# flex duct



## BayPointArchitect (Sep 23, 2020)

Applicable code: 2018 IMC
Applicable sections: 603.6.2.1 and 603.6.2.2
Flexible air connectors (a.k.a. flex duct) shall be limited in length to 14 feet.  Flexible air connectors (a.k.a. flex duct) shall not pass through any wall, floor, or ceiling.

Question:
Does your jurisdiction have any written amendments or hip-pocket practices related to the use of flex duct?  For example, my mechanical inspectors don't like to see flex duct except for the short elbow connection directly above the diffusers/return grills.  And they make verbal recommendations of six feet (shorter) while our written amendment allows for twenty-five feet (longer).  How do inspectors in your neighborhood govern flex duct?


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## mtlogcabin (Sep 23, 2020)

A flexible air connector and Flexible ducts are not the same and you must look at the label to know which one you have

FLEXIBLE AIR CONNECTOR. A conduit for transferring air between an air duct or plenum and an air terminal unit or between an air duct or plenum and an air inlet or air outlet. Such conduit is limited in its use, length and location.

603.6.1 Flexible air ducts.
Flexible air ducts, both metallic and nonmetallic, shall be tested in accordance with UL 181. Such ducts shall be listed and labeled as Class 0 or Class 1 flexible air ducts and shall be installed in accordance with Section 304.1.

603.6.1.1 Duct length.
*Flexible air ducts shall not be limited in length*.

603.6.2 Flexible air connectors.
Flexible air connectors, both metallic and nonmetallic, shall be tested in accordance with UL 181. Such connectors shall be listed and labeled as Class 0 or Class 1 flexible air connectors and shall be installed in accordance with Section 304.1.

603.6.2.1 Connector length.
*Flexible air connectors shall be limited in length to 14 feet (4267 mm).*

What is the difference between Flexible Air Ducts and Flexible Air Connectors?

UL (Underwriters Laboratories) in their *UL 181 Standard for Factory-Made Air Ducts and Air Connectors*, defines two categories of flexible "ducts"

The UL Listed Flexible Air Duct must pass all of 15 tests in the UL 181 Standard. *Air Ducts are labeled with a square or rectangular label showing their respective listing*. There is no limitation on the length of runs when using UL Listed Air Ducts.

The UL Listed Flexible Air Connector must pass only 12 of the 15 tests of the UL 181 tests, *and is labeled with a round shaped label, which states "for installation lengths not over 14 feet"*.   An installer may not increase the 14-foot limitation by using a splice between 14' sections of Air Connectors.  This length limitation is set by the requirements in NFPA 90A & 90B Standards.
https://flexibleduct.org/ADC_FAQs.asp


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## north star (Sep 23, 2020)

*# ~ #*

Here is a Link from the Forum, "Useful Links" thread:
*





						Air Duct Council - [ ADC_Inst.asp ]
					

Air Duct Council (ADC)



					flexibleduct.org
				



*
*# ~ #*


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## Sifu (Sep 23, 2020)

Most commercial mechanical plans provide limits on flex duct run-outs specific to the project.  As far as I know there is no set maximum length requirement outside of that.  SMACNA says use as short as possible, has support requirements and friction loss calculation provisions that should encourage this.


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## ICE (Sep 23, 2020)

California Mechanical Code
603.4.1 Length Limitation. [Not permitted for OSHPD 1, 1R, 2, 3, 4 & 5] Factory-made flexible air ducts and connectors shall be not more than 5 feet in length and shall not be used in lieu of rigid elbowsor fittings. Flexible air ducts shall be permitted to be used as an elbow at a terminal device.
*Exception:* Residential occupancies


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## Paul Sweet (Sep 24, 2020)

Flex duct has a much greater friction loss than metal duct, so the ducts may need to be a size larger larger if they are much over 10 or 12 ft. long.


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## BayPointArchitect (Sep 25, 2020)

Thanks everyone.  What I discovered is that many mechanical engineers make a note that flex duct may be used but limited to only 6 feet.  In that case, it is not a code requirement but a restriction imposed by the mechanical engineer - and then enforced by the field inspector when the contractor attempts to deviate from the mechanical engineer's design.

I also learned that my jurisdiction allows flex duct of unlimited length - provided that it meets all other design requirements.  However, a flex duct that is more than 25 feet needs to be insulated.


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