Have you looked at the portable ones thta just need a round hole for venting and are alot more efficient that those were too then you can just close up most of that hole and be done until you go to sell whenever .

That's a possibility that would require the least intervention with the wall....

https://www.haierappliances.com/appliance/Haier-5-000-BTU-Mechanical-Window-Air-Conditioner-for-Small-Rooms-up-to-150-sq-ft-QHEC05AC

Thanks - the challenge with window units is that they have vents on the side, which would be blocked by the wall, which is thicker than a typical window installation.

There are two units on the main floor, which is about 800 sq ft. One is a window unit, this is an in-wall one. If she won't go for a portable one with an insert, I'm likely to suggest making the opening a standard 24".

AFA casement unit...those are all taller than 14", so we'd be making the hole bigger there too, just vertically vs horizontally.

• Some Friedrich window air conditioners can be installed through a wall as described starting on page 10 of this installation manual. You would have to enlarge the opening somewhat and build a frame.
• Friedrich air conditioner specs.
• AJ Madison sells them.

Thanks, I saw that - I'll be measuring the actual current opening this weekend - if it's less than 20", I might as well expand it to 24", since that gives a lot more choices on replacement.

It sounds like you are capable of increasing size of hole so I recommend that over a portable. Portables are a joke and those without two vents (one incoming and one exhaust) are a waste of energy.