Air Systems Testing, Adjusting, and Balancing
Work from the fan outward
Air balancing follows a fixed order: verify total airflow at the air handler, then the main and branch ducts, then the terminal devices. Before touching a damper, confirm fan RPM, motor amperage, filter condition, drive alignment, and belt tension, and record static pressures across the fan, filters, and coils. A dirty filter or a slipping belt makes every downstream reading meaningless.
Duct airflow is measured with a pitot-static tube traverse using the log-linear (equal-area) method, with at least 16 points and the plane located roughly 7.5 duct diameters downstream and 3 upstream of any disturbance when the layout allows. The classic trap is averaging velocity pressures and taking one square root; velocity varies with the square root of velocity pressure, so convert each point to velocity (or average the square roots) before averaging. At standard air, V (fpm) = 4005 x sqrt(Vp in. w.c.). At high altitude or elevated air temperature, correct with V = 1096 x sqrt(Vp / density).
For adjusting, use the proportional method: begin at the least-favored terminal, set each branch to the same ratio of measured-to-design airflow, and work back toward the fan so early adjustments are not undone. Avoid using a terminal unit's volume damper as a branch balancer when it will generate noise.
Tolerances: plus or minus 10 percent of design for outlets, branches, and total flow is the usual benchmark, but always defer to the project specification.
Worked example
Average Vp = 0.09 in. w.c. gives 4005 x 0.30 = 1202 fpm. An 18 in. x 12 in. duct is 1.5 sq ft, so airflow is 1202 x 1.5 = 1803 cfm.