Premium Build: Greyscale — building a custom-looped ITX PC that pushes the form factor to its limits

Before proceeding to test the PC, we must run it through its paces a few times to see how it runs currently, to see what needs to change in the configuration.

To begin, I updated the motherboard’s BIOS, and then we set the memory to the correct EXPO profile, which had it running at the intended clock speeds in a jiffy.

With that all done and Windows freshly installed, we proceeded to set the fan curves.

Because this motherboard has no thermal probe, I decided to use Fan Control to set up a custom virtual sensor – one that calculates the average temperature of the CPU and the GPU combined. From what I saw, the CPU would push itself to its 95 °C target regardless of what I did, whereas the GPU would continue to boost within its power target, and eventually settle on a maximum temperature of about 75 °C.

Now, to get a system like this quiet, it’s essential to be aware of the basic physics of cooling with radiators. Each radiator has a certain amount of heat it can dissipate, but this can be influenced by a handful of factors.

The first, and most obvious of these is which fans are installed, their RPM setpoint, and how restricted the airflow is through this setup. In the case of this PC, we’re using top-quality fans, and the radiator at the top, although 45mm thick, has tons of breathing space. The radiator at the bottom, however, is lacking breathing space, with the intake side very close to the desk, and the exhaust side largely obstructed by the graphics card.

However, there is one other factor that affects how many watts a radiator can dissipate: temperature.

A contradictory configuration

If the coolant temperature flowing through a rad is low, you can blast a ton of air through it, but you’ll really only be dissipating a few watts. On the other hand, if the coolant is nice and hot, you can run the fans at a very low speed, and yet, they’ll expel a ton of heat.

For this reason, I had chosen to route the outlet port of the GPU block straight to the upper radiator, which would be doing the heavy lifting – it’s got way more breathing space, it’s thick, and best of all – all the heat it expels goes straight out the top of the case.

To ensure the greatest temperature delta, the intake fan on the side would supply it with a healthy amount of fresh air.

What am I trying to get at with all this information: you don’t want to run a PC like this at 100% fan speed. It’s just too noisy, and although doing so does keep the coolant temperatures a lot lower, the level of diminishing returns is significant, and the user experience becomes quite unpleasant.

For context, here’s what I mean:

With the fans at 100%, total system power consumption sits at 930 watts with a combined synthetic load on the CPU and GPU.

Drop the fan speeds on a much quieter curve, and the system finds an equilibrium load at 867 watts. The RTX 5090 runs right on the mark, and the 9950X3D simply doesn’t hit its power target anymore, instead running at 150 watts as opposed to 200.

However, all of this is only when running a synthetic load. Under these conditions, the CPU will hit 95 degrees, and throttle to keep it at this target temperature of 95 degrees. The CPU will also always be the first to throttle, simply because it has an IHS that gives it a 20-degree penalty over the GPU.

Now, to fix this, we could lower the power target on the GPU, essentially manually throttling the GPU, so that the loop has the capacity to handle the CPU without throttling, but I don’t feel this is necessary at all.

Under real-world loads, there is no performance penalty. Gaming isn’t nearly as heavy on the CPU, so it can run at full boost regardless of what the GPU is doing, and productivity workloads are generally not as ‘never-ending’ as gaming, which means the cooling loop can soak up a lot of heat before any throttling would occur.

From testing, it’s clear that in a quiet fan curve configuration, the loop can dissipate about 850 watts. Go over that, and something needs to throttle. But if you look at the gaming and productivity tests above, it’s clear that even the heaviest gaming workloads don’t exceed this figure.

If you don’t want anything to throttle, you’ll have to boost the fans to full speed, but this increases the maximum noise figure from 38.6 dBA to 54.3, which is simply put, unacceptably loud, and not remotely worth the extra 50-75 watts of cooling capacity it offers. You really don’t need to be able to handle both the CPU and GPU not throttling under simultaneous synthetic loads.