Flexible Ductwork
This is - sadly - the most popular page on the website. Apparently, Delaware is
not the only state in which installers routinely "butcher" flexible duct.


Now we come to branch ductwork, and there are two types: Metal (round, oval
and rectangular shapes are used) and “flexible air duct” (AKA “flex”); since flex
is the
single most abused product in the industry today, it’s necessary to
devote considerable time to the topic.

“ASHRAE” (American Society of Heating, Refrigerating and Air-Conditioning
Engineers, Inc.) is the authority in HVAC design; in 2004, they published a
revealing study on airflow through flexible duct. The authors (Abushakra et al)
found real world pressure drops to be
four to ten times manufacturers’
published values.

IMPORTANT NOTE: Real world resistance to flow through flex can be huge.

At issue is the degree of “
compression” of the duct: They found the only way to
replicate manufacturers’ data was to pull the ends of test specimens farther
apart than is realistically possible in the field (the specimens were “
”); when real world configurations were tested (we say there was
some degree of “compression”), resistance multiplied.

IMPORTANT NOTE:  Real world resistance to flow through flex is solely a
function of workmanship.

“ADC” (Air Diffusion Council, the people who manufacture flex and many of
their suppliers) promulgate the Industry Standard on flex:
Flexible Duct Performance and Installation Standards Fourth Edition” 2003.
Here are some quotes:

¶4.3 begins “Install flex fully extended, do not install in the compressed state or
use excess lengths” (using more than necessary to get from A to B, and
leaving the excess curling around). “This will noticeably increase friction
losses” (resistance to flow). Parenthetical phrases are ours.

¶4.3 continues “Avoid bending ducts across sharp corners or incidental
contact with metal fixtures pipes or conduits.
Radius at centerline shall be no
less than one duct diameter
.” Emphasis is ours.

¶4.8 requires supporting at no more than five foot intervals and clarifies that
connections to a main duct or air outlet are supports.

¶4.8 continues “Hanger or saddle material in contact with the flexible duct shall
be of sufficient width
to prevent any restriction of the internal diameter of the
when the weight of the supported section rests on the hanger or saddle
material.” Emphasis is ours.

¶6.3 is interesting for its caution: “Use friction loss data characteristic of flexible
duct. Do not use data for round sheet metal duct.” We all know flex has more
resistance than metal and young viewers may want to say “DUH”, but there are
contractors who don’t know there’s a difference; sad but true.
Flex consists of three
components: “vapor
barrier” or “outer
jacket”, fiberglass
insulation and the
“core” or “liner” (steel
wire “helix” encased in
foil, film or fabric).
The corrugations in
flex create a great deal
of turbulence, which
increases resistance to
flow. That turbulence
also causes airborne
dirt to settle out, as
seen here. This dirt
presents an air quality
concern (mold growth
could develop if the
duct is wetted) and a
fire hazard; those are
good reasons not to
use flex for returns (as
was done here).
The Good
This installer has
extended the duct as
much as can be
expected in the real
world, and there is
very little “wrinkling”
of the jacket or sag;
the support
compresses the
insulation, but not the
liner, and the bend is
gradual. This
approaches the
Standard of Care.
The Bad
Here’s an 8” duct (11”
OD) run through the
7” space above a
“strongback” used to
reinforce floor
trusses. We had
noted substandard
flow and used a duct
camera to locate the
restriction in this
branch duct; we then
applied the drywall
The Ugly
Here's a 6" duct (9"
OD) run through the
3½” space between
the “TJI” and the
foundation wall; this
branch supplied a
floor diffuser in front
of a large patio door,
where there were
comfort complaints.

We fixed Bad and
Ugly by having
contractors install
metal oval duct, as
should have been
done originally.
We felt the D Box
below was
restrictive to flow:
The cross sectional
area of the box
decreases as you
move away from the
large flex, and the
area of the
remaining branches
exceeded the areas
of the boxes
upstream of those

This design violates
traditional practice,
triangular D Boxes
supply only TWO
branch ducts.
More Fun With Flex
At left: Nearing the end of a 60 foot run of
branch duct, we see the flex laid between truss
chords, and VERY compressed (note the
"wrinkles"). Needless to say, the Bonus Room
supplied by this branch wasn't too comfortable.
Below: There was eight or ten feet of vertical
flex weighing on this support; a huge
restriction was thus created.
You may be tempted
to think this stuff is
funny; just remember
it was done
deliberately to fellow
human beings by
people who should
have known better.

