the results.

So the lesson I learned: watercooling is not magic. When the CPU and GPU share the same loop, the heat from the GPU can really interfere with the CPU cooling, as the CPU needs the water/waterblock to be much colder in order to transfer its heat into the loop. I need to ramp up the fans and keep the water temperature very low, in order to cool the CPU. The GPU will never struggle with cooling (even though it is more watts) and will always run at much lower temps than the CPU, because its temperature delta over water is much smaller (thanks to the bigger die size and resulting lower heat density of GPUs).

Understanding all of this ... the 64°C on the GPU, as I reported in the OP, is actually quite high! It indicates very warm water, which can't cool the CPU. The radiator is *not* supposed to feel hot to the touch! It's supposed to be close to ambient temperature!

So now I made new fan curves which aggressively ramp up the fans under GPU or CPU load and keep the fans running all the time even when idle. And now I can run a CPU stress test and my CPU temps stay around 80°C under load with max boost / no throttling! Meanwhile my GPU is now around 54°C under load, instead of 64°C. Problem solved!

I guess my dream of having a silent mini-ITX PC with high-end CPU and GPU is actually physically impossible, after all ...

 

[Eddy]

I have to admit, I didn't realise you were cooling everything with just a 240mm radiator. I probably thought it was so far-fetched that I ignored it.
The 3080Ti has a waste heat of almost 350W, plus the CPU, then you're at 400-450W waste heat. That's too much for a 240mm radiator squeezed into a mini-ITX system. That doesn't work, especially not quietly.

 

[inoine]

Well, it works at 100% fan speeds! But yes, definitely not quietly!

Under normal gaming load, the heat is much less (and the CPU usage much lower) than with stress tests, so I can get away with 70-80% fan speed, but definitely not less than that. Temps do get pretty warm.

At idle, I run 20% fan speed, which makes my PC virtually silent, which makes me very happy when I am using my PC for productivity and not gaming.

I cannot fit a bigger radiator in my case. I cannot fit a bigger pump in my case. I will have to make do ...

I guess the only things I can do to further improve my cooling would be to upgrade the pump (such as to a 3600rpm DC-LT instead of 2600rpm), which would result in more flow. And *maybe* to get a high-end CPU waterblock (such as Alphacool Core 1 instead of XPX), which might provide slightly better cooling performance.

Each of those things would reduce the temperature delta of the CPU over water by a few degrees, and that would allow the water to be a few degrees warmer without the CPU reaching its temperature limits, which would allow me to reduce my fan speeds a bit.

Now the question is do I want to spend more money on watercooling...

 

[inoine]

Also, I guess I was misled by the fact that my GPU originally came with a 120mm AIO from the factory. I got this graphics card, because it was somehow the cheapest second-hand deal I could find for a highend GPU.

So I was using my GPU watercooled even before building my custom loop now. Yes, the GPU temp was 80°C and the rad was very hot to the touch. But it was a GPU-only watercooling loop, so that was no problem. I had no idea that cooling a CPU is actually so much more difficult than cooling a GPU!

I was thinking: "if a 120mm rad can handle my 340W GPU, a 240mm rad should be a big upgrade, no?" And then I built my new loop, saw my GPU temps down to 50-60°C instead of 80°C and thought "yes! this 240mm rad is indeed amazing! i can even run at low fan speeds!"

But alas, having the CPU in the same watercooling loop ruins everything...