A close up of fan blades.
A question on Hacker News came to Quartz’ attention last week. A user asked, assuming that the inside of a room is about 30°C (86°F) and the outside is less than 20°C (68°F), “Do I place the fan so that it blows air inwards or outwards?” The lively discussion that followed inspired us to try to find an actual answer.
In this case, let’s assume that the question is about people trying to be comfortable (as opposed to keeping computers or other technology working, which is a direction the Hacker News discussion went). At 30°C, even in dry heat, it’s fair to assume that anyone would be sweating. Sweating, of course, is the wonderful thermal adaptation evolution has bequeathed us: We humans are the best at it on the planet, capable of secreting water and salts from glands all over our bodies in order to cool down. But much more comfortable is to use technology to avoid heating up in the first place.
In this case, you want to get the air in the room blowing on you. “The sensation of [air speed]”—that is to say, air blowing over you—“improves thermal comfort in a cooling situation,” says Andrew Persily, an engineer focused on indoor-air quality at the National Institutes of Standards and Technology in Gaithersburg, Maryland.
When sweat evaporates into the air, it takes heat with it, he says, and we feel cooler. Even if we’re not sweating noticeably, our bodies are constantly generating small amounts of moisture to cool off. Air that moves over our bodies in any way—wind, an air conditioner, a fan—speeds up that evaporation process, making us feel cooler. (Obviously, there’s a limit, he says: if the air speed is too high, it can just be uncomfortable.)
Fans blow air directionally, and take in air from behind them. The other way of thinking about how a fan can cool off a room, Persily says, is through heat removal. You could presumably place a fan so it blows out a window as a means of removing heat from the room. However, this only works if the hot room is connected to a cool hallway. By sucking out the hot air from the room, the negative pressure would pull in more air from wherever it could; assuming (as in this scenario) that the temperature outside is lower than inside, you may get some cooler air seeping in through the window, but majority of the air the fan would pull in would likely be from adjacent rooms or hallways.
Plus, fans, like any electronic device, generate heat as they run. If the fan’s power supply is in the room (which, if the fan is battery-powered or plugged into an outlet in the room, it is), it may be adding ever so slightly to the already stifling temperature, although likely not to the point where anyone would notice.
Ultimately, in this scenario, the best way to keep the room feeling cool is to open the window and put a fan in it facing the interior. The cooler air from outside would be blowing onto sweaty bodies, helping them evaporate sweat even faster.
If we change the scenario, and imagine, say, a warm home on a hot summer day, the answer changes a bit. In this case, the inside of the room is likely already cooler than the outside. Again, assuming the main concern is to keep bodies in the room cool, the most important factor to consider is how air movement may help sweat evaporation.
If there’s a breeze outside, opening the window would allow that breeze in, which would help with sweat evaporation and cool people down. However, wind is not usually constant, and you may end up letting out some of the room’s cooler air. One way to maintain more constant air movement is to combine an air conditioner and a fan. If you’ve got an air conditioner in one room (like your living room) and trying to circulate it to another (like your bedroom), ideally you’d put a fan in between the two rooms with the air blowing into the warmer one, Persily says.
In the absence of an air conditioner, it’s best to place a fan in a position so that it’s blowing on the people in the room, but with the window shut.
“[Fans] don’t cool the room; they cool the body because there’s more air movement,” says Persily. And specifically because they’re not cooling the room, it’s useless to leave them on when no one is occupying the space. In that case, the fan motor may actually just be adding to the heat in the room.
If nothing else, remember: cooling off isn’t necessarily about the temperature alone. Sometimes, all you need is a little air movement.
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Summer is here, and that means heat. Whether it is caused by the sun or the sticky humidity, you can expect warmth as a near-constant companion over the next few months. On one hand, this weather is amazing because it means pool time, beach, or lakeside afternoons, and lots of days with the windows open. On the other hand, few people enjoy being too hot. Using a fan in your home can increase air circulation and help make your home feel cooler. When used with the air conditioner on a lower setting, fans swiftly move cold air room to room and help save on energy bills. Before buying a new fan for summer, read the following tips to ensure your fan will be used for maximum effectiveness.
