Any reason why I can't use schedule 40 or schedule 80 PVC pipe for underslab HVAC ductwork? I have found thin wall PVC pipe (Black Max) supposedly manufactured for air duct. It is 1/3 the thickness and weight of schedule 40, and much more costly. Why shouldn't I just use schedule 40? It is thicker, stronger and less expensive. I may ask building inspector if schedule 40 is acceptable to him, but I would first like to know if there are issues about using the schedule 40 that I am unaware of. By the way, slab is high above grade with 2 feet gravel and drainage below so water/moisture in the pipe's environment is not an issue. This duct will be for cooling only. Heating will be by hydronic in slab radiant.