Airflow Measurement, Balancing, and Diagnostics
Airflow Measurement, Balancing, and Diagnostics tests the math and field judgment behind moving air: using manometers, pitot tubes, anemometers, and flow hoods; converting velocity to cfm; balancing branches; and diagnosing high static or low airflow.
Measurement fundamentals
For standard air, velocity in fpm = 4005 × √(velocity pressure in in. wc). Flow in cfm = duct area in ft² × velocity in fpm. Circular area = 0.7854 × diameter² (diameter in feet); rectangular area = width × height in feet. A pitot traverse uses a grid of points because velocity varies across the duct; a single center reading is not a reliable average. Flow hoods measure at grilles but are sensitive to edge leakage and swirl.
Balancing and diagnosis
Balance proportionally: adjust the farthest branch first, then work back, and set the main damper last. Compare measured static pressure and airflow with the fan table. High filter or coil drop, collapsed flex duct, closed damper, or undersized returns raise static and reduce cfm. Blower laws help: cfm varies with speed, static pressure with speed squared, and power with speed cubed.
Worked example
A 10-in. round duct reads 900 fpm. Diameter = 10 ÷ 12 = 0.833 ft. Area = 0.7854 × 0.833² = 0.545 ft². CFM = 0.545 × 900 = 491 cfm. If design airflow is 550 cfm, the duct is about 11% low; check for a partly closed damper, flex sag, or a low fan speed before resizing.
Traps
Do not leave inches in the area formula; convert to feet. Do not treat a centerline velocity as average velocity. Do not use a flow hood on a swirling or obstructed outlet without correcting. Do not blame the duct for low airflow before checking filter, coil, and blower speed.