# flex duck vs metal duck



## hvactech (Mar 11, 2007)

When sized correctly fex duct will indeed give the same airflow as sheet metal duct, the thermal conductivity of flex is lower than metal translating into lower heat gains or losses, its cheaper and easy to install compared to metal, at the same time flex is only suitable for attic installation, no allowed under some states codes and takes more room than sheet metal. I would pick metal work over flex without thinking twice, but that’s just my personal preference.


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## joasis (May 28, 2006)

As a builder, I specify metal ducting only when crossing areas like a vaulted ceiling, where access later would be difficult. Otherwise, it is going to be flex duct, which is quieter, IMO.


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## Mike13 (Dec 20, 2007)

If your installers are comparing metal & flex "ducK" then you need to find new installers ASAP.


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## perpetual98 (Nov 2, 2007)




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## bigMikeB (May 31, 2007)

Flex duct has more "R" value than uninsulated metal duct and can be run easier. Metal ducting offers less flow restriction (higher velocity) and doesn't wear out. You can have a 50' run of metal duct, flex maxs out at 14'.


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## handy man88 (Jan 23, 2007)

I think flex duct is only used for supply though, not for returns, in all areas except the attic.


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## bigMikeB (May 31, 2007)

handy man88 said:


> I think flex duct is only used for supply though, not for returns, in all areas except the attic.


In the real world hacks try and use it as much as they can get away with.


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## Chris Johnson (Oct 31, 2007)

Any one can install a piece of flexduct, only a true tradesman can bend and install sheetmetal.


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## bigMikeB (May 31, 2007)

Chris Johnson said:


> Any one can install a piece of flexduct, only a true tradesman can bend and install sheetmetal.


 
Exactly, and that's how I feel about plumbing with flex tubing. It's a manufacturer's way to sell more product to people that can't install the real way.


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## colevas (Nov 6, 2008)

*differnet types of flexible duct*

My understanding is that here are two types of flexible duct to be contrasted with the rigid galvanized steel.

those two types are what I have heard called " alumiflex" and " wireflex".

the former is a system with a rigid aluminum tube core with insulation, and the latter is a continuous slinky like wire with a internal mylar like lining, and externally insulated to R6 or R8. My question is, in a n uninsulated crawl space below a house setting in a mild climate, which is preferable to use as conduits for hot air from a furnace? I have heard that the alumiflex is noiser but rodent safe, whereas the wire flex is quieter but easily chewed through by rodents. Which is more efficient?
colevas


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## beenthere (Oct 11, 2008)

Flex duct, like sheet metal duct, has to be sized coreectly.

If its not, it don't matter what its made from.


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## stubborn1 (Oct 24, 2008)

I've seen too many instances where flex duct gets "abused" - sharp bends in the run reducing airflow, flex going up/down/around obstructions adding more resistance to the run, etc. One more vote here for sheet metal.


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## Marvin Gardens (Sep 30, 2008)

I prefer metal pipe. It has way more throughput secondary to more laminar flow compared to the same size flex duct.

Flex duct has too much non laminar flow with all its coils and bends. It also takes a lot more room. Insulating pipe is still smaller than flex.

Plus it's a lot easier to put in saddles and Y's.

Also I have never ripped metal pipe.


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## Rookietinner (Jan 8, 2007)

I think flex duct is great in the right applications. To say that a true craftsman only uses tin is kind of gutsy. I've seen some of the worst jobs you could imagine by guys who only use hard pipe. I also know that other trades appreciate flex for the fact that it allows them to get their stuff in a little easier than hard pipe. You may have never seen hard pipe tear but, have you ever seen it smashed by a plumbing pipe or an elbow broken off the main trunk because the hard pipe had no give? I think bottom line is the quality of contractor you use not the type off pipe you use. personally I use a sized and shop built supply and return air trunk, with flexible runs off of the trunk.


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## Drakeheating (Nov 9, 2008)

*flex duct vs. metal duct*

As an HVAC designer metal duct is prefferable over flex. Metal duct creates less drag and maintains a better velocity when installed properly. Flex ductwork is recommended in attics or spaces that are not conditioned. The pre-insulated flex duct does hold the heat and cooling better than bare metal duct. You can purchase duct wrap which would allow you to achieve the same insulation factors as flex when wrapped around the metal duct. Flex duct is alot easier to install than metal making it more cost effective at the quote phase of the project. If you are putting in a new system and the duct is being intalled in a basement or conditioned space insist on metal trunk and metal supply branches for a more effective system. Flex is ok as long as it is installed properly with the correct strapping... once flex starts to sag that is when you lose efficiency. Also you want to avoid any flattening of the flex or kinking. There is a reason why most commercial projects only allow a maximum of 6' of flex to the supply grills and the rest must be galvanized metal from the main trunk.


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## bob22 (May 28, 2008)

After reading this thread, if I were to replace the spiral wire flex duct with rigid, do I go 1" lower in diameter? My home has the flex for the branches running to floor boots.


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## beenthere (Oct 11, 2008)

bob22 said:


> After reading this thread, if I were to replace the spiral wire flex duct with rigid, do I go 1" lower in diameter? My home has the flex for the branches running to floor boots.


No, stay same size.


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## bob22 (May 28, 2008)

Would flow improve to be worth the time/investment? The span is about 15' from trunk to boot.


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## beenthere (Oct 11, 2008)

Probably.


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## ben's plumbing (Oct 6, 2011)

each has to be installed right to the way it was designed...and as beenthere said... if not sized right...dosen't matter what is used..


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## bob22 (May 28, 2008)

Thank you both!


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