Flexible air duct – maximum length

[123131415]

Dear member
What shall be maximum length of flexible air duct(used to connect diffuser and coller) as per smacna.

 

[willard3]

You can use any length you want, but it has high pressure drop and proper installation is critical.

 

[berkshire]

Most duct installations TRY to limit this to 3'-0", however as Willard says this is not cast in stone.
B.E.

You are judged not by what you know, but by what you can do.

 

[lilliput1]

My firm spec says 10 ft.

 

[GMcD]

Good practice is to use less than 2 meters or less than 6 feet. The issue is the high air friction as well as supporting the flex duct to prevent kinks and sharp bends.

 

[SAK9]

Supporting long lengths of flexible duct without deformation is almost impossible.Once it is deformed all your pressure drop calculations are out through the window.If you keep it short(<1.5m),it may keep its circular shape and possibly may not need to support it.

 

[DrRTU]

I believe smacna does not have a limit to the length but as a competent designer, you will want to limit the length based upon in house design standards, local AHJ restrictions or standard design guides such as IMC. IMC limits length 14’ but most locale codes limit the length to 7’. I see a lot of specs that limit the length to 5’. We stop at 5’ or 2’ on VAV boxes.

 

[ME27272727]

There is a difference between "flexible air duct" and "flexible air connector." Flexible air duct has no limit to allowable length, while flexible air connectors are limited to a maximum distance of 14 feet per IMC.

There is a listing labeled on the duct that determines which classification it falls under.