German overclocker der8auer demonstrated that a 110cm 3D-printed chimney can passively reduce Ryzen 7 9800X3D CPU temperatures from 90°C to 71°C. By exploiting the “stack effect” to create a pressure differential, the experiment proved that extreme vertical height can replace radiator fans, though the required footprint remains impractical for standard desktops.
PC building lore often suggests that pointing top fans outward harnesses a natural chimney effect to exhaust heat. In reality, this benefit is virtually nonexistent in standard chassis. According to calculations by der8auer, a typical 55cm tall case—assuming a 20°C room and a 40°C interior—generates a pressure difference of only 0.4 Pascal. He compares this value to the pressure one would feel under a layer of water four hundredths of a millimeter deep, which is thinner than a human hair. der8auer notes that this is functionally nothing.
The Stack Effect and the 110cm Tower
To test if natural convection could actually move air through dense cooling fins, der8auer built a modular chimney using 3D-printed funnel segments. The goal was to exploit the stack effect, also known as the chimney effect. This principle relies on differences in indoor-to-outdoor air density, which are dictated by differences in temperature and humidity. In this system, hot air not only rises but, if guided through a tube, creates more pressure that effectively draws air into the tube. The bottom of the chimney becomes a low-pressure zone, sucking in cold air that escapes through the low-pressure exit at the top.
To isolate these variables, the test rig used a Ryzen 7 9800X3D processor with heat output pegged at a constant 100 watts. The test system continued to use a water-cooling pump to circulate coolant, but the radiator fans were removed, and the chimney was mounted directly above the radiator to create airflow through natural convection.
The experiment began with a bare 240mm radiator. Without fans, the coolant settled at 61.5°C after 80 minutes, and the CPU peaked above 90°C. Visual tests with a fog machine showed that a radiator cannot “pull” air without fans attached to it, as the vapor drifted around the fins rather than being pulled through them.
| Tower Height | Coolant Temp | CPU Temp (HWiNFO) | Airflow Observation |
|---|---|---|---|
| 0 cm (Bare) | 61.5°C | >90°C | Fog drifted around fins |
| 10 cm | ~61.0°C | Not recorded | Marginal 0.5°C drop |
| 30 cm | ~56.5°C | Not recorded | Loop dropped ~5°C over 30 min |
| 110 cm | 50°C | 71°C | Fog visibly pulled upward |
The results showed that height directly impacts performance. With just a single segment stacked, there was a clearly noticeable temperature change.
The second segment created a draft, which was visualized using the smoke machine. The most dramatic shift occurred when der8auer goes for broke with the full stack,
adding four 20cm sections to reach a total height of 110cm, nearly touching the room’s ceiling. Within five minutes of assembly, water temperatures plunged to 50°C, and the CPU temperature dropped to 71°C, resulting in a 19°C temperature drop on the CPU.
Comparing Passive Convection to Mechanical Pressure
The experiment serves as a physical demonstration of why modern PCs rely on mechanical fans. While the 110cm tower successfully synthesized a pressure differential to move air, it did so by sacrificing spatial practicality. Mechanical pressure serves as a better system to design computers around than natural buoyancy. Fans achieve the same result—moving air through dense radiator fins—without requiring a full meter of vertical clearance above a desk.
This isn’t the first time the industry has flirted with passive verticality. According to reports from Hackaday and USAGoldMines, the principle has been used in the passively-cooled Power Mac G4 Cube and the SilverStone Raven series, such as the RV02, which rotates the case 90 degrees to better align with the natural rise of hot air.
Practical Limits of the Chimney Mod
Despite the 19°C drop, the conclusion remains that active cooling is the only viable path for high-wattage consumer hardware. The “big tower” is a raging success as a physics experiment, but a failure as a consumer product. To achieve the necessary airflow to cool a high-end processor without fans, a user would essentially need to redefine the meaning of a tower PC by extending it to the ceiling.
The findings suggest that for most builders, focusing on optimizing fan curves and intake placement is far more effective than banking on natural convection. While the stack effect is real, it requires a scale of verticality that is incompatible with standard workspace ergonomics.
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