Should flexible duct be used as a plenum?

Should flexible duct be used as a plenum?

• A. Yes
• B. Only if insulated
• C. No
• D. Only in short sections

Answer: (C)

Plenums are air distribution spaces that must be rigid, sealed, and noncombustible so the air can flow cleanly and be easily cleaned and inspected. Flexible duct is designed for short, connection-not-as-a-chamber runs and has seams and an inner liner that can shed fibers, trap moisture, and leak air. In a plenum, those issues create contamination, pressure loss, and potential fire and smoke concerns, which is why flexible duct should not serve as the plenum unless it is specifically listed as plenum-rated for that exact use. Even insulation on flexible duct doesn’t fix the fundamental problem of its structural and cleanliness limitations in a plenum space. The right approach is to use rigid metal duct or a listed plenum-rated assembly for the air-distribution chamber, ensuring a durable, cleanable, and code-compliant installation.

Plenums are air distribution spaces that must be rigid, sealed, and noncombustible so the air can flow cleanly and be easily cleaned and inspected. Flexible duct is designed for short, connection-not-as-a-chamber runs and has seams and an inner liner that can shed fibers, trap moisture, and leak air. In a plenum, those issues create contamination, pressure loss, and potential fire and smoke concerns, which is why flexible duct should not serve as the plenum unless it is specifically listed as plenum-rated for that exact use. Even insulation on flexible duct doesn’t fix the fundamental problem of its structural and cleanliness limitations in a plenum space. The right approach is to use rigid metal duct or a listed plenum-rated assembly for the air-distribution chamber, ensuring a durable, cleanable, and code-compliant installation.