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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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2 Answers 2

Research so far indicates this contraption.

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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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