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Old 12-19-2011, 05:36 PM   #1
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Connecting semi-rigid duct


My 30-year-old house HVAC is put together with semi-rigid duct in the attic.
http://www.homedepot.com/buy/applian...uct-64218.html

It is insulated with fiberglass sleeves, most of which is covered in grey vapor barrier.

They used cloth "duct tape," some of which is holding on, but a lot is not. I'm trying to refurbish the system by sealing it all properly. I've done just a couple, but I'm not sure I'm doing it right.

Where the hard metal connector fits into the semi-rigid, I'm using sheet metal screws with washers to secure the semi-rigid to the hard metal connector, then using both mastic and foil tape. (btw, the "mastic" they sell at HD (Masterflow) seems awfully thin compared to what I've seen on home improvement shows on TV, which looks more like toothpaste).

But where the semi-rigid fits into the metal connector, they simply cut a slit in the end, folded the semi-rigid into a slight cone, and stuck it in there. Usually the "duct tape" is not even covering the slit, and they leak like crazy.

What is the proper way to make these connections?

Can you use a crimper on semi-rigid? Is there any way to make a mechanical connection? Or do I have to rely on loads of foil tape? Or do you use a metal connector so that the semi-rigid goes around the outside of the connector, regardless of flow direction?

I know the best answer is probably to replace the semi-rigid, but I have an awful lot to do. But if that is your answer, can you give me advice as to what is easier for a homeowner to use, flex, sheet metal, or something else?

Last edited by M3 Pete; 12-20-2011 at 05:31 PM.
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Old 12-19-2011, 06:25 PM   #2
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Connecting semi-rigid duct


I have never seen that used for anything other than a bathroom exhaust venting.

I'd go back with flex or pay someone to install rigid round pipe.
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Old 12-19-2011, 06:47 PM   #3
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Connecting semi-rigid duct


Ditto.Never seen it used for supply air,only the first few feet of a drier vent. Hard pipe with an insulated sleeve or insulated flex should be what's used in an attic.
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Old 12-19-2011, 07:05 PM   #4
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Connecting semi-rigid duct


guys, the link I posted is for 7" duct. Clearly that's not dryer venting. Unless that's an awfully big dryer.

They make it in sizes from 3" all the way up to 16"

http://www.deflecto.com/advp/product...&idCategory=57

It is all insulated with sleeves.
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Old 12-19-2011, 07:06 PM   #5
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Connecting semi-rigid duct


if that is a cold attic in the winter your heat is going to cool off trying to get to the rooms and not at all the right stuff...and there are take offs that install with the direction of the air.... http://www.bestbuyheatingandaircondi...e=duct-collars
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Last edited by biggles; 12-19-2011 at 07:10 PM.
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Old 12-19-2011, 07:32 PM   #6
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Connecting semi-rigid duct


Quote:
Originally Posted by biggles View Post
if that is a cold attic in the winter your heat is going to cool off trying to get to the rooms and not at all the right stuff...and there are take offs that install with the direction of the air.... http://www.bestbuyheatingandaircondi...e=duct-collars
As I mentioned, it's all insulated.

So far, the take-offs (start collars) are not the issue, it's the Tee Wye's and registers, with ends that take a crimped pipe.

Even the supplier you linked has the semi-rigid for sale, up to 18"

http://www.bestbuyheatingandaircondi..._Code=duct-mfd

Last edited by M3 Pete; 12-19-2011 at 07:36 PM.
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Old 12-19-2011, 07:46 PM   #7
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Connecting semi-rigid duct


You may need to crimp the hard pipe end and seal with some good ul foil tape. It will be a crappy seal any way you do it. Insulated r.8 insulated flex of the proper size would be the way to go
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Old 12-20-2011, 02:35 PM   #8
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Connecting semi-rigid duct


Thanks qbert. I was thinking of using these duct couplings, which will allow me to make a mechanical connection (screws and washers) with the semi rigid on both sides.

http://www.bestbuyheatingandaircondi...uct-connectors

Not the best way, but the mechanical connection would at least allow me to mastic and tape the joint with confidence it won't come apart (or leak).

I'm using Scotch 3311, is that "good UL foil tape?"
http://www.shop3m.com/3m-scotch-foil...vp7k7wz70.html

Last edited by M3 Pete; 12-20-2011 at 02:43 PM.
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Old 12-20-2011, 05:02 PM   #9
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Connecting semi-rigid duct


Pic looked like flex duct. Code limits it to 14' I believe. Best not to use it for supply runs other then in tight spaces, or for elimination of vibration, its very restrictive to air flow.
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