That’s where this thing called equivalent length comes in. They don’t tell you what happens when you get turbulence at the fittings. Ductulators tell you how much pressure drop you’ll get in a 100 foot run of straight duct of a certain type. HVAC designers often use something called a ductulator to determine the sizing of the ducts they use, but it’s easy to misuse them, too. As you might expect, it’s the fittings that reduce air flow the most. Turbulence happens mainly when you turn the air in the duct system, and that happens at fittings.

The first is friction, or air resistance. One thing certainly bears repeating from Hill’s presentations, though, is that air flow can be reduced two ways. I do want to say a few things, though, because air flow is so important and so often misunderstood.

The combined effects, Hill said, make furnaces like “a pinhole with a hurricane going through it.” Air flow, friction rates, and equivalent lengthĪs mentioned above, Michael Chandler did a good job summarizing Hill’s 2011 presentation, so I won’t do much more with his reprise of that here. Then there’s the absence of integrated design in new construction, leaving HVAC designers and installers not nearly enough room to do their jobs well, so duct systems are smaller, too.

But he did throw out one new thing there: He said that manufacturers, in their drive to improve efficiency of their equipment, are making furnace cabinets smaller while at the same time increasing air flow. In the first half of Hill’s presentation, he reminded us about some of the fundamentals of air flow in ducts that he covered in 2011. This year, Hill did a followup presentation, reprising some of his main points but also adding two new pieces, one of which is a nice study he’s been involved with on air flow in oval ducts. (In case you weren’t reading this blog back then, I got myself invited with my 2010 article called I Don’t Need No Stinkin’ Building Science Summer Camp.) Michael Chandler ably summarized that talk in his article, Stuff I Learned at Joe Lstiburek’s House, Part 2. At my first Building Science Summer Camp in 2011, David Hill (in the photo below) gave a great presentation on some of the big problems with duct systems.

Cfm airflow in home