Christiansons Simple Guide – How to Install a Window Air Conditioner

Most window air conditioners can securely sit in the window without brackets, but those that weigh 100 pounds or more typically require these to spread the weight across a wider area. Some cities even mandate their use.

The installation kit will include an L-shaped bracket that you screw to the top of the lower window sash and side of the upper window sash, preventing them from accidentally opening.

Step 1: Measure the Window

Whether you are installing a new window air conditioner or replacing an older one, the first step is to measure the size of the window. Having accurate measurements will ensure that the unit sits snugly in the window, cooling your space efficiently. It will also help you choose a window AC that is correctly sized for your space, reducing energy costs. Gather your tools, including a tape measure, notebook and pencil, paper or cardboard to create a template of the window (optional), masking tape and a level.

With the lower sash of the window open, use the tape measure to determine the width of the opening. Write down these dimensions so you can remember them when purchasing a new window AC. You will also need to decide whether or not you want to install a support bracket, which transfers some of the weight from the AC to the window frame and sill. Assess the condition of your windowsill for signs of rot and damage, and make any necessary repairs as needed.

Step 2: Cut the Frame

The frame must be sturdy enough to hold the air conditioner and prevent it from sagging or tipping out of the window. Ensure that the bottom edge of the window is flush with the sill and clear of any debris.

A common kit includes two small L-brackets that can be used to prevent the lower window sash from being raised, but check your unit’s installation manual as some types require different brackets.

Working with an assistant, lift and set the window AC into its frame. Depending on the model of your AC, one side of the frame may have a long, vertical arm that extends into the channel that runs along the bottom of the unit. Slide this arm into place, followed by the shorter leg on the other side of the frame. This closes the gap that would otherwise allow air and insects into your home. The top of the AC’s flange should sit directly behind the lower window sash lip.

Step 3: Mount the Unit

Many window air conditioners feature flanges or rails along the bottom and top that align with the frame of the window. Screw or snap these into place, as per the manufacturer’s instructions.

While holding the AC in place, lower the window sash until its bottom edge touches the window flange or rail. Install an L-shaped sash lock, as per the manufacturer’s instructions, to keep the sash from rising too far and lifting the AC out of the window.

Most window air conditioners also come with accordion-style side panels that extend or retract to fit a variety of window widths. Screw or snap these into place, as well. If necessary, add additional foam insulating tape to close any gaps between the AC and window sash that might let cool air escape or insects or moisture pass through. This is especially important if your home is poorly ventilated.

Step 4: Secure the Unit

If the air conditioner doesn’t have a tilt feature (which helps prevent condensation and mold build-up in your room), use the included angle brackets to secure it to the bottom sill. They prevent the window from sliding open accidentally by a family member or an intruder from outside, which could cause the unit to fall out of the window.

Then, install the extendable side panels so they block the openings on either side of the window sash and screw them in according to the manufacturer’s instructions. If the AC comes with a sash lock, install it at this point too.

Some models require a slight tilt, which is why it’s important to check the manufacturer’s instructions before installing them. Finally, you should fill the gap between the lower window sash and upper window frame with foam insulating strips or weather stripping. You may need a helper at this point to hold the window open and support the weight of the AC unit while you do this.