Flexible air duct – maximum length Flexible air duct – maximum length 123131415 (Mechanical) (OP) 19 Aug 16 09:25 Dear member What shall be maximum length of flexible air duct(used to connect diffuser and coller) as per smacna. RE: Flexible air duct – maximum length willard3 (Mechanical) 19 Aug 16 12:00 You can use any length you want, but it has high pressure drop and proper installation is critical. RE: Flexible air duct – maximum length berkshire (Aeronautics) 19 Aug 16 21:39 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. RE: Flexible air duct – maximum length lilliput1 (Mechanical) 20 Aug 16 15:12 My firm spec says 10 ft. RE: Flexible air duct – maximum length GMcD (Mechanical) 20 Aug 16 20:50 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. RE: Flexible air duct – maximum length SAK9 (Mechanical) 21 Aug 16 01:25 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. RE: Flexible air duct – maximum length DrRTU (Mechanical) 22 Aug 16 02:27 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. RE: Flexible air duct – maximum length ME27272727 (Mechanical) 24 Aug 16 19:34 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.