Theatrical smoke is a very effective way to find leaks in a duct system.  A quick check will point out smoke as it spills from pressurized leak locations.  Leaks in attics and crawlspaces that can be hard-to-detect are no problem with this method. This type of detection is called theatrical for a reason - it is quite a sight to see for homeowners and builders.

Caution: theatrical smoke injected through the Model 300 test fan inlet may cause damage and void warranty. Risk increases with frequency. Instead, inject non-toxic chemical smoke into the flex-duct then reconnect the DucTester fan so it blows that smoke into the duct system.  Watch the video below to see this method used with the Power Tiny: