Sheet Metal Fabrication and Installation
This section covers sheet metal fabrication and installation: duct gauges, seams, joints, reinforcement, hangers, clearances, sealing, insulation, and fire-rated penetrations. The exam tests whether the installed duct system will stay together, stay sealed, and stay out of trouble.
Fabrication and support
Duct construction follows SMACNA pressure classes and code. Use the specified gauge for the duct size and static pressure; reinforce wide ducts; use approved seams and joints. Support horizontal rectangular duct at intervals allowed by the construction standard, and support flex duct without sagging. Keep clearance to combustibles and maintain access to dampers, coils, and fire dampers. Seal joints and seams to the required seal class; mastic and approved tape outperform duct tape.
Fire and smoke
Fire dampers are required where ducts penetrate rated assemblies and must be installed in the listed orientation with an approved sleeve and access. Do not cut a fire damper or remove a fusible link. Firestopping around penetrations must match the listed system.
Worked example
A 24 in x 12 in rectangular duct carries 800 CFM. The area is 2 ft x 1 ft = 2 ft². Velocity = 800 / 2 = 400 ft/min. If the same airflow were forced through a 12 in x 8 in duct, area = 1 ft x 0.667 ft = 0.667 ft², velocity = 1,200 ft/min, which is likely too high for a low-pressure system and will raise noise and static. The fix is a larger duct or a different layout, not more fan speed.
Traps
Using duct tape as a permanent seal; undersizing flex duct and creating sharp bends; blocking fire dampers with insulation; missing access doors; and supporting duct from ceiling grid.