Water-Cooled PC Sends Its Heat Outdoors Through a Valve-Controlled Radiator

Most PC cooling problems end at the case. This one ends at the wall. A Reddit user, u/Motor_Attention1865, got tired of his high-end workstation turning his room into a sauna, so he built a split cooling system that pushes the heat outside. It works a lot like a mini-split air conditioner, except the heat source is a water loop and the control logic is a homebrew Arduino setup. It's an unusual project, and the engineering choices behind it are worth a closer look.

Why the room was the real problem

The room already had heated floors, which meant the space was never going to cool down on its own in winter. The PC was adding heat right where the user sat, pooling under the desk even when outdoor air was 25 °F (-4 °C). His workaround was to crack open a window and regulate the temperature by hand. That works until you're gaming for hours or running a benchmark, and then it's a chore.

He chose to treat the PC's waste heat as something to be moved out of the building rather than simply vented. That's a reasonable framing. A water loop already carries heat away from components efficiently, so the question becomes where to reject it. Sending it through a radiator mounted outside removes the heat from the room entirely, instead of relocating it.

"Got tired of the pc heating the room, so i solved it sensibly.." from r/pcmasterrace

The hardware inside the case

The internal build is aggressive on its own. The CPU is a delidded Core Ultra 7 270K Plus with a Thermal Grizzly 1851 Mycro Pro direct-die water block, using Thermal Grizzly Conductonaut Extreme liquid metal between die and block. Delidding removes the integrated heat spreader, which shortens the thermal path and typically lowers CPU temperatures under load. That matters here because the outdoor loop is only useful if the internal loop can deliver heat to it efficiently.

The GPU, an RTX 4070 Ti Super, runs on a Bykski water block. The memory is also liquid-cooled: 2x24GB of G.Skill DDR5-8400 uses Bykski heat spreaders with an IceManCooler two-channel water block. Inside the case, two radiators handle the first stage of heat rejection, a 120mm unit at the back and a 360mm unit at the front, all circulated by an EK Kinetic D5 pump-reservoir.

The outdoor loop and the valve that decides everything

The external half is where the project gets interesting. It uses a 240mm radiator with two Noctua iPPC-3000 fans, which is a sensible size rather than an exotic one. The radiator itself isn't the novelty. The control system is.

A Bykski D5 pump and reservoir sits in a box next to the PC and pushes coolant outside. But the outdoor radiator isn't in the circuit by default. A motorized three-way valve decides whether coolant takes the path through the outdoor radiator or stays inside. Until the PC needs extra cooling, the valve keeps the loop indoors, which avoids pointlessly chilling the coolant when the room is already cool enough.

On startup, the PC runs a Python script that talks to an Arduino Mega. The Arduino reads three temperature probes on the coolant line and three temperature-and-humidity sensors that monitor the case and the ambient air. Based on those readings, it sets the valve position. That's a small, cheap control stack doing the job of a commercial HVAC controller, and it's a reminder of how far hobbyist microcontrollers can go when the control problem is well defined.

Condensation is the real engineering constraint

Cold is not the hard part. Condensation is. If coolant or any surface inside the loop drops below the dew point of the surrounding air, water forms on it. In a PC, that water can short components, which is why the dew point calculation is central to the whole design.

The Arduino estimates the dew point from the humidity and temperature readings inside the case and in the room. The modder set the controller to close off the outdoor radiator when the coolant approaches 10 °C above the dew point, or 5 °C during benchmarking. That margin lets the loop run well below freezing while keeping the coolant warm enough that moisture doesn't form in the PC.

The margin is a sensible way to think about the problem. Any system that pulls heat outdoors in cold weather has to manage this trade-off. Run the coolant colder and you get more cooling but more condensation risk. Run it warmer and you're safer but less effective. A fixed rule of thumb won't cover every climate, so a sensor-driven threshold like this one is the practical approach.

Coolant chemistry for sub-freezing operation

Water-based loops that run outdoors in winter need antifreeze. The builder added pure ethylene glycol to the Aquacomputer Double Protect Ultra Clear coolant, lowering its freezing point from -18 °C to -28 °C. That gives some headroom for sub-freezing ambient temperatures, although the margin still depends on how cold the outdoor radiator actually gets. Any loop that goes this route should be checked for compatibility with its tubing, fittings, and wetted metals, since glycol mixtures can behave differently from the coolant the manufacturer originally designed for the loop.

What the numbers show

The reported results are the most useful part of the project for anyone thinking about replicating it. According to the builder, the system keeps temperatures below 50 °C under heavy use. The RTX 4070 Ti sits at around 37 °C and the CPU at roughly 45 to 50 °C while gaming.

Benchmarks run hotter, which is expected. In Cinebench R23, the CPU reached 52 °C, and in 3DMark Steel Nomad, the GPU hit 48 °C. Those are still comfortable figures for both parts. The takeaway is that the outdoor loop keeps the hardware in a sustained, controlled thermal range rather than a dramatic one. The reported figures are what the builder measured, not independent test results, so they're best read as a single well-documented case.

What to take from it

Here's the thing: the cooling hardware is ordinary. Pumps, radiators, fans, and water blocks are all off-the-shelf parts. The original contribution is the control layer, a sensor-driven valve and a dew point guard that let the system run safely in conditions where a fixed loop would either freeze or sweat. That's the part that would take the most work to replicate well, and it's also the part that determines whether a setup like this stays reliable through a full winter.

For most readers, a project like this isn't a practical recommendation. It requires plumbing, a controller, multiple sensors, and a willingness to mix your own coolant. But it points at a real idea: a high-power PC is a heat source, and in a cold climate, that heat can be routed somewhere useful, or at least somewhere other than your living space. The builder's own summary captured it best. He no longer has to open a window when the PC is under load. It's a small thing, and it's a good example of solving the problem one level up from where most people look.