Anybody in the HVAC biz?
In the majority of HVAC systems I’ve seen, the shafts are usually square or rectangular in shape. Why? Wouldn’t round ones be more efficient?
Also, is closing off vents you don’t use bad for your system? Wouldn’t it make more sense to be able to cut them off BEFORE the place where they join the main shaft rather than at the terminating point (the opening)?
Square ducts provide a maximum airflow for a given wall thickness. If a wall has a 6" clearance inside, a round duct can be obviously be only 6" in diameter, but a rectangular duct can be 6" x ~14" (to fit between 16" center studs) and therefore has a larger cross section. Vents are square to fit easily onto the square ducts.
Not in the business, but rectangular ducts have nice flat sides that are easy to join to other ducts.
Closing off vents to areas not in use shouldn’t hurt the system as long as you don’t close off enough to cause the back pressure to reduce the flow to the point where the system can’t handle it. Furnaces asnd Air Conditioning units are made to work with a certain range of airflow.