How to Calculate CFM for Ductwork

Up to 30 percent of new and renovated buildings suffer from unhealthy indoor air, according to a 1996 report from the U.S. Green Building Council, which also asserts that "indoor air pollution is among the top five environmental risks to public health." Faulty air conditioning contributes to this situation, and although heating and ventilation engineers make adequate provision for ideal operating conditions, system performance deteriorates over time. Routine precautionary checks include measurement of the air flow rate in cubic feet per minute, or CFM.

Velocity pressure sensor
Drill
Differential pressure gauge
Duct tape
Calculator
Tape measure

Instructions

- 1
  Measure the velocity pressure in the duct. Turn off the ventilation system and insert the probe of the velocity pressure sensor into the duct through a measurement port. If the duct does not have a measurement port, drill one or more suitably sized holes at a convenient position in the duct. Ensure the open face of the sensor is facing into the air flow.
- 2
  Connect the sensor tubes to the pressure gauge. Turn on the system and let it stabilize. Note the reading, in inches of water, on the gauge. Write down this figure.
- 3
  Turn off the system and remove the sensor. Seal any holes that you drilled, using duct tape. If required, take further measurements by repeating the procedure.
- 4
  Calculate the square root of the velocity pressure. Multiply your answer by 4,005 and write down the result, which is the air velocity in feet per minute, or ft/min. For example, if the velocity pressure is 0.04 inches of water, the air velocity is 801 feet per minute.
- 5
  Measure the duct. For square or rectangular ducts, measure the width and height of the duct in inches. For round ducts, measure the circumference.
- 6
  Calculate the cross-sectional area. For square or rectangular ducts, multiply the width of the duct by the height to give the area in square inches. For example, if the width of the duct is 12 inches and the height 18 inches, the area is 216 square inches. Write down your answer.
- 7
  Use a different method for round ducts. Divide the circumference by 6.2832 to give the radius. Work out the square of the radius and multiply this by 3.1416 to give the area in square inches. For example, if the circumference of the duct is 30 inches, the radius is 4.775 inches and the cross-sectional area is 71.6 square inches. Make a note of your answer.
- 8
  Divide your answer by 144 to give the area in square feet. For example, if the area is 216 square inches, the cross-sectional area is 1.5 square feet.
- 9
  Multiply the air velocity in feet per minute by the cross-sectional area in square feet to give the air flow rate in cubic feet per minute. For example, if the air velocity is 801 feet per minute and the cross-sectional area is 1.5 square feet, the air flow rate is 1,201.5 CFM.
- 10
  Check each step of your work. Record the result, which is the air flow rate, expressed in CFM.

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