NADCA — the National Air Duct Cleaners Association — publishes the ACR standard for HVAC system cleanliness. The short version: source removal, negative air, agitation, and post-cleaning verification.
Source removal means physically pulling contaminants out of the system, not just knocking them loose and hoping they blow through a filter later. That requires industrial negative-air equipment and mechanical agitation — brushes and compressed-air whips.