| Flexible Ductwork |
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This is - sadly - the most popular page on the website. Apparently, Delaware is
not the only state in which installers routinely "butcher" flexible duct.
FLEXIBLE DUCT - BOON OR BANE?
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 “fully
extended”); 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
duct 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.
not the only state in which installers routinely "butcher" flexible duct.
FLEXIBLE DUCT - BOON OR BANE?
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 “fully
extended”); 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
duct 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 saw. |
| 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 shown 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 branches. This design violates traditional practice, wherein 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 support. We like to say "Nothing can touch flex but its supports and connections" |
Advanced
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.
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 shape. |
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| IMPORTANT NOTE The round shape of the core must be maintained throughout a run of flex. 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
box.
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
box.
The discolorations
shown tested positive
for "fecal coliforms,
escherichia bacteria
and enterococcus".
Perhaps foreign
matter was placed
inside the box before
it was installed.
Think about it.
shown tested positive
for "fecal coliforms,
escherichia bacteria
and enterococcus".
Perhaps foreign
matter was placed
inside the box before
it was installed.
Think about it.
| A Tale of Two D Boxes |
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
similar.
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.
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
similar.
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.
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
tape.
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
tape.
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 left we see "Good Flex": The excess that was removed; note the "Flow
Hood" used for measuring airflows.
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 left we see "Good Flex": The excess that was removed; note the "Flow
Hood" used for measuring airflows.
