Forced air systems and the duct work required to move that air are complex systems. They require a great deal of calculation to size correctly so that they operate properly and efficiently. If a duct run isn't sized correctly you can have a room which doesn't get enough cooling or heating. Or the register can be overly noisy because the air is moving through it way too fast. And that can also lead to uncomfortable draftiness. While I am not going to start installing or supply

For those of you that are building nerds like me, here are a few videos from Building Science Laboratories on... you guessed it building science principles. From Ice dams, to vapour barrier vs. air barriers, to managing humidty, to insulating in cold climates. http://learnbuildingscience.com/12-days-of-building-science/