How to Install a Window AC Unit in 6 Simple Steps

by James Fitzgerald
window ac unit

Installing a Window Air Conditioner at a Glance

  • Step 1: Assemble the unit
  • Step 2: Place in window
  • Step 3: Secure
  • Step 4: Cover opening
  • Step 5: Insulate
  • Step 6: Plug in and test

If your home or apartment isn’t equipped with central air conditioning, a window AC unit is an excellent choice for beating the summer heat. They’re relatively inexpensive, surprisingly easy to install and impressively energy-efficient.

Although the prospect of installing a heavy appliance that hangs out of your window can sound scary and intimidating, window units can be safely and easily installed if you follow the proper steps.

Things You’ll Need

  • Window AC unit
  • Tape measure
  • Utility knife
  • Power drill
  • L-bracket(s) (optional)
  • Foam insulating strip (optional)
  • Extension cord (optional)
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Things to Consider Before Installing Your Window AC Unit

Before installing — or even purchasing — your window AC unit, keep the following considerations in mind:

Size of the Room

Window AC units have different cooling capacities, measured in British Thermal Units — BTUs. Depending on the unit, the cooling capacity may fall anywhere between 5,000 and 25,000 BTUs, which will dictate the size of the room it’s able to effectively and efficiently cool.

As a general rule, you need a unit with 20 BTUs for every square foot of the room you’re going to install it in. To determine the square footage, multiply the length by the width.

So a 10-foot by 10-foot room would require a unit with a cooling capacity of approximately 2,000 BTUs, since 10 x 10 x 20 = 2,000.

Size of the Window

Fully open the window and measure the opening to ensure you purchase a unit that will fit inside.

Proximity to Power

The majority of window AC units are equipped with power cords that are 5 to 6 feet long. You may need to use an extension cord if the unit’s cord can’t reach the nearest electrical outlet.

How to Install a Window AC Unit

Step 1: Unbox and Assemble

Carefully unbox your window AC unit by cutting along the dotted lines on the box with a utility knife. Remove the Styrofoam padding and set the unit aside, taking extra care not to contact or damage the aluminum fins on the back of the AC. These fins are sharp and may cut you, and damaging them can compromise the air conditioner’s cooling efficiency.

If not already installed, attach the top and bottom mounting plates to the unit with the provided screws. Then, slide the extendable, accordion-like side panels into their corresponding tracks on each side of the AC. Depending on the model, the side panels will either clip into place or need to be secured with screws.

Step 2: Place the AC Unit in the Window

Raise up the lower, movable window panel (the “sash”), then carefully lift the unit and place it in the center of the window opening. Window AC units are usually heavy and awkward to lift, so it’s best to recruit the aid of a second person to assist with lifting the unit into place — especially if you’re not confident that you can lift it on your own.

Adjust the unit until the bottom mounting bracket is seated on the window sill’s inside lip, the side panels are aligned with the window frame, and the top mounting bracket is aligned with the bottom of the window sash. Lower the sash until it contacts the top of the unit and is seated behind the top mounting bracket. The window sash should now be holding the AC unit in place, but it’s still a good idea to have a second person maintain downward pressure on the sash until you permanently secure the unit in place.

Step 3: Secure the Unit in Place

Now it’s time to permanently secure the unit to the window. Most window units will come with one or two L-shaped brackets that attach the window sash to the window frame to prevent the sash from lifting up, possibly causing your AC unit to fall out of the window.

Place one side of the bracket on the top corner of the window sash, and the other side against the window frame. Use a power drill to drive the provided screws into the sash and frame. If the manufacturer provided two brackets, repeat these steps on the other side of the window. If none were provided, consider purchasing them online or at your local home improvement store before proceeding with the installation.

On some window air conditioners, there may also be a series of pre-drilled holes in the upper mounting bracket for securing it to the sash with screws. If that’s the case, use a power drill to drive the provided screws through the mounting bracket’s holes into the sash.

Step 4: Cover the Opening

Block the opening on either side of the unit by fully extending the side panels until they contact the window frame.

If specified by the manufacturer, secure the side panels to the frame with the provided hardware. In many cases, this will either involve driving screws through pre-drilled holes in the panels, or installing brackets to anchor the panels to the window frame.

Step 5: Insulate

Since the lower sash was raised to accommodate the unit, there will be an air gap between the lower sash and upper window panel. Many AC units come with a foam insulating strip to place in the gap and provide an airtight seal. Use a utility knife to cut this strip to length, then press it firmly into the gap.

If your window unit didn’t come with a foam insulating strip, purchase one online or at your local home improvement store.

Step 6: Test

With your AC unit properly installed, all that’s left is to plug it in and test it out. Once you confirm that it’s working properly, you can sit back and enjoy your cooler room.

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