Window fans are fundamentally different from other types of electric fans. While most fans can only cool you directly with a stream of moving air, window fans can actually cool a room by either bringing in cool air at night or exhausting hot air. The key distinguishing feature of a window fan is whether it's manually or electrically reversible. A manually reversible fan must be removed and flipped around in the window to change the direction of airflow, while an electrically reversible fan can change airflow directions with the flip of a switch. Some have two sets of blades that can run independently, so stale air can be exhausted and fresh air pulled in at the same time.
One problem with window fans is that many won't fit in a window unless the screen is removed. That means when the fan isn't running, small insects can get in through the spaces in the fan's grille. To prevent this, you may need to either cover or remove the fan when it's not in use.
We found fewer reviews for window fans than for other types. They're not covered in TheSweethome.com's comparison test, so we had to largely rely on user reviews from retail sites. These steered us toward the Bionaire Remote Control Twin Window Fan (Est. $55) as the best window fan. This electrically reversible, three-speed fan has two blades that can be controlled independently, so you can draw air in with one and exhaust it with the other. It also includes a programmable digital thermostat, which shuts the fan off automatically when the air in your room reaches your preferred temperature. You can control it through a panel on the front or with the remote control, allowing you to adjust the speed or temperature without getting out of bed. The fan is backed by a 5-year warranty.
Users at Amazon.com and Costco.com say this Bionaire window fan is quiet and draws in a good amount of air. They also say it fits securely in most windows, thanks to a sliding panel that adjusts its width from 24 to 37 inches. However, some reviews warn that this fan can't be used in casement windows.
Reports of this fan's durability are mixed. Some users say their fans have given them years of reliable service, but others report a variety of problems. The main one is that the thermostat tends to malfunction on account of a misplaced wire that heats up the fan's internal circuit board, causing the thermostat to heat up and shut itself off. It's possible to fix this by unscrewing the front panel and moving the wire, but it's a nuisance. Many users also say their fans failed within a few weeks or months as a result of blown fuses. Replacing the fuse is difficult, as it's hard to find the proper size, and some owners say it just blows again almost immediately. Users who contacted Bionaire's customer service line for help say the reps are friendly, but getting a replacement unit or parts often takes months because the company doesn't have them in stock.
The Lasko 2155A Electrically Reversible Window Fan (Est. $60) is also electrically reversible, but it has only one 16-inch blade instead of two, so it can't be set up for air exchange. Like the Bionaire fan, the Lasko is adjustable, fitting windows 26.5 to 34.5 inches wide and at least 22 inches high. The main feature that sets it apart from other window fans is its unique StormGuard, which allows the window to be closed without removing the fan. Some owners like this feature, but others fear it might make it difficult to remove the fan from the window in an emergency like a fire.
Owners at Amazon.com say this Lasko window fan produces great airflow, both for pulling in cool air and for exhausting heat and smoke. They also find its simple dial control easy to use. As for its noise level, users generally say it's fairly quiet on the low speed settings but can get loud on high. There are also numerous complaints about its durability, ranging from flimsy construction to electrical problems. The fan comes with a warranty, but it's only 1 year long.
A less expensive alternative is the Holmes HAWF2021 Twin Window Fan with Reversible Air Flow Control (Est. $20). Like other window fans, the Holmes is adjustable, though it can only fit windows up to 33.25 inches wide. Although it has twin blades, it's not electrically reversible, so the only way to switch from intake to exhaust is to pull the fan out and flip it – a feature many users find annoying. They also say it can be noisy and hard to clean, and it gets its share of complaints about durability. However, most users agree that the fan delivers good airflow at a budget price. It's backed by a 1-year warranty.