My son is buying a house in Oklahoma City. The ductwork is under the slab. During the inspection, the inspector cited that there is rust on the vent boots and some sand at the bottom of the boot. I really dont know anything about this type of duct work. Can anyone comment on the reliability of the duct work and what conditions may have caused the rust on the boots? The seller is paying for a scope tomorrow to demonstrate that the ducts are ok, but I would like to be smart enough to ask the right questions.