Airflow, Duct Design, and Indoor Unit Installation
This domain tests airflow measurement, duct design, and indoor unit installation. Expect questions on total external static pressure, fan curves, cfm per ton, sensible heat equation, drain traps, and proper coil and plenum installation.
Core rules and concepts
- Nominal airflow is about 400 cfm per ton; humid climates may use 350 cfm per ton for better dehumidification.
- Total external static pressure (TESP) is the sum of supply and return static pressures measured at the unit. Compare TESP to the fan performance table to find actual cfm.
- Sensible airflow can be estimated with cfm = Btu/h ÷ (1.08 × ΔT), where ΔT is the dry-bulb temperature difference.
- Manual D uses available static pressure, equivalent length, and friction rate to size ducts and returns.
- Install the indoor unit level with a primary drain, secondary drain or pan, a trap on the primary line, and continuous slope to the drain.
- Seal the plenum and transitions, support flex duct without kinks, and provide access for coil cleaning and filter service.
Worked example
A 3-ton system has a sensible capacity of 34,000 Btu/h and a measured supply-to-return temperature difference of 20°F. cfm = 34,000 ÷ (1.08 × 20) = 1,574 cfm, which is far above a normal 1,200 cfm for 3 tons. Either the sensible capacity is not actually 34,000 Btu/h at those conditions or the airflow is too high. If airflow is set to 1,200 cfm, the expected ΔT is 34,000 ÷ (1.08 × 1,200) ≈ 26°F. High airflow lowers ΔT and can reduce dehumidification.
Traps that cost marks
Measuring static pressure at only one port; ignoring filter pressure drop; omitting the condensate trap; installing a drain with negative slope; and accepting a flex-duct run with sharp bends or crushed sections.