I have two furnaces in my house, one for the basement and one for the rest of the house. I noticed that while using the basement furnace this winter that no air was coming out of any of the basement vents (all under cement/under padded carpeting) except the one closest to the furnace in a bathroom. And it was blowing what looks like dirt out. I have had two HVAC professionals come for other things, routine maintenance and have asked them what would be a good solution. One guy said to ditch the system and invest in a Mitsubishi splits system. The other simply had no idea if it could be repaired (easily/inexpensively).
I don't really need the bottom furnace since the other furnace actually does have some output vents in the ceiling in 3 places. But the basement is always about 3 to 5 degrees colder than the rest of the house, so the second furnace would be nice to have. I mean, the furnace works and everything, it just can't circulate because all of the vents are collapsed. Also, it's not very comforting having a broken HVAC system lying around.