Sealed my ducts and have extreme dust

In working around this house I just moved into, I have gotten around to sealing my ducts (not all the way done)and most of the plenum. All this is located in the crawl space with the furnace upstairs in a closet. Additionally after some years it appears, I replaced three flex ducts that were just laying on the ground in the crawl unconnected.

Now I have a serious dust problem every time the air conditioner kicks on.
Did I do something wrong, or was this dust just laying in the system all this time?

Also, what can be done other than running a shop vac hose down the vents as far as I can?

Thanks in advance for any advice…

you need to call a profesional duct cleaning company

The guy that does my Level II chimney inspections for me is not only a chimney sweep, but also does duct cleaning. So if you have trouble finding a “duct cleaning company” try looking at chimney sweeps.