The good news is that an alternative method of cleaning and sealing air ducts exists.
Floor vent seal.
While it certainly is possible to abandon the under slab ducts and to put ducts in soffits or an attic this.
If you do unscrew one of the floor vent covers and set it aside.
To properly seal around air vents apply caulk or expanding foam between the ducting and the sheetrock.
If you live in a house with a crawl space there is a good chance the floor vents are not sealed.
Check to make sure you have a crawl space and that you have air vents coming up through the floor.
Look into the vent right where the ductwork meets the floor.
The goal is to prevent conditioned air from escaping or unconditioned air from entering the thermal envelope.
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Is there a gap between the metal duct and the floor.
Inspect your floor vents.
Next apply weather stripping around the underside of the vent cover.