Over time, dirt, dust, pet dander, debris, and more accumulate in air ducts. And since the HVAC system works basically as a whole-home vacuum, all of that is recirculated through homes when the HVAC system runs, unless the ducts are cleaned out.

The National Air Duct Cleaners Association recently drew more than 500 people to a resort near Denver for its annual meeting and exposition.

A move into duct-cleaning work is natural for HVAC contractors looking to expand their repertoire and add another source of revenue.