Air Distribution, Ductwork, and Indoor Air Quality Study Guide
This domain, Air Distribution, Ductwork, and Indoor Air Quality, tests how air moves and how it is cleaned, ventilated, and balanced. Expect static pressure, duct sizing, equivalent length, fan laws, filters, outdoor-air ventilation, humidity control, duct leakage, and pressure effects on combustion appliances.
Core rules and concepts
- Airflow follows the fan curve: as external static pressure rises, delivered CFM falls. Measure total external static pressure with a manometer at the manufacturer’s ports and compare it with the blower table.
- Duct friction is usually selected as a friction rate, such as 0.1 in. w.c. per 100 ft equivalent length. Fittings add equivalent length, and every extra elbow costs pressure.
- Filters trade filtration efficiency (MERV) against pressure drop. A restrictive filter reduces airflow and can freeze coils or trip high limits.
- Ventilation brings outdoor air for dilution; filtration and source control handle particulates. Dehumidification is a separate load from sensible cooling.
- Return path and door undercuts matter: a starved return depressurizes rooms and can back-draft combustion appliances.
Worked example
A duct run has 100 ft of straight duct plus fittings totaling 50 ft equivalent length: 150 ft. At a design friction rate of 0.1 in. w.c./100 ft, friction loss = 0.1 × 150/100 = 0.15 in. w.c. If the blower’s available external static pressure is 0.60 in. w.c. and the filter, coil, and grilles add 0.45 in. w.c., only 0.15 in. w.c. remains for the duct-right at the limit. Reduce fittings or resize duct rather than simply raising fan speed.
Traps
Confusing velocity pressure with static pressure; reading static on only one side of the blower; using nominal filter size instead of actual pressure drop; ignoring return-side restriction; treating ventilation as filtration.