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Airflow and CFM

Understanding how to measure air flow in CFM (cubic feet per minute) or volume of air flow can be helpful when you are ready to remodel the bathroom or kitchen and are wanting to install an exhaust vent system. Most exhaust ventilation systems are rated in CFM. Generally these types of exhaust units may require some type of fresh air intake for proper exhaust operation. By performing a basic test and calculations, you can measure the air flow in CFM to help you design your next remodel or new installation of an exhaust system.
Air flow meter

Place the air flow meter inside the air duct or in front of the air register you are wanting to measure. The air flow meter will measure the speed or velocity of the moving air that is passing through the system. Generally the meter will have an LCD display or analog meter that will give a reading of the air speed or velocity in feet per minute.
Find the area of a duct that measures 1 foot wide by 2 feet high. The area of a square or rectangle is found by multiplying the width times the height. In this case we will multiply 1 foot times 2 feet for a total area of 2 square feet or 2 ft^2. 

Calculate the volume of air flow in CFM if the meter from step 1 reads 1000 feet per minute. Since CFM is a volume measurement we simply multiply 1000 feet per minute (the air speed) times 2 ft^2 (the area of the duct) and the resultant volume of air flow is equal to 2000 CFM.

Figure the CFM with the same air flow velocity of 1000 feet per minute, but for a round duct that is 1 foot in diameter. The formula for finding the area of a circle is -  Area is equal to Pi times radius squared or ( A= Pi * R^2) .  Where Pi is equal to 3.14 and the radius is half the diameter of the round duct which is equal to .5 feet.

Use the formula from the above step and insert the numbers to find the area of the round duct. The area of the duct is equal to (3.14 * .5^2)  which is .785 ft^2.

Multiply the air speed of 1000 feet per minute times .785 ft^2 and the resultant air flow volume is 785 CFM.

Words to the Wise:
Follow the manufacturers instructions for proper readings and care of the air flow meter.

When testing any air system, clean filters should be in place during any air flow test.

Some air flow meter manufacturers may require the removal of the air filters so the total circulation of the system can be evaluated.

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