• If sweltering days and nights without central air conditioning threaten your summertime comfort, consider putting a unit in the window. Installing a window AC is within the capabilities of most do-it-yourselfers if you follow the 10 tips here. You’ll get the job done safely and swiftly—and will soon be saying, “Ahhhhh!” no matter how hot it gets.

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DON’T assume you’re permitted to install a window unit.

While most single-family homeowners are free to install a window AC, if you live in an apartment, condo complex, or a neighborhood with a homeowners’ association, check with management or the HOA board before investing in a unit. Some complexes don’t allow window ACs for aesthetic reasons (they’re not the most attractive additions to a building’s exterior) or because an improperly installed unit could fall from the window, creating a safety hazard.

DO assess your windows.

Not every window is suited for an air conditioner. Almost all such appliances are designed for a double-hung window, meaning there is a fixed upper pane and a lower pane that slides up and down. Sliding windows that move from side to side, and casement windows that tilt outwards to open, aren’t options for these air conditioners.

You’ll also need an electrical outlet that accepts three-prong plugs fairly close to the window; most window units have a cord only about six feet long. Never use an extension cord for window ACs. This could void the warranty on many air conditioners and present a fire hazard if the extension cord isn’t rated for appliances.

Finally, for best performance, consider the area directly inside and outside the window. An air conditioner should be in a shady or partially shaded window, because direct, intense sunlight can decrease the unit’s efficiency by as much as 10 percent as it struggles to handle all that hot air. Also, remember that the air conditioner requires sufficient clearance for unimpeded air flow, so choose a window free of furniture, trees, shrubs, walls, or other obstructions within 20 inches of the unit on the inside or the outside of the window.

DON’T neglect to check your measurements.

There are two sets of measurements you’ll need to choose the right window air conditioner: the height and width of the window opening and the square footage of the room the unit will cool. Normally, the air conditioner’s packaging will specify both the size range of acceptable windows and the maximum square footage the unit can cool, so make sure your measurements are within the guidelines for the unit you’re considering.

To measure the window, open it as far as it goes, and then use a tape measure to determine the inches both from side-to-side and from top to bottom.

If you don’t already know the square footage of the room you hope to cool, have a helper hold the tip of the tape measure against the wall as you extend the tape to the opposite side to get the length; then repeat on the other side of the room for the width. Then multiply the two numbers to obtain the square footage. For example, if your room is 11 feet wide and 12 feet long, it is 132 square feet.

DO choose appropriate power for your space.

You’ll need two other numbers, typically indicated on an air conditioner’s packaging, to purchase the right unit: energy efficiency rating (EER) and British thermal units (BTUs), which is a measurement of the appliance’s cooling ability.

EERs can range from 8 to 12: The higher the number, the more energy-efficient the air conditioner. The U.S. Department of Energy recommends an EER of 10 or more for the best efficiency and savings on your electric bill.

The larger the room, the higher the required BTUs to cool it. If the air conditioner isn’t powerful enough for the space, it will run continuously without reaching your desired temperature. Too powerful, and the unit will tend to cycle on and off too quickly, which won’t allow enough time to reduce room humidity effectively and leads to uneven temperature throughout the space. Generally, a window unit’s BTU rating will match the manufacturer’s recommended room size, but to calculate it yourself, try the online BTU calculators here.

DON’T attempt installation without a helper.

Window air conditioners typically weigh between 50 and 100 pounds, so maneuvering the appliance into the window and then holding it in place while it’s being secured with screws requires an assistant. Installing a window AC on your own is an easy way to injure yourself, or accidentally drop the unit out the window.

Do prepare the window and install brackets, if necessary.

Before installing a window AC, open the window as far as it goes, remove the screen, and clean the windowsill, glass, and window frame. Measure the width of the windowsill, and use a pencil to mark its center.

A properly installed air conditioner should sit securely in the window without an additional support bracket. However, very heavy units (those more than 100 pounds) do require additional support and some locations, notably New York City, require the use of a bracket for all window air conditioners regardless of weight. Generally, you’ll need to buy a support bracket separately. Most home improvement centers that sell window units also sell brackets, and you’ll also find them available online. Expect to pay from $30 to $100 for a support bracket.

There are different configurations of AC brackets, but most either resemble a small shelf or a pair of “legs” that extend slightly outside the window to help hold the unit firmly in place. 

DO attach mounting rails and side panels.

Some window units come with the top and bottom mounting rails—which help hold the air conditioner securely in the window—already in place. If your unit does not, use the included screws to attach the mounting rails, making sure to tighten all screws completely.

Next, attach the accordion-style side panels by sliding them along the grooves on each side of the air conditioner. When the entire installation process is complete, the panels will fill in the gaps between the air conditioner and the sides of the window frame.

Once the mounting rails and side panels are in place, lift the window unit–with your assistant’s help–and set it in place on the windowsill. The bottom mounting rail should fit into the lower window frame, positioning the air conditioner correctly and helping hold it in place. Line up the middle of the air conditioner with the mark you made earlier indicating the center of the window. Now, have your assistant hold the window unit steady while you lower the open window sash until it sits on the top of the air conditioner, with the unit’s top mounting rail resting directly in front of the lower edge of the sash. This serves as another safeguard keeping the window unit in place.