Airflow and Duct System Performance
What this domain tests
Airflow and duct performance covers whether the airside can deliver the capacity the equipment produces. The exam targets static pressure measurement, fan laws, duct design pressure budgets and field airflow verification.
Core rules
- Fan laws: CFM varies directly with speed, static pressure with speed squared, and brake horsepower with speed cubed.
- Total external static pressure (TESP) is measured across the air handler with a clean filter and wet coil, using a manometer; compare it with the manufacturer maximum.
- Duct sizing uses available static pressure and total effective length, not a fixed 0.1 in. w.c. per 100 ft rule.
- Pressure drops add in series: filter, coil, heat exchanger, supply and return ducts, and fittings. A high drop in one component steals airflow from the whole system.
- Verify airflow by fan curve, flow hood or temperature rise. For gas heat, CFM = output Btu/h / (1.08 × ΔT); for electric heat, CFM = volts × amps × 3.413 / (1.08 × ΔT).
Worked example
An 80,000 Btu/h input, 80% AFUE furnace has a measured 50°F temperature rise. Output = 80,000 × 0.80 = 64,000 Btu/h. CFM = 64,000 / (1.08 × 50) = 1,185 CFM. If the blower is set for 1,200 CFM, the system is slightly low; check filter, coil and duct static before increasing blower speed. Using input instead of output would incorrectly give 1,481 CFM.
Traps
- Using furnace input instead of output in the temperature-rise equation.
- Measuring TESP with a dirty filter or dry coil.
- Treating duct leakage to unconditioned space the same as total leakage.
- Reading static pressure on only one side of the air handler.