Air Distribution Devices and Airflow Balance
The Air Distribution Devices and Airflow Balance domain tests how supply and return devices deliver air and how a system is balanced. Items cover diffuser and grille selection, throw, spread, drop, face velocity, free area, terminal velocity, noise criteria, dampers, and TAB procedures.
Core rules and concepts
- A register or diffuser is selected by cfm, throw, pressure drop, and NC (noise criteria), not by neck size alone.
- Face velocity is not the same as free-area velocity. Use free area when calculating cfm through a grille.
- Balance proportionally: start at the farthest branch, adjust dampers to set design flows, then recheck the fan and static pressure.
- Return air must have a path; undersized returns raise static pressure and reduce supply airflow.
- Use a balometer or flow hood at grilles and a Pitot tube or anemometer for duct traverses.
Worked example
A 12 in. × 6 in. return grille has 70% free area and a measured face velocity of 500 fpm. Nominal area is 0.5 ft², so free area is 0.5 × 0.70 = 0.35 ft². CFM = area × velocity = 0.35 × 500 = 175 cfm. Using the nominal 0.5 ft² would wrongly give 250 cfm.
Traps that cost marks
Using nominal instead of free area, taking a single velocity reading at the center of a grille, balancing by moving the diffuser damper instead of the branch damper, and ignoring fan static pressure. Another trap is failing to recheck airflow after closing a damper elsewhere.