A question on Hacker News came to Quartz’ attention last week. A user asked, assuming that the inside of a room is about 30°C (86°F) and the outside is less than 20°C (68°F), “Do I place the fan so that it blows air inwards or outwards?” The lively discussion that followed inspired us to try to find an actual answer.
In this case, let’s assume that the question is about people trying to be comfortable (as opposed to keeping computers or other technology working, which is a direction the Hacker News discussion went). At 30°C, even in dry heat, it’s fair to assume that anyone would be sweating. Sweating, of course, is the wonderful thermal adaptation evolution has bequeathed us: We humans are the best at it on the planet, capable of secreting water and salts from glands all over our bodies in order to cool down. But much more comfortable is to use technology to avoid heating up in the first place.
In this case, you want to get the air in the room blowing on you. “The sensation of [air speed]”—that is to say, air blowing over you—”improves thermal comfort in a cooling situation,” says Andrew Persily, an engineer focused on indoor-air quality at the National Institutes of Standards and Technology in Gaithersburg, Maryland.