A box fan is a compact, square device that uses a spinning blade to circulate air, producing a cooling effect. Its portable design allows you to place it in different spots, including the floor or in an open window. Depending on the level of heat outside the room, compared to the temperature inside it and whether you have multiple fans, you can face the box fan in or out of a window.
Using One Fan
If you have only one box fan, turn the fan in or out of the window in accordance with the outside temperature. If the temperature outside is cooler than the temperature inside the room, face the fan in the window so that it blows air in, which can usually be done at night and in the early morning when the air outside is normally cooler. Later in the day, as the outside temperature warms up, turn the fan to face out the window, allowing it to suck the room's warm air out, instead of drawing additional hot air in.
Circulating the Air
Because fans don't actually make the air cooler but simply redistribute the air around a room, open a window or door opposite the fan, when you have the fan facing in or out of the window, which allows the air to circulate through the room and, even if the air outside is warm, the fan may create a cooling effect through a breeze being formed. Also, if the fan faces out, the open window or door draws in fresh, cooler air from the hallway outside the room or from a lower level while the fan blades pull the room's warm air out.
Using Two Fans
If you have two fans available, you can use them for a cheap method of cross-ventilating the room. Open a window on one side of the room, and place a box fan in it, facing out. On the room's opposite end, open a window and install the second fan in it, facing in. One fan actively pulls in fresh air while the other fan draws out the stale, warm air already in the room.
Multi-level Fan Use
If you have a two-story house and you have multiple fans to work with, position the fans in windows both upstairs and downstairs. Place one or two fans in upstairs windows with the fans facing out. The fan blades will draw out the warmer air that is rising to the upper level from the lower level, moving it outside. Place one or two fans on the lower level in the window, facing in, so that fresh air is simultaneously drawn into the house.