Have not seen this before. I have sub slabslab Transite system from the "80s.
The supply plenum extends through the basement slab and feeds a sub slab “ductwork”. (pic1) House was built in 1999.
The individual take-offs are openings in what appears to be schedule 20 PVC.(pic 2) A large family room and two bedrooms where ducted this way.
Are there any concerns with the metal duct going through the floor?
Are they any concerns with this type of duct system?