Modder der8auer recently demonstrated that a 110cm 3D-printed chimney can drop CPU temperatures by 19°C without using fans. By exploiting the “stack effect,” the experiment successfully cooled an AMD Ryzen 7 9800X3D by creating a natural pressure differential that pulls cool air through a radiator.
The notion that pointing top fans outward helps hot air escape is a common piece of PC building advice, but it often ignores the actual physics of the chimney effect. While hot air rising is simply convection, a true chimney works by trapping that rising air in a tall channel, which then drags cool air in from the bottom. The taller the channel, the stronger the pull.
Most PC cases lack the height to make this effect meaningful. According to calculations shared by der8auer, a case 55 cm tall in a 20°C room with a 40°C interior produces only 0.4 Pascal of pressure—a force thinner than a human hair. To find a measurable result, the modder turned to 3D printing and extreme height.
The Ryzen 7 9800X3D Test Rig
To isolate the variable of airflow, der8auer used a Ryzen 7 9800X3D CPU capped at a constant 100 watts. This ensured the heat entering the water loop remained stable. The setup featured a 240 mm radiator with no fans, propped up to allow air access from below. Initially, without any chimney structure, the water settled at 61.5°C, and the CPU peaked above 90°C.
A fog machine was used to visualize the airflow. In the baseline state, the fog drifted around the radiator fins rather than through them, proving that a radiator cannot effectively pull air on its own without mechanical assistance.
Scaling the 3D-Printed PLA Tower
The experiment proceeded in stages, using 3D-printed PLA shrouds to incrementally increase the height of the airflow channel. Each addition provided a measurable, though varying, impact on temperature.
| Tower Height | Temperature Impact | Observations |
|---|---|---|
| 10 cm | -0.5°C | Barely functional; unexpected slight drop. |
| 30 cm | -5°C | Temperatures dropped over a 30-minute stabilization period. |
| 110 cm | -19°C (CPU) | Water hit 50°C within five minutes; fog visibly pulled through radiator. |
The final assembly consisted of an 80 cm block of four sections added to the previous 30 cm, bringing the total height to 110 cm. At this height, the water temperature plummeted to 50°C, and HWiNFO recorded the CPU temperature dropping to 71°C.
The Mechanics of the Stack Effect
Also known as the chimney effect, the stack effect relies on differences in air density caused by temperature and humidity. This creates a low-pressure zone at the bottom of the chimney, which sucks in cold air that then escapes through the top.
This is essentially what modern PC fans do: they synthesize air pressure to create a pressure differential. The experiment proves that while natural buoyancy can be catalyzed into an effective cooling system, it requires a massive amount of vertical space to compete with mechanical pressure.
Passive Cooling Precedents and Trade-offs
While a 110 cm tower is impractical for most desks, the principle of using hot air’s natural ascent is not new. The Power Mac G4 Cube and the SilverStone Raven series (such as the RV02, which rotates the motherboard 90 degrees) have both employed similar logic to enhance passive airflow.
The trade-off, however, is space. To achieve a 19°C drop, the user would need over a meter of vertical clearance. Mechanical fans achieve the same pressure difference without the need for a towering PLA structure.
Practicality vs. Physics
The experiment serves as a successful proof of concept for the stack effect, but the conclusion remains pragmatic.
The results highlight a critical reality of PC thermals: natural convection is a weak force in small enclosures. While the “chimney effect” is a real physical phenomenon, its utility in consumer electronics is limited by the physical constraints of the home office. For those interested in extreme modding, the project demonstrates that significant passive gains are possible, provided one has the ceiling height to accommodate them.
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