Ventilation, Air Distribution, and Ductwork
This domain blends indoor air quality, duct design, and pressure balance. Know the difference between ventilation (intentional outdoor air supply), infiltration (unintentional leakage in), and exfiltration. Understand ASHRAE 62.2-style residential ventilation rates based on floor area and bedroom count, and why tight homes need mechanical ventilation. Recognize that bath and kitchen exhaust must be ducted outdoors, not into attics, and that makeup air matters for large exhaust appliances.
Duct design: know friction rate, equivalent length, and why fittings cost more pressure than straight runs. Be able to explain the equal-friction method and the concept of a ductulator. Understand that return air paths matter as much as supply; a starved return raises static pressure and reduces delivered airflow. Static pressure testing (supply plus return, measured in inches of water column) is the diagnostic backbone.
Traps: confusing velocity pressure with static pressure, ignoring duct leakage to unconditioned space, and assuming bigger ducts always solve comfort complaints when the real issue is balancing or a restrictive filter. Also watch for questions that confuse cfm (volume flow) with fpm (velocity).
Rule of thumb: Residential supply trunks are often sized near 0.1 in. w.c. per 100 ft of effective length; total external static pressure on a typical residential air handler should stay within the manufacturer's rated limit (often 0.5 in. w.c.). Worked example: a return grille sized too small shows high return static; enlarging the return and adding a filter grille lowers total static and restores airflow.