Air Distribution: Airflow, Static Pressure and Proven Numbers
Airflow, static pressure and the numbers that prove it
Air distribution questions test whether you can prove airflow rather than guess at it. Know total external static pressure (TESP): measure with a manometer using the correct pressure taps, with a clean filter and a wet coil, and compare the result to the blower performance table. Residential equipment is commonly rated at 0.5 in. wc.; a reading above the table rating means the ducts, coil, filter or registers are undersized or restricted.
Know the fan laws: cfm varies directly with speed, static pressure with the square of speed, and horsepower with the cube. Know the friction-rate method (about 0.1 in. wc. per 100 ft of equivalent length), supply velocities of roughly 700-900 fpm, return velocities of 600-700 fpm, and the 400 cfm/ton rule.
Traps: taking static pressure in the return plenum only, reading a blower table without correcting for a wet coil and filter, forgetting that a high-MERV filter or bypass humidifier changes TESP, and confusing face velocity with duct velocity. Also know the sensible heat equation: cfm = sensible Btu/hr ÷ (1.08 x ΔT).
Worked example: a 3-ton system must move about 1,200 cfm. A 16 x 25 in. filter grille has 2.78 ft² of face area, so 1,200 ÷ 2.78 ≈ 432 fpm at the grille, which is acceptable. The same 1,200 cfm forced through a 12 in. round trunk (0.785 ft²) is about 1,528 fpm, far too fast and a guaranteed source of noise and high static pressure.