Flex vs. Aluminum ducts


I am having an HVAC system installed in my basement. I would like to know what is the consensus/general opinion regarding the use of Flex vs. Metal ducts? Which should i use?
I have had 3 companyies tell me i should use metal as much as possible and use flex where necessary. Another two tell me everyone is using flex these days and there is no disadvantage.
Please advise.
thank you
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farseer wrote:

metal passes air with less restiction so it can lead to long term $ savings.....
flexible saves bg bucks on install
since yours is a new install what system did you have previously?
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I did not have a system before. I am replacing hot water heating pipes...
snipped-for-privacy@aol.com wrote:

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WHY???????
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Doug Miller (alphageek at milmac dot com)
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snipped-for-privacy@aol.com wrote:

care to elaborate, perhaps with some cites?

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minimize spam. Our true address is of the form snipped-for-privacy@prodigy.net.
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See the following:
http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/96/960307.html http://www.energyfederation.org/consumer/default.php/cPath/86_744_998 http://www.toolbase.org/PDF/DesignGuides/doe_airdistributionsysteminstallation.pdf
Typical/example excerpt:
"Sheet metal ducts have a smooth interior surface that offers the least resistance to air flow. . . . flexible duct is easily torn, crushed, pinched, or damaged during installation. It has the highest resistance to air flow."
For more technical study (including Reynolds numbers of the test cases) of flex ducts in residential HVAC, see:
"A Study of Pressure Losses in Residential Air Distribution Systems" at
http://eetd.lbl.gov/ie/pdf/LBNL-49700.pdf
From "Conclusions":
"The flexible duct elbows have much higher local loss coefficients (due to increased roughness) than those reported for sheet metal elbows."
and
"The results from the tested air distribution system components were used in the complete duct system analysis and compared with available data from the literature. The comparison showed that our new data provide an improved estimate of the pressure drop for individual sections. Both the new data and existing ACCA procedures gave reasonable (less then 20% error) results for the total pressure drop. The ASHRAE calculation methods give large (20% to 40%) underpredictions."
----------------------------------------------------------- CJT wrote:

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I have flex as a result of a major remodel done a few years ago. The idiot contractor connected them using ordinary duct tape and did not provide adequate support for each run. On at least 3 occasions the flex seperated from junction boxes and I had to fix them. (the crawl space was being heated better then the house).
If I had known then what I no now I wouldn't have allowed this crap to ever be installed. If it's installed 100% correctly then I guess it's a pretty good idea but that's a big "if". I'd say Flex is totally unforgiving to poor installation procedures. The other problem if that flex is round. Sometimes rectangular will be less of an obstruction in areas where form factor is a consideration.
I'd recommend 22 gauge galv. ducts in a heartbeat.
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Properly sized and installed flex is just as good as metal duct and is far cheaper to install.
farseer wrote:

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On 19 Jan 2007 09:42:23 -0800, "jamesgangnc"

So you seem to know a whole lot about friction loss in flex compared to metal ductwork james? And of course we all know that flex in a Phoenix attic will last forever where as the metal duct work will melt in a heart beat. duhh Bubba
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Well the way I see it Flex vs. Metal there is couple differences cans: yes it does restrict some of air flow. pros: is less loses of air temp. like metal it does not conduct air temperature on outer wall, there for not likely getting the condensation on outer wall and it is cheaper installation. Securing the flex duct to distribution duct clamps are made for that. Three pros. One cons. you pick? from Dido

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Thanks for all the replies. I am leaning towards "Metal and flex where necessary". I've see cases where these flex get crushed, easily damaged and the connections fall apart due to they taping they use...don't want to be ripping up walls to fix these things in a few years.
farseer wrote:

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Flex ducting has a higher friction loss, and tends to sag, which exaserbates the problem. We have several condos in my area that used flex ducting and most have air flow problems.
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Respectfully, Bob

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I have a 12 year old house with flex ducting in the for the second floor system and it seriously sucks. The resistance on the long runs makes it impossible to get any kind of balanced airflow.
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