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Old 08-05-2010, 09:37 PM   #1
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Metal duct or flex?


I have a home that was built in 1965. I'm not sure how long the existing duct work has been in place, but it's all round metal duct with insulation wrapped around it. It's a single level, 1600 sq ft home with the duct work in the crawl space. I had to go under my house a few weeks ago to run some TV cable and noticed some of the insulation around my duct work was coming off and it all looks really old and has condensation really bad coming through. So I called a local HVAC company to look at it and give me a price on replacing the insulation. I got a price of $730 to tear out the old and put in new insulation, which I thought sounded fine. But they also gave me a price of $650 to take out the metal duct and replace with flex duct.
After doing some research online, it appears that metal duct seems to have better air flow. But I also like the idea of replacing the duct, which I'm sure contains lots of dirt and dust, with new "clean" duct. I would like some other thoughts on this. Is the metal duct so much better that it would be worth paying more money to have the insulation re-done? Or would the flex be just as good.

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Old 08-05-2010, 10:18 PM   #2
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Metal duct or flex?


You need a 7" flex duct to move as much air as a 6" sheet metal duct.

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Old 08-05-2010, 10:23 PM   #3
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Metal duct or flex?


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Originally Posted by beenthere View Post
You need a 7" flex duct to move as much air as a 6" sheet metal duct.
Flex will also raise the noise level, correct?
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Old 08-05-2010, 10:35 PM   #4
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Metal duct or flex?


Flex often reduces the noise level. Until its under sized past a certain amount.

Flex is used as a noise attenuator on sheet metal systems. Often the last 3 to 5' of a supply run, is done in flex to absorb the noise that sheet metal can have.
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Old 08-06-2010, 06:09 AM   #5
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Metal duct or flex?


So in your opinion, would you make the change to flex or re-insulate the metal?
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Old 08-06-2010, 06:12 AM   #6
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Metal duct or flex?


If the system isn't loud, I'd re-insulate.
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Old 08-06-2010, 06:20 AM   #7
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Metal duct or flex?


I'm doing the opposite, replacing flex with metal duct.

My crawl space and attic has R4.2 insulated flex duct. I'm slowly replacing the flex with R8 wrapped metal duct. I have a heap pump and know a good amount of heat/cooled air temperature is lost through the R4.2 insulation. Plus all the droops and kinks in the flex restricts air flow.

Did your contractor quote mention what R value would be used to re-wrap the metal pipe? I'm sure the R value varies by location and building code, but I wouldn't go less than R8.

Also, can you tell if the seams, areas where the pipes are joined, have a sealant installed. Correct term I believe is mastic. Very important to seal both the supply and return ducts. Did the contractors quote mention this?

If neither was addressed, mentioned, recommended, I'd get another quote, actually two more quotes.

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