Just a note.
The NFPA (National fire code, not adopted by all states) explicitly requires any ducting passing through a floor be protected with an automatic fire damper. There is an exception for UL listed furnaces that have a 165F automatic shutdown and there is no interruption in the ducting from the furnace..
Just a note, my inspector follows the NFPA code and my insurer voids my fire insurance if the inspector doesn't sign off.
Actually automatic fire dampers are not hard to come by, they are often just a short piece duct with a spring loaded damper held open with a link that melts at 165F, closing the duct.
Just a note re fire insurance, check your area!