PC modder der8auer recently demonstrated that a 110cm 3D-printed chimney can reduce Ryzen 7 9800X3D CPU temperatures by 19°C without using fans. By exploiting the “stack effect,” the passive cooling system dropped temperatures from 90°C to 71°C, proving that natural convection can effectively dissipate heat if given enough vertical scale.
Modern PC cooling relies almost entirely on mechanical pressure—fans that force air across heatsinks. However, a recent experiment by der8auer challenges this norm by utilizing the stack effect, also known as the chimney effect. This principle relies on differences in air density caused by temperature and humidity, where warm air inside a tube creates a pressure difference that draws cooler air into the bottom of the system.
The 110cm Modular Chimney Design
The experimental setup utilized an AMD Ryzen 7 9800X3D CPU, a processor noted for running hot due to its 3D V-Cache design acting as a thermal blanket. To test passive dissipation, der8auer removed the fans from a 240mm AIO radiator and replaced them with 3D-printed modular funnel segments.
The mod was built in stages to determine the minimum height required for the stack effect to become viable. Adding another 20cm segment to reach a total height of 30cm resulted in a further 5°C decrease over a 30-minute stabilization period.
The most significant results occurred when the total height reached 110cm, achieved by adding four 20cm parts to the existing structure. At this height, the chimney was nearly as tall as the modder’s ceiling. This configuration dropped the CPU temperature from 90°C down to 71°C.
Visualizing Airflow with Fog
To prove that the system was actually moving air without mechanical assistance, der8auer used a fog machine. The visual evidence highlighted the failure of natural convection without a guide; without the chimney, hot air dissipating from the radiator simply fell back onto the components.
Once the 110cm tower was installed, the fog machine demonstrated that air was being actively pulled through the radiator and sucked upward through the chimney.
Thermal Data and Water Temperature
The experiment didn’t just track the CPU; it monitored the coolant temperature via probes in the fittings. The thermal drop was rapid once the full chimney was deployed. While the water was at 61°C during the 30cm test, it dropped below 55°C almost immediately upon installing the 110cm version, eventually hitting 50°C within five minutes.
| Chimney Height | CPU Temp Change | Water Temp / Observation |
|---|---|---|
| 10cm | -0.5°C | Barely functional |
| 30cm | -5°C (additional) | 61°C water temp |
| 110cm | -19°C (total) | Dropped to 50°C in 5 mins |
Practicality and Hardware Limitations
Despite the success of the physics experiment, the solution is entirely impractical for consumer use. Because the chimney requires massive vertical clearance to create the necessary pressure differential, it cannot fit inside any standard computer chassis. The entire setup had to be assembled on a test bench.
This creates a stark contrast between theoretical efficiency and real-world application. While the “stack effect” is a proven method of moving heat, the physical footprint required to replace a few 120mm fans is prohibitive. Wccftech noted that the height requirement makes it impossible to outfit the enclosures in a standard casing.
Precedents in Passive Case Design
While a 110cm 3D-printed tube is an extreme example, the principle of using rising hot air for cooling has appeared in commercial hardware before. Hackaday pointed to the Power Mac G4 Cube and the SilverStone Raven series (such as the RV02), which rotates the motherboard 90 degrees to better align with the natural upward flow of heat.
The experiment serves as a reminder that the goal of modern fans is simply to synthesize the same pressure differential that a tall chimney achieves naturally. By scaling the chimney to an absurd length, der8auer demonstrated that the “boring” efficiency of fans is essentially a compact version of a centuries-old architectural technique.
