Modifications may be required (such as removing brackets and side accordion panels) but any "window" unit probably can be installed as through-the-wall.

To sktn77a: Unfortunately, it is humid here -- Connecticut, a few miles from the shore. I'm not sure what the capacity of the old unit is.

To dadoes: Yes, I thought of that. A few 5,000 BTU window units are available, which isn't too much bigger than I need.

The problem is that window units tend to exhaust through the side, so the wall would partly block the exhaust. Maybe I could have a glass-less window from plywood or the like, something that wouldn't be as thick as a wall, but that sounds like a real kludge. The existing opening isn't that much bigger than a new window a/c, so I don't have a lot of space to work with. I'd probably have to make a window sill to support it, too.

That doesn't really sound better than an oversize a/c, with all its problems.

Maybe I should get my existing unit repaired. It seems silly to put money into a 20-year-old a/c, but there doesn't seem to be a good new option. Or I could just live with the problem (which is that it no longer cools very well on a hot day.)

I also thought of an inverter-type, variable speed A/C. That would be nice, but they don't seem to make that in less than 12,000 BTU, I think.

Majority of window-mount units I've seen pull air in from the external side grill(s) and blow it over the condenser and out the back ... but you are correct that the sides need to be clear for the air intake.

The window units I've seen exhaust out the back. They intake from the sides.

You can buy (at least you used to be able to) AC window brackets. Basically screws into the wall below and braces at a 45* angle.

I agree. The side louvers are intakes. Sorry.

Still, they shouldn't be blocked by the wall, as dadoes said. The spec sheet for a 5,000 BTU window a/c says it's 16" wide and my opening is 20" so, unless I enlarge the opening, I'd have only 2" on a side for the intake airflow. I'll have to measure the a/c to see how much of the louvers will be shrouded by the wall.

You're also right about the brackets. I forgot that I had used one on another a/c years ago. That will help, if I go this way.

Thanks.

Why can't you buy an 8,000 BTU model and use it on the lowest speed? This LG model on Amazon has 3 cooling speeds. It also has a dry mode that is supposed to work as a dehumidifier.

Great! I was looking for something like that, but couldn't find it.

The one issue is that, at 8,000 BTU, it's a little bigger than my existing opening. I'll ask a carpenter what would be involved in enlarging the opening.

I also question if window and through the wall units actually deliver their cooling rating. It may be my perception but I think the manufacturers can get away with inflating their numbers.