The gas line shown
is not an approved

We like to say
"Nothing can touch
flex but its
supports and

Next, we see two installations comprising the equivalent of three 90 degree
bends, and they're not made smoothly in either case.

Resistance to flow through bends is expressed in feet of straight flex duct.
Each 90 degree bend, if PROPERLY MADE equates to about ten feet of flex
duct, so this little arrangement would normally equal 30 feet of flex duct.

Given the crimping at the lower left, it's probably 100 feet or more.

The image at the lower right is titled "STUPID" in our file. It's about the same
number of bends as shown in the previous image, but these bends are
much more crimped.

Call this arrangement 150 feet or more.

The "fix" for both installations was to replace the flex bends with round metal
"adjustable elbows"; that reduced the respective resistances to flow by 70
feet and 120 feet. Airflow improved dramatically.
We haven't mentioned
"sag". ADC para. 4.8
limits sag in flex to 1/2"
per foot of spacing
between supports:
2-1/2" between
supports that are five
feet apart; that's
INCHES, not feet!
There is no Industry
Standard for the
gage (thickness) of
the wire used for the
helix; and
manufacturers do not
publish the gage of
their wire.

We felt the wire used
in this product was of
such a light gage (so
thin) that it could not
support the weight of
the duct itself.

Note how the flex has
settled into an oval

The round shape of
the core must be
throughout a run of

Supports and bends
must not cause the
core to be out of
round or restricted
in any way.
25 feet of flex is
shipped in a box less
than three feet long.

The tool shown is used
to tighten the plastic
clamps required to
secure the
connections of flex.
We like to examine
inside the D Boxes
we see, and here's a
typical finding: The
installer didn't do
everything possible
to conceal the
underlying untreated
fiberglass at cut
edges. In fact, this
installer appears to
have left a chunk of
fiberglass inside the
The discolorations
shown tested positive
for "
fecal coliforms,
escherichia bacteria

Perhaps foreign
matter  was placed
inside the box before
it was installed.

Think about it.
A Tale of Two
D Boxe
The Flexible Duct System - A Concept Whose Time Has Gone

Flexible duct systems are characterized by LOTS of flex: It's everywhere; there
are no rectangular ducts.

These systems present multiple opportunities for abuse, and home buyers
must resist buying homes with these systems unless they're certain air
deliveries are within spec; to do otherwise is to risk poor comfort levels, high
energy bills and high repair costs.

Unscrupulous contractors are promoting "Flexible Duct Systems" (AKA
"Radial Systems"), because they're cheap to install. These systems consist
of a supply plenum (think "box"), constructed of metal or board; main supply
ductwork is large flex ducts connecting to "Distribution Boxes" (AKA "D
Boxes"), which in turn supply multiple branch flex ducts. Return ductwork is

The studies and the 2009 Standard of Care sounded the death knell for
these systems but the people advocating them don't know it.

Wake up, people! The resistance to flow through your duct systems is
four to ten times your design! How can you possibly deliver design flow?

The heart of a flexible duct system is the "D Box" (That's "D" as in
"Distribution"); here we see a large flex at right supplying several smaller
ducts through a  triangular D Box.

Note that in normal designs, one large flex supplies only two or at most three
smaller ones; triangular boxes are used for two branches and square boxes
are used for three.
Typical Flex Repair:
Remove Excess Length and Eliminate a Bend

This branch duct (below left) was originally routed between the two trusses in
the background, and then curved back to an outlet between the two trusses in
the foreground; we disconnected it and relocated it for a more direct route,
ending up with the excess length shown.
At the right, we attached an adjustable elbow to the existing collar and
included a short length of round duct to eliminate a flex bend and provide a
means to securely attach the now shortened flex. Note that we've pulled the
jacket and insulation back to expose the liner; we'll shorten the liner so we
can use the jacket and insulation to cover the elbow.

Next (below left) we pulled the liner over the round duct, and secured it with
two duct clamps (We were on a roll - Only one clamp is required) and UL181
Above right we see the nearly finished product after pulling the insulation and
jacket over the elbow; all that remains is to tape the jacket to the duct wrap, as
was done with the branch at the lower right of the image. Note the tool used to
tighten and then remove unused length from the 36" long duct clamps.

At the l
eft we see "Good Flex": The excess that was removed; note the "Flow
Hood" used for measuring airflows.