Air Distribution
Air Distribution
NATE’s Air Distribution domain tests whether an installer can deliver design airflow quietly, at the correct static pressure, and with a balanced return path. Questions often involve duct sizing, fan laws, register selection, and field balancing.
Core rules
- Sensible airflow: CFM = Btu/h ÷ (1.08 × ΔT). Total airflow uses enthalpy: Btu/h = 4.5 × CFM × Δh.
- Use the equal-friction method at about 0.1 in. w.c. per 100 ft of equivalent length; keep supply trunk velocity roughly 600–900 fpm and branches 400–700 fpm.
- Total external static pressure is measured across the air handler and includes filters, grilles, and ductwork. Compare it with the manufacturer’s fan table, which already accounts for internal coil and cabinet losses.
- Fan laws: CFM varies with RPM, static pressure with RPM squared, and brake horsepower with RPM cubed.
Worked example
A 3-ton (36,000 Btu/h) sensible load has a 20°F temperature rise. CFM = 36,000 ÷ (1.08 × 20) = 1,667 CFM. If the measured airflow were only 1,200 CFM, the same load would produce ΔT = 36,000 ÷ (1.08 × 1,200) = 27.8°F, a sign of low airflow. An 8-inch round branch at 700 fpm carries about 244 CFM (area 0.349 ft² × 700), so this system needs several branches, not one large run.
Traps
Measuring only supply static and ignoring the return; comparing ESP to a table without the actual filter; treating 400 CFM/ton as a code requirement; and forgetting that a restrictive filter raises static and lowers airflow.