Air Distribution, Ductwork, and Indoor Air Quality
What the exam expects
Air side questions test static pressure, airflow, duct sizing, and the components that shape airstreams: trunk and branch ducts, takeoffs, dampers, registers, diffusers, and returns. Know that total external static pressure is the sum of supply and return static, that friction rate drives duct sizing (equal-friction method), and that flexible duct must be fully extended with gentle bends to avoid added resistance.
Psychrometrics belongs here too: dry bulb, wet bulb, dew point, relative humidity, and the sensible heat equation. Indoor air quality covers ventilation air, filtration (MERV ratings), humidity control, and contaminant sources. Know that a coil removes latent heat only when it operates below the dew point, and that oversized equipment can leave a space cold and clammy.
Traps
- Reading static pressure with a dirty filter installed and calling it design airflow.
- Confusing velocity pressure with static pressure.
- Assuming a higher MERV filter is free - pressure drop and blower capacity matter.
- Sizing ducts by guesswork rather than by friction rate and available static.
- Ignoring return-side restriction, which shows up as high TESP.
Worked example
Sensible capacity = 1.08 x CFM x temperature difference. A system moving 1,200 CFM with a 20°F return-to-supply difference delivers 1.08 x 1,200 x 20 = 25,920 Btu/h sensible. If measured delta-T is only 14°F, sensible capacity has dropped to about 18,100 Btu/h - check airflow before adding refrigerant.