Buying a new fan for warmer weather? Decide before you buy what you need from a fan, and where it will go in your space. Fans come in many shapes and sizes, meaning that it isn’t a one-model-fits-all mentality. Before you purchase a fan, take a moment to look at your room. How big is it? How tall is the ceiling? How many windows are there? Where do you want to set the fan – on the floor, in a window, or mounted on a wall? All of these factors can influence which fan you purchase. Spaces like living rooms, family rooms, and dens may benefit from a tower fan, pedestal fan, wall-mount fan, bladeless fan, or an air circulator fan. Bedrooms tend to be smaller, and have a variety of options (visit Choosing the Right Fan for Your Bedroom for more info.) There are multiple fans that can suit your needs, and placement is just as important as fan type or size. (For more info on the best types for hot weather, visit 5 Types of Fans to Use in Your Home This Summer.) Knowing where you want to place your fan can inform the fan you buy.
A basic science lesson is that cold air is naturally heavier than warm, meaning it will sink to the area near the floor while the hot air occupies the higher space. If you have a room with no breeze, this is how the temperature will settle 100 percent of the time. The good news, however, is that even cold air is relatively light and easily influenced by outside sources. Placing a fan in a room will disrupt that balance and can force the cooler air to go where you want it to be. However, fan placement matters. Place your fan so that it faces the opposite wall from where most of the activity takes place in your space. This approach will drive the air to the surface, where it will bounce off, mingling with the rest of the air and cooling the space. Ideally, if lower temperatures are your main goal, you may want to choose a fan that’s lower to the ground so that it scoops up all that refreshing chilliness along the way.
There’s nothing like a fresh breeze, but opening up a window for fresh air when it’s hot outside can be anything but pleasant. Too much heat, too many bugs, and too much noise can ruin the effect. A box fan or a window fan solves the problem. A box fan (like the Weather-Shield® Select 20″ Box Fan with Thermostat) can be used on the floor or in a window. Positioning the fan in a window circulates fresh air faster, and cools down the intense heat of a room. The quiet settings create a gentle white noise that can drown out street noise, and circulated air discourages bugs. Box fans with Weather-Shield® can be used rain or shine; these fans come with an isolated fan motor that protects from rainy weather. (Although it is recommended you move the box fan from the window during heavy rains and winds, since the wind can blow water into electrical components and damage your fan.) Window fans work even better at circulating fresh air from outside. Fans like the 16″ Window Fan with E-Z-Dial Ventilation
fit securely and tightly in the open window, keeping all bugs outside where they belong. A twin fan, like the Electrically Reversible Twin Window Fan with Remote Control, can simultaneously pull fresh air in while pushing out stale air, creating circulating air that’s crisp and clean.
The furniture in your space will help in determining where you should put your fan. Placing a table fan or a window fan will depend on the room’s décor and arrangement. Table fans will need a clean flat surface close to an electrical outlet to operate. If the cord of the fan impedes walkways or causes a tripping hazard, it’s not the right placement. Window fans should not be blocked by furniture, and cannot help circulate air if a sofa or curtains are in the way. Always point fans away from tables or desks with loose papers or light objects; picking up a mini-tornado of paperwork isn’t relaxing. Fans are most effective when they have an unobstructed pathway in front of their blades. A wall-mount fan is placed high above any air obstacles. Many come with a tilt-head feature (like the 12″ Oscillating Wall-Mount Fan with Anti-Rust Grills and the Air Flexor® Remote Control High Velocity Fan) so that the force of the fan can be pointed in the direction where it is most needed. Do your best to give your fan this path so that it may carry the air farther, increasing its effectiveness in your house.