1. Duct Design
Presented by
Dave Janquart

2. Overview of Topics
• Duct design
• Duct insulation
• SMACNA Standards

3. Factors Influencing Duct Design
• Equipment Losses
• Air Velocity
• Duct Material
• Duct Size & Shape
• Length of Duct

4. Friction Losses
FURNACE
NO SCALE
SUPPLY
FAN
COIL
OR
FURN
ACE
R.A.
BOX
FILTER
S.P. 0.07
RETURNAIR
RAG CONT
G35W
S.P. 0.045
SAG CONT 32W
S.P. 0.04 A coil
.25” sp

5. Air Distribution System
Components
• Supply fan
• Supply Ductwork
• Transition fittings
• Discharge grills
• Return grills
• Return ductwork

6. Calculate Air System Pressure
Losses
• The air pressure losses from both the
supply and return duct systems plus
the loss through external a/c coil is
often referred to as the external static
pressure.
• Manufacturers generally publish fan
ratings based on external static
pressure (internal losses due to coils,
filters, etc. are usually accounted for in
the cataloged fan ratings).

7. Air Velocity in Duct Design
• As Air Velocity Increases Noise Increases
• Increasing Air Velocity Causes More
Turbulence
• Recommended Velocity 700-900 FPM

8. Duct Material Roughness
– Duct material roughness refers to the inside surface of
the duct material.
– The rougher the surface, the higher the friction loss.
– Most duct sizing tables use the roughness factor for
smooth, galvanized sheet metal as the reference value.
– The back side of the Duct Calculator has a conversion
table for converting the material roughness from
smooth sheet metal to other materials.

9. Select Duct Material (sheet metal,
fiberglass, flex)
• Decide which duct construction material to
use.
• Common choices: fiberglass duct board,
galvanized sheet metal, “flex” vinyl coated
with helical wire core.
• Each material has advantages.
– First cost (price & ease of installation)
– Performance (friction loss, fan energy)
– Acoustic properties
– Thermal properties (heat gain & loss;
internal/external insulation)

10. Round vs. Rectangular Duct
of Equal Friction Rate
40.7 in.
3 ft.
3 ft.
2 ft.
4-1/2 ft.
1-1/2 ft.
6 ft.
1 ft. 9 ft.
9.0 ft2
9.0 ft2
9.0 ft2
9.03 ft2
10.65 ft.
Ratio of
Perimeter
to Area
Equivalent
Round Duct
Friction
at
15,000 CFM
Perimeter
1.18:1 40.7 in. .07 in. / 100 ft.
1.33:112 ft. 39.4 in. .086 in. / 100 ft.
13 ft. 1.45:1 38.7 in. .095 in. / 100 ft.
15 ft. 1.67:1 37.2 in. .113 in. / 100 ft.
20 ft. 2.22:1 34.5 in. .156 in. / 100 ft.
Figure 8

11. Fitting Influences
• Selection of the proper fittings is critical

14. Design Friction Rate
• When sizing a duct system the
designer will usually choose a design
friction rate.
• This is the desired friction loss in
inches w.g. per 100 ft. of equivalent
length (in. w.g./100 ft. E.L.) of duct.
• The design friction rate is generally
determined based on the velocity of
the air in the first section of ductwork.
• Most designers use a design friction
rate somewhere between 0.08 and
0.10 in. w.g. per 100 ft. E.L.

16. Determining Friction Rate/100 ft.
• Equipment pressure drop
• Duct pressure drop
• Terminal device pressure drop

17. Terminal Device Pressure Drop
• Pressure loss for supply air grill
• Pressure loss for return air grill

18. Friction Losses
FURNACE
NO SCALE
SUPPLY
FAN
COIL
OR
FURN
ACE
R.A.
BOX
FILTER
S.P. 0.07
RETURNAIR
RAG CONT
G35W
S.P. 0.045
SAG CONT 32W
S.P. 0.04 A coil
.25” sp

19. Equal Friction Method
• Same friction loss per foot of ductwork
• Most commonly used method
• Accomplished by use of a Ductulator
• Requires less balancing
– Important to have balance dampers at each
outlet
• Results in reasonable duct sizes

20. Velocity Scale
Velocity
Pressure
Airflow
(CFM)
Friction
Loss
Round Duct
Diameter
Equivalent
Rectangular
Duct Sizes
Figure 7
Duct Calculator

21. Duct Calculator

22. Duct Calculator (Reverse)

23. Recommended & Maximum
Duct Velocity Ranges
Design toward smaller end of range for quieter systems
and toward larger end of range to minimize duct sizes.

