Duct Systems and Ventilation
Duct construction and materials
Know duct material standards, minimum thickness and support spacing, sealing requirements, and restrictions on using building cavities as ducts. Concepts include duct sealing (joints, seams, and penetrations), support intervals, and protection where ducts pass through fire-rated assemblies. Flexible duct has limits on length, sag, and support.
Ventilation and exhaust
Understand the distinction between supply, return, and exhaust; the requirement for outdoor air where the code requires it; and exhaust requirements for specific occupancies (bathrooms, kitchens, hazardous operations). Know that exhaust systems for hazardous or grease-laden air must not be combined with other systems and must discharge to a safe location.
Traps
- Treating a panned joist or building cavity as an approved duct without meeting the code conditions.
- Under-supporting flexible duct or allowing sharp bends that restrict airflow.
- Routing return air through a bathroom, kitchen, or garage - generally prohibited.
- Forgetting that duct penetrations of fire-resistive assemblies need approved protection.
- Mixing exhaust from different hazard classes in one system.
Worked example
A designer proposes using a floor cavity as a return plenum. Is that automatic? No - the cavity must be constructed, sealed, and separated per code, and must not contain prohibited materials or communicate with certain spaces. Rule of thumb: a cavity is not a duct unless it meets duct requirements.
Rule of thumb
Seal every joint you can see; support flexible duct so it stays straight and uncompressed.