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Old 02-11-2013, 02:04 PM   #1
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Installing flexible heating duct


I'm replacing some old heating duct with the latest flexible insulated duct. The attachments are 6" but the guy at the a home depot said you want to use 7" flex for easier installation. Is this correct? I plan to use mastic sealer and zip ties.

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Old 02-11-2013, 02:14 PM   #2
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Installing flexible heating duct


That would make for one loose connection.
I've never seen mastic used on a flew line, cable tie and foil tape is what I use.
May need one of these to help taper the ends of the adapters.
http://www.shoplocal.com/dl-50534845...0160&newzone=y

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Old 02-11-2013, 03:02 PM   #3
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Installing flexible heating duct


Thanks for the quick reply. I'll return the box of 7" flex and work with the 6".
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Old 02-11-2013, 03:43 PM   #4
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Installing flexible heating duct


maybe he was recommending the 7 inch to get the same airflow since the flex duct is more restrictive than smooth pipe. why not just replace it with smooth pipe?
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Old 02-11-2013, 04:03 PM   #5
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Installing flexible heating duct


Quote:
Originally Posted by ovahimba View Post
I'm replacing some old heating duct...
why? is something wrong with what is there?
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Old 02-11-2013, 05:55 PM   #6
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Installing flexible heating duct


Get 7 inch take offs and cut them in, and also get 7 to 6" reducers for the register boots. 6" flex will only flow about 80% of the air that 6" hard pipe will.
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Old 02-11-2013, 08:19 PM   #7
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Installing flexible heating duct


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Originally Posted by TarheelTerp View Post
why? is something wrong with what is there?
My main problem is the old insulation is falling off of the aluminum duct. When the heat is on in the living space above, it feels warm in the space below which I guess means leaks.

I'm sure smooth pipe is more efficient but I need to make many bends to go through stud walls. Also I wasn't looking forward to hand wrapping the insulation on the hard pipe, which I thought wouldn't be as air tight as the factory insulated flex.

At this point I might redo all the ducts in the lower space.

Since I have your attention, do I need to reinsulate that distribution box in the photo?
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Old 02-11-2013, 09:13 PM   #8
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Installing flexible heating duct


Quote:
Originally Posted by ovahimba View Post
My main problem is the old insulation is falling off of the aluminum duct. When the heat is on in the living space above, it feels warm in the space below which I guess means leaks.
Gotcha (now that I see the pic).
I thought you had real sheet metal and was going to talk you out of disturbing it.

Get sheet metal fittings for all the bends and take offs...
and limit the flex to straight runs only. Keep it WELL supported.

Quote:
...do I need to reinsulate that distribution box in the photo?
By the time you finish everything else you'll need to.
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Old 02-12-2013, 10:02 AM   #9
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Installing flexible heating duct


fairly classic flex duct insulation wearing out after X number of years. The conditions in the area that the ducts were run in and the age of the materials used has contributed to the failure of the outer wrap / insulation. You have expandable aluminum flex duct. This is far different then the flex duct of today. Here is a link to a product similar to what you have. http://www.novaflex.com/productcart/...&idproduct=565

The best thing you could do is change our the flex to steel. This will be more efficient and will last 30 plus years. Yes, Insulation will be hard to do, but in the end you will have a quality installation.

that being said, you stated that you dont want to use steel. so standard plastic flex duct is what your looking at using. as others have suggested, you could used ridgid steel at all elbows and takeoffs etc and use the flex duct for straight runs only. The main issue with the new flex duct is that it must be pulled tight in order to work correctly. This put stress and strain on all the rigid joints that in my experiance, causes the joints to fail/leak. Additionally, there are standards for how much sag the duct can have in a 5 foot run. The percentages of air flow loss are suprisingly large, if the sag is introduced. The best installation method I can suggest from your pictures is to create a supported shelf under the flex duct, thus no sags. Bottom line is that it will move air, how well and for how long depends on the installation.

I would suggest some serious google work on how to size and install flex ducts.

good luck and I hope this helped.
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Old 02-12-2013, 10:31 AM   #10
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Installing flexible heating duct


Thanks all for the helpful information. With a 30 year old house, the maintenance and repairs never seem to end. Its great to have a resource like this forum.

Bobinphx, I'll probably insert 1x4 material under the flex to keep it from sagging between the support straps. Thanks.
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Old 02-12-2013, 11:59 AM   #11
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Installing flexible heating duct


Good plan.... supported very will from above, that will really help!!!! also, read up on how to best connect and seal the rigid parts of your system.

I also forgot to add that adding more insulation is a good idea. flex from the box can run from r4 to r8, so adding more on the outside might help. and I would for sure re-wrap the plenum.

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