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.
See the following:
"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
"The flexible duct elbows have much higher local loss coefficients (due
to increased roughness) than those reported for sheet metal elbows."
"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."
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
I'd recommend 22 gauge galv. ducts in a heartbeat.
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
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?
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
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