Derek Cole explains why distant rooms often get poor airflow: oversized ductwork loses pressure before air reaches the end. The fix is "reducing" trunk-line ducts, gradually shrinking them along the run to maintain pressure. Think of it like putting your thumb on a hose to spray farther.
Chapters
And that is a problem in most homes we've gone to. So if you go to like a Zaxby's or a Red Robin and you look up above in the ceiling, you'll see the duct system. And what you'll see is a properly sized duct. You'll see it going big, small, smaller, smaller all the way down. That's called reducing. That is a proper way to do it. That way the air pressure gets all the way down to the end of that run of trunk line so you can have air at the end. That is the proper way to do it. It's the way we do it. That's the way it should be done. Any engineered drawings you see, that is the way it's done. And that is a problem in most homes we've gone to because the back bedroom or the side over here is not getting the airflow well. It's because the pressure has dropped.. And if you think about it, it's kind of like a water hose. If you have the water hose at home and you just set it there, turn the water on, the water comes out, kind of drips out. If you put your thumb over the outlet, it will spray a lot further, right? Same situation. It's because of the pressure. So air is just like water and should be treated that way. So you want to make sure you keep the pressure up.