The interior of your house is hot, the exterior cool. What would the most efficient orientation be for a box fan? Pushing hot air out of a window or pulling cool air in through it? (Solved)*

Secondarily, should the fan be outside or inside the window?

Asking for a friend. (Me, help. I can’t remember physics, it’s too goddamn hot)

Edit: I’ve opened all the windows and set up a fan a short distance from the window pointing out and it’s reduced the temperature to near-tolerable levels. Unfortunately, venting through the attic was a no-go but moving things aside might make it an option next time. Thank you all, I appreciate the help!

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Well, the folks over at noctua have fan shenanigans down to a fine science, and since I have generally forgotten the last rabbit hole I went down, I'd say get two fans and have one blowing in and the other out, lol.
- This is the wrong application, noctua are looking for static pressure, what OP wants is to maximise airflow. Mathias wandel has some pertinent videos on the subject, the ideal is a large fan about a metre or two away from the window, depending on the size of the fan, blowing hot air outside.
  
  Why so far from the window?
  
  I think you get the benefit of the airflow pulling in air along its edge to push out with it.
  Like pushing a bunch of water with a broom removes not only the water in front of the broom but also more water due to surface tension and inertia
  
  It's called the venturi effect.
  
  It's called the linguini effect.
  
  Ohhhhhh the bernoulli effect?
  
  no no, it’s the Magnum effect.
  
  Good ole vermicelli effect
  
  In this case it's called the brunelli effect
  
  Ah yeah, the Dom Mazetti effect
  
  Its called the Chimichurri effect
  
  The Bozzetto effect?
  
  It's called the kimchi effect
  
  It’s called the butterfly effect.
  
  Oh yeah the bruscietta effect
  
  It creates an effect where surrounding air is pulled with the stream due to the Bernoulli effect. It's the same thing that pulls shower curtains in when you shower. If the fan is a bit inside the window, surrounding indoor air gets pulled into the stream, moving more air out in total.
  
  Most of the replies are correct overall. But Mattias Wandel had identified another important factor. Fans don't just "stuck from the back" and blow out the front. They sort of suck from everywhere including the edges. This means that if you're putting a fan right up against the window blowing in you're sucking some room air as well. If you're putting the fan right up against the window blowing out, some of the air you're blowing out comes from outside in the first place.
  The ideal is then to blow out from 2-5ft away. This way all the air that's blown is room air, and it's all blown out. The venturi effect plays a small part. The Bernoulli effect? Equations? Explains why the air stays in a "tube" for a few feet away from the fan. Technically also explains the venturi effect..... Everything is Bernoulli....
  https://www.youtube.com/watch?v=1L2ef1CP-yw
  
  This was a wonderful comment to read and video to watch, thank you
  
  A fan creates kinda long "channel" in front of it and behind it, where the air is moving rather fast (at the speed that the fan creates). Imagine the narrow beam of light from a flashlight. You want that channel to go right through the window to have a good amount of air flowing.
  I don't know that you need to keep a distance from the window. I think 1-2m is just more practical than having the fan right inside the window frame.
  
  do you happen to know whether this is significantly different for a blower-style fan that has its air intake on one side? for example
  https://lasko.com/collections/air-movers-blowers/products/lasko-12-pivoting-pro-high-velocity-utility-blower-fan-with-3-speeds-u12104-gray
  
  I don't know the type, so I am judging it from that picture.
  It shows a rather small area where the air comes out. I imagine that it creates high speed, but the "channel" would have the exact same size and shape as that small area where the air comes out. So I'd consider it as less effective.
  A fan with a large diameter could probably move more air.
  
  No, it should be roughly the same.
  The thing you're trying to take advantage of when placing a fan a meter or two from a window is that when you have a moving mass of air and it passes by static air, it "drags" the static air with it. As a result you move more air mass that what was blown by the fan directly out of the fan blades.
  A blower fan still blows air in a direction, and can likely take an even bigger advantage than a traditional box fan since a blower fan's output direction will be more orderly aka closer to a laminar flow than a box fan. But the real advantage of the blower style fan is that its enclosure allows you to direct both the intake and exhaust portions of the air being moved.
  
  Same reason why blowing on your hand close to your mouth is warm, but farther away is cool.
  
  Can you explain that? With one fan pushing air in, and one fan pushing air out, you should be achieving higher airflow than you would with a single fan pushing air either in or out. Especially if you only have the one window open. I know that when I open windows on both sides of my house, I get better airflow than if I have windows on only one side open.

59 comments