How to Install Horizontal Air Conditioners

Slide open the lower sash of the window into which you are installing the air conditioner.

Measure for the mounting brackets on the window sill, according to the manufacturer's instructions, if the unit will extend more than 12 inches outside the sill. Use a tape measure, and mark the appropriate spots on the sill for the mounting bracket's screws, using a pencil.

Install one end of the mounting brackets to the window sill, and the other end to the exterior wall, on the outside. Fasten the bracket with a screw, using a screwdriver or a handheld power drill. Use wood screws if the frame is wood or vinyl; use sheet metal screws on a metal frame.

Place a level on top of the mounting brackets to make sure they are level, allowing the air conditioner to slope about 1/4 inch toward the outside. Adjust the brackets, according to the manufacturer's instructions, which normally means turning a center screw on the bracket, using the screwdriver.

Open the window wider than the air conditioner is high to allow ample room. Lift the unit and set it in place inside the window. Lower the window on top of the air conditioner unit to hold it in place.

Slide open the expandable, accordion-type extensions on each side of the air conditioner unit, closing the gap between the unit and the window frame. Screw the included screws through the holes in each extension, and into the window sash, to fasten the extensions to the sash.

Screw one small angle bracket to the left side of the upper sash, and one to the right side, using a screw and screwdriver.

Cut the foam weather stripping, which normally comes with new units, to the right length, using a utility knife. Press the foam stripping into the gap between the lower sash and the window to keep out leaks.

Place the level on top of the air conditioner unit, on the outside of the window. Make sure the unit's back side has a slight tilt toward the outside.