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3. INSTALLATION (
CONT
3.4 C
D
ALCULATING THE
UCT SIZE
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.
D
UCT
D
IAMETER
4˝ (102
5˝ (127
6˝ (152
7˝ (178
8˝ (203
NOTE: Examples 3.4.1 and 3.4.2 use imperial measures.
The same calculation applies to metric measures.
3.4.1 E
XAMPLE OF CALCULATION
P
: My installation requires two exhaust registers (one for the kitchen, one for the bathroom).
ROBLEM
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
: Simplified method. (For a more detailed method of calculating duct size, refer to the ASHRAE
OLUTION
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 away enough 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.
3.4.2 E
XAMPLE OF A DESIGN FOR A FULLY DUCTED SYSTEM FOR A UNIT HAVING A HIGH SPEED PERFORMANCE OF
5˝
5˝ø
65
CFM
6˝ø
129
CFM
6˝ø
93
CFM
VI0004
'
)
D
R
ECOMMENDED
)
40
19
/
MM
CFM
L
S
)
75
35
/
MM
CFM
L
S
)
120
57
/
MM
CFM
L
S
)
185
87
/
MM
CFM
L
S
)
260
123
/
MM
CFM
L
5˝
4˝
5˝ø
4˝ø
64
CFM
42
CFM
4˝
6˝
7˝
7˝ø
222
CFM
A
F
IR
LOW
68
3/
60
M
H
CFM
127
3/
110
M
H
CFM
204
3/
180
M
H
CFM
314
3/
270
M
H
CFM
442
3/
380
S
M
H
CFM
4˝
4˝ø
42
CFM
4˝ø
42
CFM
4˝
6˝ø
84
CFM
6˝
7˝
7˝ø
222
8
M
A
F
AXIMUM
IR
LOW
28
/
102
3/
L
S
M
H
52
/
187
3/
L
S
M
H
85
/
306
3/
L
S
M
H
127
/
459
3/
L
S
M
H
179
/
645
3/
L
S
M
H
E
ND
B
RANCHES
5˝ø
70
CFM
VI0003
222
CFM
4˝
6˝ø
96
CFM
6˝
6˝
6˝ø
138
CFM
CFM
M
B
AIN
RANCH
6˝ø 140
CFM
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