A Redditor has drilled holes in a wall and routed a gaming PC's liquid cooling loop to a radiator outside, where it was -4 at the time of posting.
The poster calls it "just a proof of concept." A Bykski water block sits on the GPU, and the Intel Core Ultra 7 270K Plus gets direct-die cooling from a Thermal Grizzly 1851 Mycro Pro block. Direct-die means the CPU's metal lid was removed so the block touches the silicon. The two blocks share one loop. A second pump sends some of the water outside and back in when the PC needs extra cooling.
The GPU is listed as an RTX 4070 Ti, but the poster's own quotes say 4070 Ti Super, so the exact card is unclear.
The hard part is not the cold. Water that arrives too chilly causes condensation on the components, and the poster put it bluntly: the build is about keeping the water cold, "but not too cold." Arduino boards calculate the dew point, the temperature below which moisture forms, and keep the loop above it. A Python script starts at boot, takes over from the Arduino's own auto mode, and links to HWi and Fancontrol for finer control.
The poster reports the GPU at about 48 °C under load and the CPU at 52 °C. In CS2, the GPU sits at 34 °C and the CPU at 45-50 °C. No indoor baseline is given, so the outdoor radiator's share of those numbers is unknown.
That is the useful lesson. A normal custom loop keeps its radiator in the case or the room, so every watt the PC burns ends up in the air you breathe. Moving the radiator outside removes that heat from the room, which was the stated motive, and any extra headroom for overclocking is a side effect. It also ties the setup to the weather, and it turns a cooling problem into a control problem, since the system must warm itself up as much as it cools down.
The poster's next target is the number two spot on 3DMark's Steel Nomad benchmark for their card.