My house is two floors, though I think it was originally a one-story cape cod, and the upstairs was a single semi-second floor/attic. Anyways, the attic is now higher up, but the walls still slope on the sides. The air/heating unit is in the crawl space, duct work runs directly to vents/returns on the first floor, but for the second floor there are these two really large plastic(?) pipes that run up to the second floor from which I believe the ductwork is distributed. What are these called?
Vertical duct work. In general, if forced air, heated, cooled or even unconditioned, goes through it it is called duct work, unless it is an accessory / fitting such as a grille, diffuser, register etc. It's called duct work or vertical duct work if you would like.
