Use the table below to ensure that the ducts you intend to install will be carrying air flows at or under the recommended values.
Avoid installing ducts that will have to carry air flows near the maximum values and never install a duct if its air flow exceeds
the maximum value.
3. INSTALLATION (
CONT
’
D
)
3.4 C
ALCULATING
THE
D
UCT
SIZE
8
VI0003
P
ROBLEM
:
My installation requires two exhaust registers (one for the kitchen, one for the bathroom).
I will connect these registers to a main duct which will connect to the unit (high speed performance
value of 140 cfm). What size of duct should I use for the main exhaust duct and for the two end
branches leading to the registers? (See illustration at right.)
S
OLUTION
:
Simplified method. (For a more detailed method of calculating duct size refer to the ASHRAE
or HRAI HANDBOOK).
Main duct: Table above indicates a 6” Ø duct: Recommended air flow: 120 cfm; maximum air flow:
180 cfm. The high speed air flow of 140 cfm is close enough to the recommended value (120) and
far enough away from the maximum value (180). Therefore a 6” Ø duct or larger is an appropriate
choice for the main exhaust duct.
End branches: Each end branch will have to transport an air flow of 70 cfm (140 divided by 2).
Table above indicates a 5” Ø duct: Recommended air flow: 75 cfm; maximum air flow: 110 cfm. The
high speed air flow of 70 cfm is close enough to the recommended value (75) and far enough away
from the maximum value (110). Therefore a 5” Ø duct or larger is an appropriate choice for the 2
end branches.
NOTE: A 4” Ø duct would have been too small because the maximum acceptable value for a 4” Ø
duct is 60 cfm.
3.4.1 E
XAMPLE
OF
CALCULATION
VI0004
E
ND
B
RANCHES
5˝ø
70
CFM
M
AIN
B
RANCH
6˝ø 140
CFM
D
UCT
D
IAMETER
R
ECOMMENDED
A
IR
F
LOW
M
AXIMUM
A
IR
F
LOW
4˝ (102
MM
)
40
CFM
19
L
/
S
68
M
3/
H
60
CFM
28
L
/
S
102
M
3/
H
5˝ (127
MM
)
75
CFM
35
L
/
S
127
M
3/
H
110
CFM
52
L
/
S
187
M
3/
H
6˝ (152
MM
)
120
CFM
57
L
/
S
204
M
3/
H
180
CFM
85
L
/
S
306
M
3/
H
7˝ (178
MM
)
185
CFM
87
L
/
S
314
M
3/
H
270
CFM
127
L
/
S
459
M
3/
H
8˝ (203
MM
)
260
CFM
123
L
/
S
442
M
3/
H
380
CFM
179
L
/
S
645
M
3/
H
NOTE: Examples 3.4.1 and 3.4.2 use imperial measures.
The same calculation applies to metric measures.
3.4.2 E
XAMPLE
OF
A
DESIGN
FOR
A
FULLY
DICTED
SYSTEM
FOR
A
UNIT
HAVING
A
HIGH
SPEED
PERFORMANCE
OF
222
CFM
5˝ø
65
CFM
5˝ø
64
CFM
4˝ø
42
CFM
4˝ø
42
CFM
4˝ø
42
CFM
6˝ø
96
CFM
6˝ø
84
CFM
6˝ø
138
CFM
7˝ø
222
CFM
7˝ø
222
CFM
6˝ø
93
CFM
6˝ø
129
CFM
5˝
5˝
6˝
6˝
6˝
6˝
7˝
7˝
4˝
4˝
4˝
4˝
4˝
