Duct Systems and Air Distribution
This domain tests duct construction and air distribution: materials, thickness and reinforcement, seams and joints, supports, dampers, access, clearances, plenum use, insulation, and testing. Questions often ask whether a duct assembly is acceptable for its location and pressure class.
Core rules
- Ducts must be constructed of approved materials and be adequate for the pressure class and installation environment. Gauge, reinforcement, and joint method depend on duct size, pressure, and location.
- Joints and seams must be sealed appropriately for the pressure class; tape alone is not a universal sealant. Flexible duct must be installed with gradual bends, limited sag, and proper support.
- Support ducts at code intervals and within required distances of joints and fittings; do not support duct from ceiling grid or from other trades' hangers unless approved.
- Fire dampers are required where ducts penetrate fire-resistive assemblies, and smoke dampers where smoke barriers require them. Dampers need approved access.
- Duct clearance to combustibles, insulation requirements, and use of concealed spaces or return plenums are restricted. Undercut framing or use of a stud cavity as a return requires specific approval.
Worked example
A 20 in by 10 in supply duct crosses a one-hour rated corridor wall. The plan shows a volume damper but no fire damper and no access panel. The correct review response is to require a listed fire damper at the rated assembly, installed per its listing, with an accessible inspection opening. A volume damper does not satisfy fire-resistance requirements.
Traps
Using duct tape as the only seal on higher-pressure duct; compressing flexible duct at fittings; hanging duct from ceiling grid; omitting damper access; and assuming any concealed space can be used as a return plenum.