Having just read the story by Jim Wheeler about indoor air quality causes me great concern. Most of his remarks and observations may have been acknowledged as true 15 years ago, but they are certainly not held true today. Claiming that galvanized ductwork holds no medium for microbial growth is just plain misinformation. Good filters catch most of the contaminants but not all. Thes contaminants tend to fall in the ductwork and settle to the bottom. They are excellant food for microbials. The A/C coils are usually wet and trap dirt. This is the IDEAL breeding ground. While it is true that cleaning fiberglass ductboard is difficult, it can be done without weakening the integrity of the duct lining.
I won't discuss the MANY problems with the total house flex duct systems. Let's just say runs over say 3 feet are not good for IAQ or your energy budget.
I am NADCA certified as an ASCS. Maybe Mr. Wheeler should check his facts before he writes another article.
His HGTV acticle can be viewed here:
http://www.hgtvpro.com/hpro/dj_construction/article/0,2...0156_3924472,00.html