Crystal ball: Some pieces of PC building advice have circulated so long that nobody bothers checking them anymore. One is the chimney effect: the claim that a case works like a flue, and that pointing your top fans outward gives hot air a natural boost on the way up. German overclocker der8auer decided the best way to test it was to actually build one.
Building one meant understanding what a chimney actually does, which isn't quite what the forum posts assume. Hot air rising is just convection, and it happens around anything warm. A chimney works because that rising air is trapped inside a tall channel, so as it climbs it drags cool air in behind it through the bottom. The taller the channel, the harder it pulls.
A normal case doesn't have much height to work with. der8auer ran the math on a case 55 cm tall, assuming a 20°C room and a 40°C interior, and landed on 0.4 Pascal. He compares that to the pressure you'd feel under a layer of water four hundredths of a millimeter deep – thinner than a human hair.
Since that's functionally nothing, he went hunting for height. His test rig ran a custom water loop around a Ryzen 7 9800X3D, capped at a constant 100 watts so the heat entering the loop stayed the same throughout.
The 240 mm radiator had no fans attached and sat propped up off the bench so air could reach it from below. After an hour and twenty minutes, the water settled at 61.5°C, and fog blown underneath drifted around the fins instead of through them.
Onto that stalled radiator, der8auer began stacking 3D-printed PLA shrouds. The first was only 10 cm and still shaved off half a degree, which he admits he didn't expect. A 20 cm section brought the tower to 30 cm total, and between the two the loop gave up around five degrees over the next half hour while the room temperature never moved.
Then came the big one: an 80 cm block of four sections that brought the whole assembly to 110 cm and nearly reached his ceiling. Water temps fell below 55°C before he'd finished bolting it together, hitting 50°C within five minutes. This time the fog got visibly pulled up through the radiator, exactly as if fans were running above it.
Those water numbers matter for what they do to the chip underneath. The Ryzen had been peaking above 90°C with the radiator bare; HWiNFO put it at 71°C once the tower was standing, a swing of nearly 19 degrees with no fan moving air anywhere.
Turns out, to no one's surprise, fans are still the sensible option, as der8auer himself concludes. They chase the same pressure difference without demanding a full meter of vertical clearance above your desk.
Der8auer previously converted a $7,000 RTX 5090 Lightning Z to custom water cooling.
der8auer tests PC chimney effect, dropping CPU temps by 19°C with a 110cm 3D-printed tower
