Source removal is the method NADCA requires for a real duct cleaning: the technician mechanically agitates the inside of the ductwork — with brushes, air whips, or compressed-air tools — while the whole system is held under continuous negative pressure by a powerful vacuum. The loosened debris is pulled out at the source rather than pushed around the home.
This is the dividing line between a legitimate job and a 'blow-and-go' visit. Fogging a sanitizer or running a shop vac at a few registers moves dust without removing it. Source removal captures it. Because it's thorough, a whole-home source-removal clean typically runs two to five hours — a job quoted at under an hour is a red flag.
Ask how the provider creates negative pressure (a truck-mounted or large portable HEPA vacuum), how they agitate each run, and whether they'll show before/after photos. Those answers tell you whether you're paying for source removal or for theater.