The 2 Primary Causes of Reduced Air Flow in Ducts
Two things. Just two things in your ducts are responsible for giving the blower in your furnace or air handler a hard time. They make the blower push against more pressure, thus reducing air flow or increasing energy use, depending on blower type. They cut the amount of air that gets delivered to the rooms. And they can be reduced but not eliminated. Do you know what they are?
Maybe you're thinking it's flex duct that's not pulled tight or not using rigid elbows or maybe even the dreaded ductopus. Those things are related, but we need to go back further. We want the root causes. This is basic physics I'm talking about. Maybe looking at the image below, a view through a piece of flaccid flex duct liner, will give you an idea of what's to blame.
Friction
The first cause of reduced air flow is friction. When air moving through a duct rubs against the inner surfaces of that duct, it loses energy. It slows down. Its pressure drops. The more it rubs, the more those things happen. It's like walking down a busy sidewalk with your shoulder rubbing against the buildings.
The amount of friction depends on the nature of the material the duct is made of, how it was installed, how dirty it is, and how fast the air is moving. The photo above shows flex duct that's not pulled tight at all. Even though you can't see it all that well, you can tell that there's probably going to be a lot of rubbing when air moves through that duct. The same flex duct pulled tight is shown below. It still looks a bit rough but is much better than the one above. A piece of rigid metal duct would provide a much smoother surface.
Turbulence
The other primary cause of reduced air flow is turbulence. This one is a kind of friction of the air rubbing against itself. The main cause of turbulence within ducts is turning the air. When you send air through a 90° turn, the type of fitting you use to do so can make a big difference.
The diagram below is to Understanding the Friction. In both of the 90° elbows, the air enters nice and smoothly. That's laminar flow. When it makes the turn, however, notice that the air in the elbow with the curved inside edge (the throat) results in less turbulence. The elbow with the square throat produces more turbulence. Pick your fittings carefully!
Friction rates and pressure drops
The result of friction and turbulence, as I said above, is that you get a drop in the pressure. As air moves through a supply duct, the pressure created by the fan behind it keeps it moving. The farther it travels down the duct, though, the more that pressure is reduced by friction and turbulence. That's true in good duct systems as well as bad.
Both of these causes, friction and turbulence, are included in the friction rates given for various types of ducts and fittings. As the word 'rate' indicates, the friction rate doesn't tell the whole story. You've got to combine it with something else to figure out what the whole pressure drop is.
When designing and installing ducts, you've got to know about this stuff. Friction and turbulence play a big role in whether a duct system does what it's supposed to or not. We've got this stuff quantified.