In this short podcast episode, Zach joins Bryan to answer a common duct maintenance question: Should I fog or sanitize my ducts? The National Air Duct Cleaners Association (NADCA) outlines the uses of chemical products. Adding chemicals is usually a last resort in flex or sheet metal (externally wrapped) duct systems; they should NOT be used on ducts with internal liners or are made of duct board or any other material with porous surfaces. Fog is typically applied while a vacuum is running at the opposite end of the duct. It liquefies on surfaces, and a lot of contractors don't go in and dry off the duct surfaces. It's difficult to coat the entire interior of a duct system with the chemical to make it as effective as possible, and the residue creates a moist, sticky film that actually encourages microbial growth in the ductwork. On top of that, some people may be sensitive to the chemicals. In many cases, replacing ductwork that has been confirmed to have microbial growth is more reliable. Chemical duct sanitation often has more of a perceived benefit than long-term results. The use of chemical cleaners in ductwork is similar to that of using some line set flushes without forcing a solid pig through the line set to pick up the residue afterward. Ultimately, the customer has to make an informed decision based on the problem and knowledge of the benefits and drawbacks of chemical solutions. NADCA's white papers, including "Chemical Product Applications in HVAC Systems," are available at https://nadca.com/industry-professionals/nadca-white-papers. Have a question that you want us to answer on the podcast? Submit your questions at https://www.speakpipe.com/hvacschool. Purchase your tickets or learn more about the 6th Annual HVACR Training Symposium at https://hvacrschool.com/symposium. Subscribe to our podcast on your iPhone or Android. Subscribe to our YouTube channel. Check out our handy calculators here or on the HVAC School Mobile App for Apple and Android.