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Building Science

The Best Velocity for Moving Air Through Ducts, Part 2

There's more to duct sizing than low resistance to air flow

What is the best velocity for moving air through ducts? Photo: Energy Vanguard

This is the second post in a 2-part series on the physics of moving air through ducts. If you missed it, click here for The Best Velocity for Moving air Through Ducts, Part 1.

The first thing to know about the velocity of air moving through ducts is that the slower you get the air moving, the better it is for air flow. That was the main point of my last article. It asked the question, is low velocity bad for air flow in ducts? And the answer was that, in terms of air flow, you really can’t get the air moving through the ducts too slowly.

But that’s not the end of the story. If it were, you’d always try to get the lowest velocity possible by using the biggest ducts that fit the space without blowing the budget. There’s another relevant fact, however, and ignoring it can lead to trouble.

In moving air through a duct system, we want good airflow, but remember that the object isn’t just to move air throughout the house. It’s to move heated air in winter and cooled air in summer. When that conditioned air is moving through the ducts, the second law of thermodynamics comes into play because we have a temperature difference between the inside and outside of the ducts.

The second law of thermodynamics says that when you have objects at different temperatures, heat flows from the warmer to the cooler object. In winter, the warm air in our ducts can lose heat to the surroundings. In summer, the cool air gains heat from the surroundings.

And the amount of heat that flows between the duct and its surroundings depends on three things:

The equation that ties these things together is:

Heat flow equation

Q here is the…

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  1. MKCF | | #1

    Thank you for this informative article! It's very hard to find this sort of information if you're not in HVAC school, I have discovered.
    I'm in the process of designing a heat system for my old-house remodel using Mitsubishi heat pumps. My plan requires a short linear duct upstairs so I can heat multiple rooms with a single air handling unit (SEZ-KD15NA4). Both the air handler and the duct will be in the conditioned space: the unit will be in a closet and the duct will be in a soffit.
    I read your great 2016 article series on duct design. The one thing I couldn't piece out from it, was how to connect the BTU requirements for each room to the number of CFM needed. "MANUAL S". Does your next article explain this one?
    Another rule of thumb popped up, which is 400 CFM = 12,000 BTU.
    However, after reading this it seems like it might be ok to use this rule of thumb in order to get the proportions, and then simply scale the ducts as big as one can aesthetically stand, keeping them in proportion to each other. Bigger ducts would keep the static pressure low as well as the velocity. Correct?

    Thanks again,

  2. User avater
    Dana Dorsett | | #2

    It seems like you might be conflating 400 cfm (volume rate) and 400 fpm (velocity). You only get 400 fpm out of a 400 cfm flow when the cross section of the is 1 square foot.

    The 400 cfm/12,000 BTU rule of thumb is for latent load handling on cooling equipment. On single speed air conditioners if you cut that ratio in half the evaporator coil is prone to icing up, if doubled the latent cooling goes way down, delivering cool-but-clammy conditions indoors.

    But sure, a bigger duct yields lower velocity, which reduces the static pressure induced by the duct.

  3. MKCF | | #3

    So, my approach to duct sizing might actually work in this case? Nice big grilles might be a good idea too?

    1. User avater
      Dana Dorsett | | #4

      Big grilles on the return registers is fine, but you may need to taper down to a smaller size/greater velocity and use smaller supply grilles to get sufficient throw for good mixing.

  4. MKCF | | #5

    Thank you!!

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