The short answer to how you install a conventional window air conditioner in a casement window is you don't, but removing the window and making some removeable modifications to the frame worked well. I closed the upper part of the opening with plywood. Then I mounted the AC unit in a frame that would just slip into the opening. A few side panels closed it off. It can be installed in less than 5 minutes.

Removing the window is a simple procedure but if you have to do it from inside the house it's heavy and awkward and you can't stop in the middle - be warned.

2x3's were screwed to the window frame to attach everything to. The jambs, head and sill of the window frame all angle out so the outsides of the 2x3's were beveled to match, leaving square and flat inside surfaces to attach to.

I made a separate "sill" piece that sits in the bottom of the opening for the AC to sit on. It's not attached to anything but a lip in front keeps it in place. The AC is solidly attached to a frame that sits on the sill. The weight of the unit keeps the top of the frame pressed against the upper part of the opening, the same as it would in a double-hung window. Two plywood filler pieces close the openings on either side.

This was done about 3 years ago for the guest bedroom when we were concerned that we might have to isolate someone with a contagious disease and didn't want the central AC to mix the air from that room with the rest of the house. The left filler panel can be replaced with an exhaust fan, which would plug into a 12-volt power supply with a speed controller.

We've never installed this system except to test it but it sure was pleasant in that room with its own AC for a little while. SWMBO said "No way" to putting it in our bedroom because of the blocked window, but some glass or plastic in the upper part would look a lot better and I might try again.

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