Radiator placement question

Agall · October 16, 2023

1 hour ago, dugacs said:

Yes, but in that case, the radiator gets hot air so the cpu may not be well-cooled, also, my case can handle fans on the outside of the case with the front panel being on, so I can make double layer of fans for the radiator

One thing to also understand is that temperature in this context isn't what you're controlling for, its energy in vs energy out. CPU temperature is a balancing result of that equation, with the controlling variables being specific material/design properties of the cooler itself and flow (fan speed and pump speed).

CPUs like the 7800x3D run exceptionally 'hot' because of the thermodynamics involved with 3D v-cache, where it'll regularly reach its thermal limit while only drawing 45W. Increasing some of the heat transfer variables do help, but you're fundamentally limited by the extra heat transfer layers that inevitably create a higher difference in temperature between the heat source and heatsink.

As I mentioned before, it's difficult to actually mess things up as long as you're following the basic rules of the case, being front intake and top/rear exhaust. If you want to front mount your radiator, even in the 'no no' configuration, then sure, it'll probably be fine for a few years. The only limitation would be if you're running the absolute highest wattage components available, like an RTX 4090 and 13900k, speaking as someone with a slightly more power efficient CPU but a 600W overclocked RTX 4090. I don't even run water cooling, and only have 4 case fans and the system remains practically silent, because it's been designed/configured/tuned by myself who ran mITX top specced systems with crazy DIY mods for +7 years.

I'll go deeper then into the fluid dynamics and thermodynamics of my suggested configuration, for understanding.

The internal volume of the case is proportional to heat capacity, something mITX substantially lacks. Where as long as you have sufficient airflow in and airflow out, ideally slightly positive on a full sized case, then the internal temperature of the air in the case won't rise by much to dramatically affect CPU temperatures and won't affect gaming performance.

Slightly positive pressure is achieved by the resistance to the top exhaust by the radiator, which I equate to 0.5-0.75 of a whole fan, depending on the fan used and its nominal speed. That would put your system at ~ 3:2.5, being slightly positive pressure, which would allow any excess deficient to mitigate the dead spot around the rear end of the GPU (that I circled in blue) to exhaust out the rear. If you compare that to the latter setup where you're running a front mounted radiator, you're deprecating the airflow of all 3 of your intake fans, while having 3 normal exhaust. That would put it at ~2.25:3 being more negative proportionally than positive. That would increase the intake of the GPU from the rear of the case which is fine, but will increase dust and potentially turbulence.

I've personally tested this configuration with an EKWB 280mm elite AIO in my setup in a few cases, including a Fractal Design Torrent and two cases similar to yours, the Corsair 4000D and FD North with an RTX 4090 overclocked to 600W. This just being recently, my previous setup having dual 240mm AIOs with a 6900 XT and 5800x3D, I've ran a 240mm Corsair AIO on systems as far back as my 4820k, 4930k, 4790k builds way back in 2013/2014 with the same theory, including DIY mounted 120mm AIOs on the SLI graphic cards.

TLDR: A CPU will be plenty cooled with a top mounted 240mm radiator, even with an overclocked RTX 4090's 600W of thermal output, excluding the 13900k specifically. Top mounting is better for the fluid dynamics and longevity of the system and is unlikely to impact CPU and GPU performance in gaming. Worst case scenario, you're a few degrees higher on the CPU and proportionally less on the GPU, allowing for better GPU overclocking or lower fan speeds on the GPU fans, which are almost always louder than 120mm case fans.

The 13900k caveat is because that CPU is a power hungry 250W monster (if you let it) and its incredibly difficult to keep from thermal throttling without exotic modifications.

RevGAM · October 16, 2023

30 minutes ago, Agall said:

Slightly positive pressure is achieved by the resistance to the top exhaust by the radiato

You can also maintain positive pressure by throttling the exhaust fans, choosing weaker fans for exhaust or much stronger ones for intake, adding more intake, or flipping an exhaust to intake.

Agall · October 16, 2023

30 minutes ago, RevGAM said:

You can also maintain positive pressure by throttling the exhaust fans, choosing weaker fans for exhaust or much stronger ones for intake, adding more intake, or flipping an exhaust to intake.

No matter what I think you have to tune the fans depending on the hardware chosen, including the fans. Reversing the fans to compensate in my experience isn't worth it, but I have recommended people reversing the bottom fan of the side mount on cases like the 5000D as an intake to increase the airflow to the lower part of the case for the GPU. The front and top mounts however are pretty much required to be a certain way based on the case design, but that only truly matters at the high end of power consumption, +500W or so.