Air Distribution
Air Distribution covers how supply air travels from the blower to the occupied space with the right quantity, velocity and mixing. The exam tests conversion between CFM, Btu/h and temperature difference, static-pressure reading, the fan laws, and diagnosis of the complaints that show up as hot rooms and high bills.
Airflow benchmarks
- Nominal airflow is 400 CFM per ton; the workable range is 350–450 CFM/ton. Below about 350 the coil runs cold and loses latent control; above about 450 dehumidification suffers and the blower gets noisy.
- Sensible heat equation: Btuh (sensible) = 1.08 × CFM × ΔT, where 1.08 = 60 min × 0.075 lb/ft³ × 0.24 Btu/lb·°F.
- Total external static pressure (TESP) is measured across the blower with both taps - supply plenum and return plenum - filter and coil in place. Residential equipment is typically rated near 0.5 in. w.c.; 0.8–1.0 in. w.c. signals a restrictive duct system, filter or coil.
Fan laws
CFM varies directly with RPM, static pressure with the square, brake horsepower with the cube. Speed changes are powerful and easy to overdo.
Worked example
A blower delivers 1,200 CFM at 0.50 in. w.c. You raise speed from 1,000 to 1,150 RPM; ratio = 1.15. New CFM = 1,200 × 1.15 = 1,380. New static = 0.50 × 1.15² = 0.66 in. w.c. New horsepower = original × 1.15³ = 1.52×, a 52% motor load increase for 15% more air. That is why duct improvements or a different motor, not just a faster pulley, are often the real answer.
Traps
Substituting total capacity for sensible capacity in the 1.08 equation; measuring static pressure with the filter removed, which hides the restriction; and assuming a whistling register means too much air, when it usually means too little duct and high velocity.