Mechanical Code…
Any tube or conduit for transmission of air, fumes, vapors, or dusts. This definition shall not include:
(1) A vent, vent connector, or chimney connector.
(2) Any tube or conduit wherein the pressure of the air exceeds one (1) pound per square inch.
(3) The air passages of listed self-contained systems.
(Uniform Mechanical Code® 206.0)