I live in an upstairs apartment with very small windows in the bedrooms. In the summer, it can be over 10 degrees hotter in the apartment than it is outside. I shopped around extensively for a duel fan that would fit in our 13.5" window frames. Almost every other brand is at least 14". This fan fits in the window perfectly! Each fan has independent controls, so that they can be set at different speeds, turned on or off independently, and set so that one intakes and one exhausts to circulate the air. Both fans can do either intake or exhaust. There is also a rotatable grate in front of each fan to direct the air flow. It has a thermostat which will turn off the fans when the desired temperature is reached and back on when the room gets too warm again. I bought my first one almost four years ago, and it stayed in the window constantly from late spring through fall. I let the thermostat keep the animals cool while I was away without having to worry that they would get too cold. After four years of faithful service, the right side fan is beginning to squeak a bit, but it still runs fine. I purchased my second fan last year, and it comes with a bracket that you can screw into the window casing which will allow the fan to be hung sideways in the window, allowing the accordion-style side panels to close off the gaps in Chicago-style windows that one of the other reviewers complained about. I'm so happy with this fan that it is already on my wish list again so I can put one in the other window!Get more detail about Window Fan.