Upstairs A/C issue......

Can you install a ceiling fan at that location? Keep that high space circulating downward so it doesn't get hot in the first place?

I get what you are trying to do but an exhaust fan to suck air out of climate controlled space seems a bit extreme. Surely there are a few better options even if you can't use a ceiling fan. Something doesn't seem right.

I’m no AC technician, but I think your issue is more about poor air circulation. I don’t think you want to expel air because your home will need to then pull in air…also from the exterior.

Probably what you want to do, is better circulate it. A return on the first floor to the second floor unit is one idea. A ceiling fan that moves air around between first and 2nd would likely help also. Not sure if there are other solutions?

I would not do this. Atleast the warmer air is presumably dry. Whatever you suck out has to be replaced and it will be hot AND humid which is worse in my view. I don’t know what the answer to this problem is. I had the same situation and my upstairs cool air would fall down the stairs and warmer air would rise up from the bottom floor. As a result my smaller upstairs unit would run much more than my larger downstairs. During the day I would keep my upstairs a few degrees higher than downstairs and it seemed to help keep the upstairs from running so much but if people are up there they don’t like it.

The only other solution I can see is to build a wall and frame in a door that can be closed to keep the two layers separate.