Duct Materials, Components, and Fabrication
The Duct Materials, Components, and Fabrication domain tests your knowledge of what ducts are made of, how they are joined, and which components belong in a system. Items cover galvanized steel gauges, spiral and rectangular duct, flexible duct listings, duct board, dampers, access doors, turning vanes, and SMACNA fabrication practice.
Core rules and concepts
- Galvanized steel is the default for most commercial and residential ducts; gauge is selected from SMACNA tables using duct size, pressure class, and reinforcement.
- Rectangular ducts use Pittsburgh locks, drive slips, TDC/TDF joints, and cross-breaks or reinforcements. Spiral round duct uses gasketed slip joints and couplings.
- Flexible duct must be listed to UL 181 and installed fully extended, with gentle bends and support per the listing.
- Fire dampers must be listed for the rated assembly and installed with the correct sleeve, clearances, and access.
- Volume dampers, turning vanes, takeoffs, boots, registers, and access doors are selected for function, not just size.
Worked example
A 48 in. × 12 in. low-pressure trunk is proposed in 26-gauge steel. The 48-in. dimension is wide and may exceed the unreinforced span allowed by the SMACNA gauge table at that pressure class. The fabricator checks the table, then selects a heavier gauge or adds a TDC joint and cross-break. The lesson is that gauge is not chosen by habit; dimension and pressure drive it.
Traps that cost marks
Using 26-gauge for every rectangular duct, installing a fire damper without access, mixing flexible duct brands with unlisted tape, and forgetting that turning vanes must be installed in the direction of airflow. Another trap is selecting a damper that cannot be locked or accessed.