NADCA — the National Air Duct Cleaners Association — is the trade body that sets the professional standard for cleaning HVAC systems. It publishes the ACR Standard, certifies individual technicians, and requires member companies to carry insurance and follow its cleaning methods.
For a homeowner, NADCA membership is the single most useful filter. Air-duct cleaning is not a state-licensed trade in most of the country, so there is no government list of qualified providers. A NADCA member has agreed to clean to a written standard and can be looked up on the association's directory — the everyday operator running a '$99 whole-home' ad usually cannot.
Membership is not a guarantee of a perfect job, but it screens out the bait-and-switch end of the market. Pair it with a written, per-component scope (supply, return, trunk, and air handler) and evidence of source removal — mechanical agitation plus a HEPA-filtered vacuum — and you have the two signals that separate a real cleaning from a cosmetic one.