Hi. On a hot day should you put a fan in the window
facing out, to blow the air out or in, to blow air around inside? Thanks!
If you face the fan out, you’ll get a nice air exchange, provided that you have two openings: one to let the old air out, and one to let fresh air in. This is most effective if there is a reasonably straight shot from the in window to the out window, and if there are not a lot of other windows open at the same time. The idea is to create a forced stream of air. You can test whether it’s working or not by waving some smoke nearby. It should move quickly toward the out window.
Alternatively, you can just turn the fan toward yourself to increase the evaporation of your perspiration. Blowing the air around inside will also help in this way. If the humidity is low, it might help a lot. Sooner or later, though, I think you’re going to want some fresh air.
This question came up often during college. (ahem 
)
Facing the fan out of the window acts as an exhaust fan, pulling air (and perhaps some unwanted gases) out of the room which will be replaced by “fresh” air. Of course, this works best if there is another window open, as explained by Peregrine.
This method is better than blowing air into a room to exchange air. This is the same principle in use with whole house fans. Hot, funky air is pulled out of the house via the fan and replaced with cool fresh air from outside.
If you grew up in Virginia (or some other place in the hot, humid South), you’d know that you turn the fan so that it blows out and then you open another window to get cross ventilation, BUT YOU ONLY DO THIS AT NIGHT. If you do it during the day you’re pulling the air out of the house through one window and hot, humid air in through the other. During the day you take the fan out of the window and put it in the floor to circulate the air.
I’ve found that using the fan to draw in air (at night, when the air is cool) will cool down the room faster. However, it will only effect the room where the fan is.
Using the fan to exhaust will take longer to cool down, but it will cool down the entire house.
So it’s your call. If you’re in a one-room apartment, it’s probably better to draw the cool air in.
You have to work with the wind as fighting it could stall the air. With that in mind as long as the temp outside (ignoring humidity) is cooler then inside put the fan in the window. When temp increases, close the window (and the blinds to block the sun) and have it blow on/around you. Then when it starts cooling again put it back in hte window.
I find that facing the fan toward the room gives the most immediate cool relief. I’ve watched the thermometer drop significantly faster that way. My tests have been with a 20" Lasko box fan.
Facing it out might work best if the fan seals off the window and creates suction throughout the room or house, ideally with another window or door as the intake. It also depends on the nature of the airflow; whether it’s in a straight line or has to bend around corners, and the total volume of air you’re trying to cool.
The power of the fan also matters, of course. You need strong suction to move air out of a room fast enough to get quickly noticeable cooling. Using smoke (maybe an incense stick) to display the airflow would be a good test.
Keep in mind that drawing in cool air will make the existing warm air cooler, like pouring cool water into warm water, and all the while, warm air will still escape around the fan if it doesn’t seal the window. I’ve had unsatisfactory results with aiming fans outward, but I know it works in industrial settings where the fan seals an opening and creates suction, like a bathroom fan removing steam. Or, your whole house fan example. It’s all about context and specific equipment.
Just a technical note: This thread has only 7 responses, but over 25,000 views. This is a really hot topic!
And it only took 14 years to do it!
To cool multiple rooms at night, blow air out; seal around the fan, and open windows in the other rooms you want to cool.
With only one room to cool, take your pick, whether you want the fan blowing on you or a more gentle airflow. In either case open another window somewhere.
Anyone who lived in hot climates before air conditioning knows this.
When I lived in a hot climate (with no A/C) we, and everyone else, had ceiling fans. Houses were designed to be cool and had wide verandas and lots of windows. Yes we had to put up with the flying bugs, but they were easily dealt with.
Why is pulling the air out more effective than pushing the air in?
It isn’t. Go back and read plnnr’s response.
? He clearly says to blow the fan out (at night).
It depends; IF you have another open window or two you will draw the cooler night air into the house and exhaust the warmer air which will tend to cool the entire house. Ideally, you would want two fans, one blowing cool air into the house, one exhausting the warm air. It really depends on the layout. I’ve used two fans, one to blow cool air from the basement up and then out of the master bedroom at the far end of the house and it worked well enough.
As noted above, for immediate relief or a smaller room like an apartment it might be better to blow cooler outside air in, or circulate it until the outside air is cooler than the air in the room.
I’m not explaining myself very well, sorry. 
I can easily see why two fans–one blowing in, one blowing out–would be be better than other combos, or edge cases (like trying to cool just one room) where one set-up is better than another.
Given a situation where you have one fan with two open windows and multiple rooms why is it generally considered better to have the fan blow out? Is there something about fluid dynamics that makes pulling better than pushing?
What about a reversible ceiling fan? Push air down toward you or pull it up to the ceiling? I apologize if this sparks another 14 years of replies…
The key is to do both. One fan pulling air out and another at the opposite end of the house pushing it in. I used that for years-- works great. Fans are cheap, so no reason to limit yourself to just one!