I think the large ducts are all 16".I've struggled to find ducts in the past. I did get lucky last year and bought 2 pallets of new 12" ducts but they are hard to find. I don't have any extra as of today but that might change shortly once I get all my systems worked out. I will keep you in mind once I know what I will have to keep. The ones that I have not been able to find in quantity are the 18" returns. If you or anyone else find these I would also be a buyer for these. On a side note, I will be posting in the classified 50+ new UEC 60a equipment/load side panel mount plugs very shortly if you are planning on keeping your generator using all military cabling. (this is not meant to be an advertisement or classified ad, only meant to be informative)