15
5.0 Installation
(cont’d)
5.3 Calculating Duct Size
(cont’d)
VI0002
5.3.1
Example for calculation:
Problem:
My installation requires two exhaust registers (one for the kitchen, one for the
bathroom). I will connect these registers to a main duct connected 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 Figure 9.)
Solution:
Simplified method. (For a more detailed method of calculating duct size refer to
ASHRAE HANDBOOK.)
Main duct: Table indicates for a 6ӯ duct: Recommended Airflow:120 cfm; Maximum
Airflow: 180 cfm. The high speed airflow of 140 cfm is close enough to the recommended
value (120) and far enough away from the maximum value (180). Therefore a 6ӯ duct
or larger is an appropriate choice for the main exhaust duct.
End branches: Each end branch will have to transport an airflow of 70 cfm (140 divided
by 2). The table indicates for a 5ӯ duct: Recommended Airflow: 75 cfm; Maximum
Airflow: 110 cfm.The high speed airflow of 70 cfm is close enough to the recommended
value (75) and far enough away from the maximum value (110). Therefore a 5ӯ duct or
larger is an appropriate choice for the 2 end branches.
NOTE: A 4ӯ duct would have been too small because the maximum acceptable value
for a 4ӯ duct is 60 cfm.
VI0001
end
branches
main branch
6ӯ 140 cfm
5ӯ
70 cfm
140 cfm
Figure 9
4ӯ 42 cfm
6ӯ 129 cfm
5ӯ 65 cfm
5ӯ 64 cfm
6ӯ 93 cfm
5”
6”
7”
7”
6”
6”
6”
4”
4”
4”
4”
7ӯ 222 cfm
4ӯ 42 cfm
6ӯ 84 cfm
6ӯ 96 cfm
6ӯ 138 cfm
5.3.2
Example of a design for a fully ducted system for a unit having a high speed performance of 222 cfm
(see Figure 10).
5”
Figure 10
5.4.1
Fully Ducted System (as illustrated in section 4.1)
Stale air exhaust ductwork:
• Install registers in areas where contaminants are produced: kitchen, bathrooms, laundry rooms, etc.
• Install registers at 6 to 12 inches (152 to 305 mm) from the ceiling on an interior wall OR install them in the ceiling.
• Install the kitchen register at least 6 feet (1.8 m) from the oven.
• If possible, measure the velocity of the air flowing through the registers. If the velocity is higher than 400 ft/min then the register
type is too small. Replace it with a larger one.
Fresh air distribution ductwork:
• Install registers in bedrooms, dining room, living room and basement.
• Install registers either in the ceiling or high on the walls with air flow directed toward the ceiling. (The cooler air will then cross
the upper part of the room, and mix with room air before descending to occupants level.)
• If a register must be floor installed, direct the airflow toward the wall.
5.4 Installing Ductwork and Registers
WARNING
!
Never install a stale air exhaust register in a room where a combustion device is, such as a gas furnace or a gas
water heater or a fireplace.
