Errrr . . . returns . . . not so much.
I’ll describe the HVAC system.
The house is a 1911 “American Four-Square”, one of the most common types of old house here.
https://www.oregonlive.com/hg/2016/09/four-square_portland_houses_hu.html
The layout is typical: ground floor has foyer, living, dining, kitchen, bathroom. Second floor has four bedrooms, bathroom. Attic is one room.
Like an old car, the house didn’t come “from the factory” with AC. It was heated with fireplaces; I don’t know if there was originally any other heat source. In the late 1990s, it was retrofitted with central heat/air. The furnace/AC is in the basement. Each room on the ground floor has a roughly 12” x 12” supply vent in the floor, there is a large return in the living room floor. Ducting runs to the second floor through a vertical duct right at the center of the house, boxed into the wall between kitchen and dining room. Each room on the second floor has a roughly 6” x 12” supply vent in the floor, and no return. The attic has a roughly 6” x 12” supply vent in the floor, and a similarly sized return in one of the knee walls, that goes where I don’t know - there are not access doors in those knee walls.
So I am reading more about house AC, and starting to get the impression that since the bedrooms have no air returns, the central AC just can’t work well in those rooms if people sleep with their doors closed, as people tend to do. (I’d thought the problem was just the smaller supply vents - which I still think are inadequate, since the AC doesn’t do much in the bedrooms even with all doors open.) I am also starting to realize that my air circulator idea will only bring cold basement air into one part of the second floor, which doesn’t solve the problem of cooling the bedrooms when their doors are closed.
Hmm. I could fit door returns, but they wouldn’t look right on these old solid wood doors.
Maybe I shouldn’t be so quick to write off mini-splits. If I use units with two wall units, I can do all the bedrooms with two such units, putting cold air directly into the bedrooms with no need for returns.