Allison Bailes recently posted a pair of articles on his esteemed Energy Vanguard blog about the ideal velocity of air through ducts. The latest, "The Best Velocity for Moving Air Through Ducts" explains that, while the ideal velocity is the slowest possible (which translates to the largest duct size that will fit through a given space) the slower the air moves, the more prone to heat gain or heat loss it becomes. Therefore, where the duct is located plays a big role in how it should be sized.
As Allison writes: As the conditioned air moves through a duct, it gains or loses heat in proportion to those three factors [surface area, temperature difference and insulation level]. But that just tells you how many BTUs go into or out of the duct in an hour. The other factor is how much air is involved in picking up each BTU. The factor that governs this is velocity:
The slower the air moves in a duct, the more BTUs each cubic foot gains or loses.
And it's not just time of contact that's responsible. To get the air moving slower, we need bigger ducts so there's more surface area, too. The upshot of all this is that when you're sizing ducts, you have to consider the space those ducts are in.
That's where Mike MacFarland's way cool duct sizing tool (a chart) comes in, and Allison provides a link to download the tool. The chart allows you to find the duct diameter that gives you the right velocity and air flow rate (cfm), depending on where it is located - exposed in an unconditioned attic, deeply buried in an unconditioned attic, or running through conditioned space.
To learn more and download Mike's duct sizing tool, click here.