24. Things to Consider
• Aspect ratio no more than 3:1
• Residential 2 times # of 6” RD take offs
plus 2” for ducts that are 8” high
• .5 cfm per Sq Ft
• No more than100 cfm per 6” RD

25. Round vs. Rectangular Duct
of Equal Friction Rate
40.7 in.
3 ft.
3 ft.
2 ft.
4-1/2 ft.
1-1/2 ft.
6 ft.
1 ft. 9 ft.
9.0 ft2
9.0 ft2
9.0 ft2
9.03 ft2
10.65 ft.
Ratio of
Perimeter
to Area
Equivalent
Round Duct
Friction
at
15,000 CFM
Perimeter
1.18:1 40.7 in. .07 in. / 100 ft.
1.33:112 ft. 39.4 in. .086 in. / 100 ft.
13 ft. 1.45:1 38.7 in. .095 in. / 100 ft.
15 ft. 1.67:1 37.2 in. .113 in. / 100 ft.
20 ft. 2.22:1 34.5 in. .156 in. / 100 ft.
Figure 8

31. GAS
DRYER
(N.I.C.)
WASHER
(N.I.C.)
120 SQ FT
160 SQ FT
60 SQ FT
120 SQ FT
450 SQ FT
90 SQ FT
120 SQ FT
50 SQ FT
190 SQ FT
50 SF
90 SQ FT

32. GAS
DRYER
(N.I.C.)
WASHER
(N.I.C.)
120 SQ FT
160 SQ FT
60 SQ FT
120 SQ FT
450 SQ FT
90 SQ FT
120 SQ FT
50 SQ FT
190 SQ FT
50 SF
90 SQ FT
95 CFM
45 CFM
25CFM
45 CFM
60 CFM
225 CFM
25 CFM
60 CFM
80 CFM
30 CFM
60 CFM

33. GAS
DRYER
(N.I.C.)
WASHER
(N.I.C.)
120 SQ FT
160 SQ FT
60 SQ FT
120 SQ FT
450 SQ FT
90 SQ FT
120 SQ FT
50 SQ FT
190 SQ FT
50 SF
90 SQ FT
95 CFM
45 CFM
25CFM
45 CFM
60 CFM
225 CFM
25 CFM
60 CFM
80 CFM
30 CFM
60 CFM

34. GAS
DRYER
(N.I.C.)
WASHER
(N.I.C.)
120 SQ FT
160 SQ FT
60 SQ FT
120 SQ FT
450 SQ FT
90 SQ FT
120 SQ FT
50 SQ FT
190 SQ FT
50 SF
90 SQ FT
95 CFM
45 CFM
25CFM
45 CFM
60 CFM
225 CFM
25 CFM
60 CFM
80 CFM
30 CFM
60 CFM

35. GAS
DRYER
(N.I.C.)
WASHER
(N.I.C.)
120 SQ FT
160 SQ FT
60 SQ FT
120 SQ FT
450 SQ FT
90 SQ FT
120 SQ FT
50 SQ FT
190 SQ FT
50 SF
90 SQ FT
95 CFM
45 CFM
25CFM
45 CFM
60 CFM
225 CFM
25 CFM
60 CFM
80 CFM
30 CFM
60 CFM
12/8
18/8
8/8
8/8

36. GAS
DRYER
(N.I.C.)
WASHER
(N.I.C.)
120 SQ FT
160 SQ FT
60 SQ FT
120 SQ FT
450 SQ FT
90 SQ FT
120 SQ FT
50 SQ FT
190 SQ FT
50 SF
90 SQ FT
95 CFM
45 CFM
25CFM
45 CFM
60 CFM225 CFM
25 CFM
60 CFM
80 CFM
30 CFM
60 CFM
8/8
12/8
8/8

37. Select and Adjust Fan
• The total static pressure is the pressure that the fan
must overcome to deliver the required amount of air.
This total amount is also used to select the required fan
motor size or fan speed.
• Perform air balance next. Per Comm 23.18
• Measured airflow values should be compared against
the required values.
• Adjust fan and balancing dampers as necessary to
deliver the correct amount of air to each zone.

40. Duct Insulation
• 22.17 UDC Duct system insulation. (1)
except as provided in sub. (4), all heating
and cooling duct systems, or portions
thereof, that are located in unheated or
uncooled spaces respectively, shall be
provided with insulation with a thermal
resistance of at least R-5.

41. Duct Insulation cont.
• R-4.2 and R-6 are common industry
standards for insulated flex
• R-5 and R-8 are common industry standards
for duct wrap

42. Installation of Underground
Ducts SMACNA Standards

43. Installation of Underground
Ducts cont.

44. Installation of Underground
Ducts cont.
• 23.08 UDC supply air ducts shall be
insulated with a moistureproof material
having a resistance value of a least R-5.

45. Underground Duct cont.

46. Underground Ducts cont.
• Uninsulated duct will require 152 btuh per
linear foot of duct for heating
• 1” insulated duct will require 91 btuh per
linear foot of duct for heating (40% less
energy)
• 2” insulated duct will require 74 btuh per
linear foot of duct for heating (51% less
energy)