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How to Install a Window AC Unit

If your home doesn’t have central air conditioning, you don’t have to sweat it out all summer long. Window-mounted air conditioners cool, circulate and dehumidify the air to make your home a comfortable abode.

Step 1: Measure the Window

Use a tape measure to measure the bottom half of the window in which you want your air conditioner to go. Open the bottom sash all the way up when measuring. You don’t want to get the wrong size unit and find this out after you’ve lugged it around and heaved it up into the window. Be sure that there is an available outlet near the window to plug in the air conditioner.


Make sure the window frame and sill are sturdy enough to support the air conditioner. If the sill or other part of the window is cracked or damaged, repair it first before you put the unit in place, or choose another window. Window air conditioners are heavy. If not securely positioned in place, they can fall out of the window and potentially injure someone or damage property.

Don’t use an extension cord with a window air conditioner. The unit should be plugged directly into a proper outlet.

Window air conditioners come in a standard size to fit normal-sized window frames, but their size can change a fraction depending on the BTU output of the unit; for example, the more powerful your air conditioner is, the bigger it will be.

Keep in mind that window-mounted air conditioners are meant to work most effectively with vertical-closing, double-hung windows.

Step 2: Buy the Right Air Conditioner

When choosing a window air conditioner, your selection should be based upon the unit’s British Thermal Unit (BTU) rating — its cooling capacity. Window units range in size, usually from 5,000-BTU models to 14,000 BTUs (for normal home voltage).

Calculate the number of BTUs needed to cool a space by multiplying the square feet of the room by 25 and then adding 1,000 for every window, plus 400 for each person in the home. Example: A 10-ft. square (10′ x 10′) room with two windows and three people in the home would mean that 5,700 BTUs are needed.

You should strategically decide which room and window you want to use. You can get the most cooling potential and efficiency by placing the unit in a room that is connected to an adjoining space via an open doorway. When doing this, you want to count the square footage of both rooms as one space. Then, purchase an air conditioner with the necessary output. Air from your window unit won’t go around corners or through closed doors — plan unit placement accordingly.


It is very important to get the right size of air conditioner. One that has too much output will leave air feeling clammy and damp, whereas one that is too small will not remove warm air and cool down the space when it is very hot outside.

Get more than one unit if you want to cool down more space. While one air conditioner can sometimes cool more area than just the room it’s in, to get better coverage, have at least two units to effectively cool down the most space.

Try to choose a window free of direct sunlight. The sun’s hot rays beating down on the unit can make it work less efficiently and effectively.

Also, it’s a good idea to check the Energy Efficiency Rating (EER) of the unit. If the EER is not listed, it can be calculated by dividing the unit’s BTU output by the number of watts it requires to operate. A high EER rating is good because it means the air conditioner will run more efficiently, but the higher rating will come at a cost. If you plan to have the unit on most of the summer, it will pay to have a higher EER.

Step 3: Install Support/Mounting Bracket

support bracket helps keep the air conditioner securely in place and is much more convenient and safer than propping up the air conditioner on the outside sill with bricks, as many people do. Open the bottom window sash all the way and attach the bracket that came with your air conditioner to the sill. How the bracket and air conditioner will be installed will depend on the manufacturer and model, but generally they are attached with screws to the sill, so keep a screwdriver or power screwdriver handy. Follow all manufacturer instructions. If your air conditioner did not include a bracket, you can find these at your local True Value store. To be safe, go for the most heavy-duty bracket with the highest weight capacity.


Follow all installation directions from the manufacturer closely with no exceptions. Any deviance from the instructions could result in poor installation, and thus the air conditioner potentially slipping from the window frame and falling below, injuring someone or damaging the window frame/sill or other property.

Step 4: Install Air Conditioner

Ask someone to give you a hand with this part if possible. Not only will you be lifting the air conditioner into place, but also you will need to steady it and position it. It will be much easier to position the air conditioner if you have assistance from an extra set of hands.

Position the unit by sliding it onto the support bracket and fastening it, according to the installation instructions. Close the window down as far as you can, making sure the air conditioner is flush with the window on the inside.

If the installation kit came with angle brackets, use them. These L-shaped brackets keep the air conditioner from forcing the window open with its weight. You simply use the supplied screws to attach one side of the bracket to the upper sash; the other side rests against the bottom sash, holding it in place. You can also buy these brackets at your local True Value hardware store.

Many air conditioner models include extendable “accordion” side panels that close the gap between the sides of the window and the air conditioner’s width to make a seal. If yours does not include these panels, kits are available for purchase at your local True Value hardware store. Side panels are attached to the window sash with screws for added security and safety. Follow the specific manufacturer’s instructions on how to attach them.

Step 5: Finish Up

Now that the air conditioner is securely in the window, you’re almost done. Attach the drainage hose that came with your air conditioner. Cut a length of foam weather stripping to fit and apply it between the lower and upper sash since there will be a space there from the window being open. This will help seal it from the outside and cut down on leakage, making it run more efficiently. If your air conditioner is new it may also include this weather stripping with its installation kit.

You can also apply caulk at the edges between the air conditioner, side panels and the window frame to seal spaces that can let warm air in and the cooler air escape.

Good job; you’re done! Take a break and cool off.

Project Shopping List

Here’s what you’ll need to complete this project successfully.

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