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Hey Ed, Why Do We Reduce Trunk Ducts?

In this edition of "Hey Ed," Ed talks about why we reduce trunk ducts.

Hey Ed, why do we reduce trunk ducts? We reduce trunk ducts in an extended plenum-type duct system to make it easier to balance. That wasn’t the answer that you were expecting, was it? You were expecting me to say because it keeps velocity up. I’m not going to say that’s wrong, but it’s not the primary reason we do it. It’s for easier balancing. Keeping velocity up is nowhere near anything I’m concerned with ever in a ducted system. I know a couple people are out there like, "Whoa, what did he just say?" I’m just going to flat-out say it: there’s no such thing as velocity that’s too low in a ducted system. There’s definitely velocity that is excessive. 900 feet per minute in the supply is where we max out, 700 in the return, but there is never going to be a problem with velocity through a ducted system that’s too low. What we care about is volume in the branch run. As long as we have the proper volume in the branch run and the register was selected properly, it’s going through a boot that is the proper size to go with that register. We will always get the proper throw and spread out of the register. And that’s the way I see it.

Ed Janowiak is the Manager of HVAC Design Education at ACCA.

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