Duct Installation and Assembly
The Duct Installation and Assembly domain tests whether you can put a designed system together so it performs as designed. Expect support spacing, joint assembly, airflow direction, flex duct routing, fire damper installation, access, and coordination with other trades.
Core rules and concepts
- Support horizontal rectangular duct at intervals allowed by SMACNA and the job specifications, commonly 8 ft; round duct commonly 10 ft; flexible duct commonly 4–5 ft and within 2 ft of a fitting, per the listing.
- Assemble joints fully seated and in the correct airflow direction. Use the specified sealant or gasket; do not rely on tape alone where mastic is required.
- Route flex duct fully extended with long-radius bends; avoid sharp 90s, compression, and sagging.
- Install fire dampers in listed assemblies with required sleeves, clearances, and access panels.
- Provide access to dampers, smoke detectors, and fire dampers. Isolate duct from vibration with flexible connectors where specified.
Worked example
A 25-ft flex run must turn 90° twice to reach a ceiling diffuser. The installer shortens the run to 15 ft, replaces one tight bend with a sweep, and supports the duct every 4 ft. The revised route has lower equivalent length and less sag, so the branch can deliver its design cfm at the available static pressure.
Traps that cost marks
Hanging flex duct from a single strap, compressing it behind a joist, using sharp elbows, and supporting rectangular duct only at joints. Another is installing a fire damper upside down or without access, which fails inspection even if airflow is fine. Also watch for ducts that block service access to equipment.