How to Make a Window Air Conditioner Fit a Large Window

Adding a window-mounted air conditioner to your home is a quick and easy way to keep a room cool during high-temperature days. Mounting the air conditioner requires a window large and sturdy enough to hold the unit in place and a power outlet capable of dealing with the electrical requirements of the unit. Placing a unit in a window that's too large, however, could cause difficulties in fitting the air conditioner in place. Overcome these fitting problems with the installation of air conditioner extensions placed on the side of the unit to provide the necessary seal between the unit and the window edges.

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Extensions ensure that a window air conditioner fits snugly in a larger window.

Step 1

Install the mounting hardware, which is needed to hold the air conditioner in place, provided by the air conditioner manufacturer. Larger units require external brackets to be placed onto the window sill to provide a wide enough perch for the unit to sit on. Secure the brackets into place using screws to attach brackets onto the outside of the window sill and the outside wall beneath the window. Smaller units rest on a sill mounted support shaped as a bar that rests directly across the length of the sill and is secured by screws. Follow the manufacturer instructions for precise placement of the mounting hardware in relation to your window pane.

Step 2

Open the window so that it is slightly wider than the air conditioner, and then lift the air conditioner onto the sill into place. Get help with lifting larger units. Slide the air conditioner along the window sill until the unit clicks into the mounting hardware. The majority of the unit should hang outside of the window.

Step 3

Close the window down onto the air conditioner until the window sash lies straight along the top.

Step 4

Pull the window extensions from the side of the air conditioner unit. They should stretch into place extending to the sides of the window frame. Secure the extensions into place by screwing them to the movable window sash and the window frame. Screw the top of the permanently opened window sash to the top sash using an L-bracket to keep the window from wiggling in place. Once the extensions and sash are secured the air conditioner should fit snugly in the large window opening.

Step 5

Add a line of caulk to the outside of the window along the air conditioner edges to insulate the window opening. Add further insulation inside the window by placing foam weather stripping at the top of the now opened window sash where the open sash meets the upper window pane.