How to Install a Casement Window Air Conditioner

Exterior view of open casement windows on home
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  • 01 of 08

    Casement A/C Unit Installation

    Casement Slider Window AC
    Frigidaire / Electrolux

    When you are hot and lacking central air and the proud owner of slider or casement windows, you have an interesting trifecta of cooling issues. You probably already know that most window-unit A/Cs were made for windows that open up and down, such as double-hung windows. Only a few A/C units are vertically structured to accommodate sliders and casements, and these models are all expensive.

    To make matters even more challenging, after you spend the money on this expensive machine, you now have to figure out how to install it. While these instructions use a very standard unit as its model (Frigidaire Casement/Slider Room Air Conditioner, ranging from 8,000 to 12,000 Btu), it can pertain to most other casement or slider style A/C units. The only difference would be the horizontal measurements mentioned later and how they correlate to the window frame.

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  • 02 of 08

    Install Support Base and Platform on Window Sill

    These instructions assume that the unit will be installed on the right side of the window.

    You will begin by installing a wooden support base of your own making on the window sill, and then the metal manufacturer-supplied platform assembly on top of that support base. The reason for this user-created wooden support is so that the window unit is not resting all of its weight on the thin edge of the window frame, which is not structurally supportive.

    Support Base

    • Width: Cut wood boards the width of the unit, plus another couple of inches to account for the window frame on the right side. This would make it about 17". Functionally, it doesn't matter if you cut a little too long. From an aesthetic standpoint, you want it to be roughly the width of your window unit A/C.
    • Height: Board height should be as tall as the height of your window's bottom track. The idea is to avoid having the window track support the unit.  

    Platform Assembly

    Hook the platform assembly over the front edge of your support base, with the long end protruding outside. Drive a screw into the center hole of the platform assembly, sinking it firmly into your support base.

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  • 03 of 08

    Determine the Center Point

    This is the single most important point of this installation: determining the center point of the eventual window opening.

    Start by finding the leading edge on the right side of the window. The official instructions might call it the "window jamb," but it isn't clear what window jamb means. Stating it as "leading edge" makes a little more sense. In other words, you need to find the left-most point of the window frame. Your frame may have several edges, but it will have only one edge that will butt up against the side of the A/C unit.

    It helps immensely to hold a square or drywall T-square against that leading edge so that you can see where the line falls across your support base.

    From this line, measure leftward 8 11/16" and mark that point. This is where you sink the platform assembly screw mentioned in the previous step.

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  • 04 of 08

    Install and Adjust Platform Assembly

    Go outside. Put the platform assembly together per manufacturer instructions. Drive a screw into your siding, provided you have a solid base. If you don't, you'll have to first secure a support board with exterior grade plywood or similar material.

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  • 05 of 08

    Position A/C Unit in the Window

    Go back into the house. Open up the window to the width of the A/C, plus another four or five inches.

    Lift the A/C and slowly push it through the window opening and onto the platform assembly. Be very careful, as the A/C is prone to slipping off at this point.

    Close the window sash against the left side of the A/C. Make sure the sash is behind the metal bracket that is attached to the side of the unit. It is this metal bracket pressing against the sash that holds the A/C in place.

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  • 06 of 08

    Make Sure A/C Tilts Downward Slightly in Back

    All air conditioners, including your casement/slider unit, extract moisture from the air, process it in the machine, then expel it. When you have a full-sized permanent, central unit, water discharge happens without you even knowing about it.

    With window units, water is collected and discharged. But with these units, the water drains out of the back.

    You want the unit to tip slightly downwards toward the back (outside). Many manufacturers recommend a one-quarter inch drop from front to back. Keep in mind, too, that after you have that quarter-inch drop set up with your platform assembly, the drop will increase when you place the heavy unit on top of the assembly.

    However, some units are designed with an integrated slope and should be installed level. Check the product manual to be sure. 

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  • 07 of 08

    Fill in A/C Window Unit Top Gap

    Before removing the panel frame, measure for height.

    Pull the panel frame up and off completely. This panel frame needs to be filled in. Otherwise, your house will be open to the outside.

    One solution is to purchase a clear acrylic or polycarbonate sheet, which you can find at Home Depot, Lowe's, or another home improvement store. If you don't want clear plastic, any type of sheeting that is light enough will work.

    For the Frigidaire Casement units:

    • Width: Using a plastic cutting tool (usually found in the Plexiglas and acrylic section), cut a window that is 15" wide. You only have a 1/4" to 1/2" tolerance, so be exact about this.
    • Height: Measure the height of the opening. Then add 1/4" plus another 1/4" to account for the panel frame grooves in which the acrylic will rest. So, if your window opening is 16" tall, you need to add 1/4" and another 1/4" for a total of 16 1/2".
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  • 08 of 08


    Below are commonly asked FAQs about casement window air conditioner units.

    Is It Really Complicated to Install?

    No. Once you understand the product's broad concepts, you've got it. Looking at the instruction sheet may be daunting, but in the end, this is very simple.

    Does the Unit Install From Inside the House or Outside?

    You can install the unit itself completely from inside your house. However, you do need to drive one screw into the side of your house, approximately one foot below the level of your window sill. You will need to do this from the outside. Leaning over the window, while inside, is not only dangerous, but you will not be able to place the hole precisely.

    What Holds the Air Conditioning Unit in Place?

    It is a redundant support system. The back (outside) is held up by the metal brace called a platform assembly that you install on the house siding. The front is held up by a wood support base that you create with your own materials and place on the window sill. The unit should then clear the lower edge of your window, but only barely. The greater the clearance, the more space you need to fill in or accept as an open-air gap.

    How Do You Fill the Air Gap Above the Unit?

    Some units may come with a thin plastic board to fill in that area. If not, inexpensive plastic sheeting is available at your local home improvement store.

    How Do You Lift Such a Heavy Unit?

    You need a partner to lift it. While these units are not all that heavy, they are unwieldy, and you can hurt yourself trying to pick them up. Some hints, if you decide to lift it yourself:

    • Hold the unit on opposite corners. So, looking at the unit from the front, your left hand is on the upper left back corner and right hand on the lower front right corner.
    • Don't lift straight to the window.  Have an intermediate "landing pad," such as an old table.