Here is the senario: Cape code with a finished second story. The house has forced air (oil) and no central A/C. The second story has three rooms (small) and a bathroom (small, too). Two of the bedrooms and the bathroom has vents. the vents seems to have been installed in the cheap because they are connected to the main duct in the basement at perpendicular angles. The third bedroom has no vents. There is no return on the second floor.
During the winter, the furnance does supply heat to the rooms upstairs and if I am at the bottom of my stairs (on the main floor) I can feel the cold air coming down due to the return on the main floor, so the second floor is not bad in the winter (except the bedroom with no vent - it can get cold in there on really cold days)
I want to install central A/C. I've been told many different options:
1) "The second story will cool down, but will be humid because there is no return installed up there - you can buy an air handler and a second central A/C just for that floor"
2) "Just use window A/Cs"
3) "It'll be fine"
4) "It'll be fine, but you might need a dehumidier"
5) "Use ductless on the second story"
6) "Fix your ducts"