: Why do you feel cold air from an air-conditioned room coming out under a closed door into a warmer room or corridor?

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Room has air-conditioning going, cooler than corridor. Much cooler.

I can feel the cold air coming out from under the closed door when standing in front of it.

Why?

In: 3

Cold air is denser that warm air, and it leaks under the door in the same manner that water would leak under the door

The AC unit is causing positive pressure within the room. Positive pressure is when the pressure is greater than ambient. Since the pressure is higher in the room with AC, the air is constantly trying to equalize the pressure between the room and hallway. To do so it rushes out of the room through any crack or hole it can.

Cool Air sinks to the bottom of the floor. And since your AC exhaust usually isn’t placed on the floor the newly cooled air sinks on top of the already cool (but a bit warmer air since the floor is reheating the air). As such you get a high pressure zone as the coolest air is trying to sink down onto the ground. This squeezes the cool air which is already on the ground through the small gap below the door into the warmer hallway.

Also worth noting that air will naturally spread out and mix so you’d expect a cooler zone next to the door where there’s a small gap (providing there’s little circulation in the room to break up the air).

Do you have central air conditioning? If so then somewhere in your house, usually near the thermostat, there is an air intake that brings room air into the AC evaporator to cool. Then the cool air blows into the room and has to make its way back to the intake.