Whole House Humidifiers for Ductless Furnaces?
Your installation sounds as if your house originally had a coal furnace installed many years ago. Your description sounds like my grand mother's house. That being said, If the supply pipe is a rectangle with a flat surface, and you have a return air duct, almost any bypass type humidifier could be used. If the supply pipe is round, you would have to have a sheet metal shop make you a transition piece that would adapt to the round pipe and give you a flat surface to mount the humidifier on.This may be a lot for the average home owner to do but yes, it can and has been done.