Hard Metal Vs. Flexible Ducts - HVAC - DIY Chatroom Home Improvement Forum

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Old 05-02-2008, 09:42 AM   #1
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hard metal vs. flexible ducts

Hey, we're adding an addition for my Mom to move in and the HVAC contractor has asked me if I really need hard metal ducts. We live in Louisiana(VERY HOT AND HUMID). We used to have flexible ducts in the older portion of our home and changed them to hard metal. I have seen no real change in energy usage and in a couple of rooms we have a much harder time regulating the temp. So, I guess my question is... Are hard metal ducts that much better? What are the real advantages? Any help would be appreciated.


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Old 05-02-2008, 07:02 PM   #2
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Flex duct is a little less efficient simply because of the rigid interior which causes turbulence, rather than a nice smooth flow.

However, more often than not, in a residential setting you won't notice any significant difference in the two. Flex ducts are much easier to run and usually more cost effective so I would probably just go with that.

You might also bring the main trunk line in with hard pipe and use flex runs to the individual vents, which will improve the performance and keep costs down.


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Old 05-02-2008, 11:54 PM   #3
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You can use flex, but it needs to be installed correctly and sized correctly. The problem with flex is as jumpinjack said is turbulance. Insist they oversize the runs by one size. If they want to use 6" make it 7" and so on. 6" metal allows 100 cfm's 6" flex allows from 50-75 cfm's. Installing flex is easy, but it needs to be stretched fairly tight and not kinked anywhere. My preference is metal with insulation wrapped. The metal last forever.
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Old 05-17-2008, 10:01 AM   #4
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Originally Posted by sgthvac View Post
My preference is metal with insulation wrapped. The metal last forever.
I second that. When you are using flex dogs, cats or wild life in general have a keen ability for finding a way to tear open and get into the ducting system. They want to keep cool in the summer and warm in the winter and they do that by ripping your duct work.
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Old 05-18-2008, 08:23 AM   #5
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Around here the maximum length allowed for flex is 15ft. There is a wire imbedded in flex duct that spirals along its length, keeping the duct erect and round. This "spiral" wire creates resistance in the duct, because it is not smooth and flush, and reduces airflow.

Most manufacturers, and codes do not allow the main trunk (duct) to be flex, but if your takeoffs to the individual vents are 15ft or less the labor and material savings are considerable. It is standard practice that the first section of the takeoff is insulated metal, with a balancing damper, so that the airflow can be adjusted, or "balanced" throughout the home. Also make sure the flex that is installed is insulated flex, theres a big difference.

I am assuming that you dont have a basement, and will be installing the ductwork in an attic space. I cant recall the last time I saw my dog in the attic. If its going to be in a crawl space, insulated metal is best due to its durability and logevity.

Good Luck.

Last edited by 8 Ball; 05-18-2008 at 08:39 AM.
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