The stack effect — also known as the chimney effect — is the basic principle that hot air not only rises, but if guided through a tube, the rising hot air will create more pressure that will effectively draw air more effectively into the tube. This is not only great for a chimney, but also a useful principle if you seek to cool something like a PC in a more passive manner. Although there’s a lot of fun physics behind the stack effect that you can run the numbers on, the more practical demonstration here using 3D printed funnel segments for the radiator and various thermometers provides a very hands-on feeling for what you can expect from this approach. After this he goes for broke with the full stack and a resulting 19°C temperature drop on the CPU. The basic idea of making use of the fact that hot air rises, and maybe also banking on the stack effect for some free passive cooling, clearly isn’t so crazy.