Placing a fan in an open window is a well-known trick for cooling a room, but the direction in which you position it determines whether that trick actually works. The answer depends on the time of day and where the cooler air is located at that moment.
During the day: blow warm air outside
If it is warmer outside than inside, there is little point in blowing that hot air inside with a fan. Instead, place the fan with its back to the window, so that it exhausts the warm indoor air outside. In another part of the house, preferably on the shaded side, crack open a window so that cooler air can flow in. This creates airflow instead of just circulating warm air.
In the evening and at night: draw cool air inside
As soon as the outdoor temperature drops below the indoor temperature, usually in the evening or at night, you can turn the fan around. Position it with the front facing the window, so that it actively blows the cooler outdoor air inside. This works best if a door or another window is open on the other side of the room, so that the warm air has an exit and real air circulation is created instead of a dead-end airflow.
Two windows, even better results
If you have the option to open two windows on different sides of the room, a fan will work even more effectively. Place it at one window to blow air inside and use the other window as an exhaust. This cross-ventilation moves much more air than a single fan in a room with only one opening can achieve.
Pay attention to the time
The most common mistake is leaving the fan facing the same direction all day, even though the outdoor temperature can fluctuate considerably during the day. Check in the morning and evening whether the outdoor air is still cooler or warmer than the air inside, and adjust the direction of the fan accordingly.
Direct answer: during the day, when it is warmer outside, place the fan with its back to the window to exhaust warm air. As soon as it is cooler outside than inside, usually in the evening and at night, turn it around so that it blows cool air inside.
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