flex duct vs collar diameter


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Old 08-22-07, 10:25 AM
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flex duct vs collar diameter

What inside diameter flex duct should I be using to connect to a 16" collar? I've tried without success to fit a nonmetallic, insulated, 16" ID flex duct over a 16" collar. It seems like I need either a larger diameter flex duct or some installation tip on how to get a 16" flex duct to fit a 16" collar.
 
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Old 08-22-07, 11:56 AM
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I should work. Might have to crimp start collar to reduce diameter. Notch with snips. Flex duct has no ability to stretch.
 
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Old 08-22-07, 02:15 PM
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Thanks TimJimBob

Appreciate your response, it says the nominal 16" flex duct should fit the nominal 16" collar. I agree the flex duct doesn't stretch. The only degree of freedom is to gradually decrease the diameter of the collar until the flex duct core can fit over collar. I'll try that next.
 
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Old 08-23-07, 02:52 PM
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Mundane subject but interesting numbers

Original return air 16" ID, 7' length nonmetallic, insulated, flex duct installed 2/78 had to be replaced because inner wire broke collapsing duct, inside core vinyl ripped blocking return air flow, causing loss of cooling and loss of air movement acoustics could only hear motor acoustics in air handler unit. Replaced return flex duct with 16" ATCO Rubber Products Class 1, R 4.2, UPC #070, UL 181. Had problem getting new flex duct to fit into old collars. Old collar had adjustable circumference/diameter range from C=51 1/16" (D=16.25"), to C=51 5/8" (16.43"). Measured circumference of new flex duct is C=50" (D=15.9"). Had to retrofit old collars so mid-range of adjustment would give D=15.8" (C=49 5/8"), that is I had to shorten the circumference of the collar by about 1 1/2" to get the new flex duct to work. Was surprised that present day 16" flex duct measures almost exactly 16" core ID, whereas 16" flex duct bought in 1978 measures 16 1/4" core ID requiring removal and retrofit of old collars to accomodate new flex duct.
 
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Old 08-23-07, 09:29 PM
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Wink

On any flex pipe to a metal collar. Most code's calls for the inside linner to be pull on the the collar and a band on it. The the outer insulation pulled over that and another band on it . All ends to be sealed up with duct mastic
 
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Old 08-24-07, 05:37 AM
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Thanks Ed

For the rework, I used the procedure you outlined and written up in most of the published Installation Instructions, that is using mastic and drawstrap on inner core over collar just below collar bead, then another drawstrap over outer jacket below first drawstrap.

The original flex duct installation done in 1978 before I purchased the house didn't use mastic, used duct tape on inner core over collar, no drawstrap on either inner core or outer jacket. (But it seemed to work OK for 29 years.)
 
 

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