Rule of thumb: if it is a short flexible piece connecting equipment, a terminal unit, or a diffuser boot, review it as a connector. If it is the actual air path from point A to point B, review it as duct.
Plain English:
603.6 Flexible Air Ducts and Flexible Air Connectors
Flexible air ducts and flexible air connectors are not the same thing.
Both metallic and nonmetallic types must comply with the subsections that apply to each.
603.6.1 Flexible Air Ducts
Flexible air ducts must be tested per UL 181.
They must be listed and labeled as Class 0 or Class 1 flexible air ducts.
In plain terms, flex duct is part of the actual duct run.
603.6.1.1 Duct Length
Flexible air ducts are not limited in length by this section.
That is not a design recommendation. It only means this section does not set a max length.
603.6.2 Flexible Air Connectors
Flexible air connectors must also be tested per UL 181.
They must be listed and labeled as Class 0 or Class 1 flexible air connectors.
In plain terms, a connector is a short connection piece, not a substitute for the duct run.
603.6.2.1-603.6.2.2 Connector Limits
Flexible air connectors are limited to 14 feet.
They cannot pass through any wall, floor, or ceiling.
603.6.3-603.6.4 Temperature and Clearance
Design air temperature in flex ducts and connectors must be under 250 degrees F.
Minimum clearance to appliances must follow the manufacturer's instructions.
On Plans: Why it matters
Plans often call everything "flex duct," but the code distinction matters. If a connector is being used as a duct run, exceeds 14 feet, or crosses a wall, floor, or ceiling, that is a straightforward redline and often a field correction.
Code Path: Where to show it
M-001: flex duct / flex connector notes and limits
M-101: identify where flex duct is used versus flex connector
M-501: details at equipment, terminal units, diffusers, and penetrations
M-601: UL 181 and Class 0 / Class 1 basis
Equipment notes: appliance clearance requirements
Check: Do
Distinguish flex duct from flex connector on the drawings
Show connector lengths where misuse is possible
Keep connectors out of wall, floor, and ceiling penetrations
Review Risk: Don't
Don't treat flex ducts and flex connectors as interchangeable
Don't run flex connectors longer than 14 feet
Don't allow flex connectors to pass through walls, floors, or ceilings
Field Tip:
When reviewing a set, trace each flexible segment and ask one question: is this the connection, or is it the duct? That usually tells you which IMC 603.6 rules apply.
Masterbuild QA Lens
Flexible duct use is restricted (603.6) to 5 ft maximum per connection in most applications, with no sharp bends that reduce the internal diameter. Confirm the permit set does not show flexible duct as a substitute for rigid duct on long runs, that the length and number of flexible connections are within the limit, and that the listing for the flexible duct is cited on M-001.
Drawing / Submittal Check
For 603.6-603.6.4 (flexible ducts and connectors): the drawings must show every flexible duct location with its maximum length and the maximum number of bends. Flexible duct longer than the code maximum must be replaced with rigid duct on the drawings before submittal. Show the flexible duct diameter - flexible duct must match the rigid duct diameter at each end without collapsing the liner.
Common Review Risk
Flexible duct errors are the most common duct system inspection failure. Flexible duct longer than the allowed maximum (7 feet under most standards), compressed or kinked flexible duct, and flexible duct used in concealed locations where it is prohibited are all common findings. A field walk during rough-in specifically to check flexible duct lengths and routing prevents most inspection failures.
When To Escalate
Escalate when flexible duct non-conformance has been identified by the inspector and the remedy requires opening finished ceilings in an occupied space. Flexible duct corrections in occupied spaces require coordination with the building owner, the tenant, and the inspector to schedule the work without disrupting operations.
Duct System Coordination
Flexible duct requirements (603.6-603.6.4) limit installation length, bend radius, and support spacing. Show flexible duct maximum lengths on the plans and flag any connection that exceeds the straight-length limit.