Unsealed Joints in Return Ducts 

Well said, and great video! This is a very good point that gets missed often. I'd also like to add that issues like these will cause pressurization problems as well, since the air in the home entering the return no longer equals the air being distributed by the supply vents, causing positive pressure. Likewise, you cause a negative pressure in the area where the leakage is occurring (in this case however, the garage probably feels no effect because it's not tight enough and the leakage isn't strong enough).

We have to smoke test duct systems when the FAU and/or duct system is in the garage here in CA. So we are not bringing in off gassing chemicals into the hvac system. I’ve conducted duct tests in the past before this code and a homeowner opened their garage, started their car. As I was running my airflow testing for the evaporator coil. My CO belt monitor was at 8ppm and in the garage reached as high as 25 with it open.