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How to Install a Window AC Unit


The exterior of a house with a brand new AC unit in the window.

Window air conditioning units may seem intimidating, but they’re actually quite easy to install. With a bit of background knowledge, the right tools, and a guide to show the way, cooler temperatures are on the horizon—even when it’s boiling outside.

When the hot weather hits, we all need a bit of reprieve. Unfortunately, many older houses and apartments don’t have central air, which means window AC units are an absolute necessity. But these units can be heavy, and the idea of hanging an expensive appliance out of a window can seem intimidating. With this guide on how to install a window AC unit, you’ll be able to cool down in no time.

How to Mount an AC Unit in a Window

You’ll need a few items in order to install a window AC unit correctly. Most DIYers will have them on hand already:

A sturdy stool or bench to place the AC unit onUtility knifePower drillScrewdriverTorpedo levelTape measureIt can also be handy to have a helper for larger AC units, as they can be heavy. One person can hold the unit while the other lowers the window to lock it in place.

What to Know About Window AC Unit Installation

Older homes might not be great candidates for window AC units. This is especially true if the sills aren’t in good shape or the sashes aren’t as structurally sound as they used to be. A window-mount AC unit can cause more damage, and damaged windows can prevent the installer from securing it in place.

Before attempting to install a window AC unit, ensure that it will fit. If the AC unit is new, check the specs on the box to ensure that it’s the right size for your window. If the air conditioner is pre-owned, measure the unit itself, as well as the distance the side panels extend, and compare the dimensions to the window opening.

Also, understand that window AC units come in different outputs, known as BTUs. Different BTU values can cool differently-sized spaces. The best resource is the manufacturer, but here’s a quick, rough guide:

5,000 to 6,500 BTUs for rooms 100 to 300 square feet in size7,000 to 8,500 BTUs for rooms 250 to 500 square feet in size9,800 to 12,500 BTUs for rooms 500 to 900 square feet in size Window AC units are designed to work in double- or single-hung windows. Casement and sliding windows will complicate things and require additional hardware. Be sure that there is an outlet near whichever window you choose, or purchase an AC-rated extension cord.

Steps to Install a Window AC Unit:

Step 1: Prepare the window for the AC unit

The first step in installing a window AC unit is to prepare the window. Remove any items from the window sill, open the window completely (from the bottom), and remove the screen. It’s also a good idea to place the stool or bench near the window, to provide a nearby place to rest the AC unit without putting it on the floor.

Some alarm sensors might get in the way, so be sure to assess yours. Temporarily removing them might be in order.

Step 2: Unbox and assemble the window-mount AC unit

Window AC units typically come in boxes with Styrofoam bottoms. Place the box on the stool so you’re not working on the ground. Cut the cardboard along the dotted line to remove the box and reveal the AC unit.

The back of the window AC unit will contain aluminum fins, and they’re sharp. They’re also easy to crush, which will reduce the AC unit’s effectiveness. Ensure yours aren’t crushed and be careful when removing the unit from the box.

Install the extendable side panels and corresponding brackets according to the instructions. This will usually require a screwdriver. If you use a drill, lower its torque to prevent it from stripping the AC’s housing.

Step 3: Place the AC unit in the window opening

With the brackets attached, face the front of the window AC unit and lift it from the left and right sides. Air conditioners can be heavy, so be sure to lift with your legs as much as possible to prevent back injury.

All window AC units have a bracket of some sort on their bottom side. This is often the bracket into which the extendable side panels slide. This bracket must sit on the outside of the window’s bottom lip. There is also a bracket on the top of the unit, and it must be inside the window.

With these points in mind, lift the air conditioner and place it in the window. Steady the unit with one hand while lowering the window sash with the other. If you have a helper, ask them to lower the sash for you. Be sure that the back of the top bracket sits flush against the bottom of the window sash before letting go of the AC unit.

Check to ensure that the unit is centered in the window. Place a torpedo level on top to ensure that it’s level side to side, as well as pitched backward so water will drain outside, rather than in.

Step 4: Extend and secure the side panels

With the AC unit sitting properly in the window, extend the side panels so they block the openings on either side completely. Follow the manufacturer’s instructions for securing the side panels. This will involve either attaching brackets to the window sash or running screws through holes in the panels and into the window sash.

Step 5: Screw the window AC unit in place

The unit is mostly in place at this point, but manufacturers typically include brackets to anchor it down.

The installation kit will almost certainly include at least one L-shaped bracket. This bracket keeps the window sashes locked in place, and it installs where they meet. Follow the manufacturer’s instructions to ensure it’s installed properly; installation generally involves placing the bracket on top of the lower sash and attaching it to the upper sash with a screw. This will lock the AC unit in place and prevent it from falling out of the window.

Step 6: Plug in the window AC unit

The only thing left to do is plug the AC unit into an outlet and turn it on. Don’t be alarmed if it takes a few minutes to produce cool air; AC condensers often need a few minutes of running before they kick on and pump refrigerant through the lines.

With your window AC unit installed, you’ll be able to benefit from cooler, more comfortable temperatures when the weather is brutal. Kick back, grab a cold drink, and enjoy the cold air pumping from your newly installed air conditioner.

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