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As my contractors finish installing ductwork and the air handler for my central AC system, I would like to have a measure of how many CFMs come out of each register, as a function of how the system is designed. In other words, if there is significant disparity among solidly quantifiable measures (coming out of some metering device as opposed to putting a hand and saying "it's not blowing much"), I will send the contractors to reconfigure the ductwork until I get a proper distribution of flow.

Is there a metering device I can use to measure this flow?

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3 Answers 3

Use an Anemometer to measure the air velocity, then multiply that by the cross sectional area of the duct.

Circular Duct

Air Velocity x (πr²) = Air Volume
4583.67 feet/minute x 3.14159265359 x ((4 inches / 12)/2)^2 = cubic feet/minute
4583.67 x 3.14159265359 x 0.02777777777777777778 = CFM
4583.67 x 0.08726646259972222222222222222222 = CFM
400.00066662446875833333333333333 cubic feet/minute

Rectangular Duct

Air Velocity x Length x Width = Air Volume
1772.31 feet/minute x (3 1/4 inches / 12) x (10 inches / 12) = cubic feet/minute
1772.31 x 0.2708333333333333333333333333 x 0.8333333333333333333333333333 = CFM
1772.31 x 0.22569444444444444444444444444444 = CFM
400.00052083333333333333333333333 cubic feet/minute

Or pick up a handheld digital anemometer, and let it do the calculations for you.

handheld digital anemometer

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Research so far indicates this contraption.

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Get an anemometer for $40 on the web and download the related app for your smartphone (free!) Website even has a video showing you how to use it.


the app will track total cfm. the next step is determine the output of your furnace to determine leakage etc.

Seal all ducts, balance your house with the main branches and enjoy!

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