Brochure

40 41
Sizing
Information and
Instructions
Sizing and selecting a Ceiling
Mounted Fan:
Proper sizing requires that you determine
the needed CFM, the square footage of the
room or home, and the length and type of
duct.
Example: Sizing for an 8 ft x 10 ft x 8 ft
ceiling bathroom using 12 foot long, 4 inch
diameter aluminum flex duct, one elbow,
one wall cap.
Step 1: Determine application
Bathroom = 1 CFM/square foot
Step 2: Calculate the area to be ventilated
in square feet.
Assuming an 8 ft ceiling: room length x
width = area in square feet
8 ft x 10 ft = 80 sq ft
Step 3: Calculate your required CFM
1 CFM x 80 sq ft = 80 CFM
Step 4: Use the Equivalent Duct Length
chart above to calculate duct run.
4a. 12 ft aluminum flex duct x 1.25 = 15 ft
4b. One elbow = 15 ft EDL
4c. One wall cap = 30 ft EDL
15 ft + 15 ft + 30 ft = 60 ft EDL
This is the equivalent duct length (or
resistance) the fan must overcome to
move air through the duct to the outside.
Step 5: Review models in catalog pages
to find a model with desired feature.
Features may include light fixture, heater or
low-profile housing.
Note: Check with your local building
inspector to confirm that these methods
are accepted in your area.
FIG. B
FIG. A
Equivalent Duct Length (EDL): The
Equivalent Duct Length Table (Figure B)
shows you how to calculate the equivalent
straight duct length in order to overcome
static pressure. The EDL chart helps
ensure fan performs as expected under
the airflow resistance caused by the listed
components.
A ventilating fan’s performance is plotted on
a graph called a performance curve. The
performance curve shows airflow in cubic
feet per minute (CFM) along the horizontal
axis and static pressure (resistance) along
the vertical axis. Figure A shows how a
performance curve works. The fan with
a “Closed Duct” has high static pressure
and no airflow; and the fan with “No Duct”
has low static pressure and high airflow. In
reality, an installed fan will be somewhere in
between these two points.
Performance Curves are listed on pages
26-33.
Equivalent Duct Length
Duct Diameter
3" 4" 6" 8"
Duct Smooth Metal Same as measured duct length
Material
FlexAluminum 1.25xductlength 1.25 X duct length 1.5 X duct length 1.5 X duct length
InsulatedFlex 1.5 X duct length 1.5 X duct length 1.75 X duct length 1.75 X duct length
Terminal Wall Cap 30 feet 30 feet 40 feet 40 feet
Device Roof Jack 30 feet 30 feet 40 feet 40 feet
Elbow Adjustable 15 feet 15 feet 20 feet 20 feet
Closed or Blocked duct work —A closed or blocked exhaust
port results in highest static pressure and zero air flow.
Extended Duct work —Long duct length and
elbows increase static pressure and reduce air flow.
Duct work —Duct increases static
pressure and reduces air flow.
Short Duct work —Short
duct allows low static
pressure and higher air flow.
No Duct work —
No duct equals
zero static pressure
and maximum
air flow.
Air Flow (CFM)
Static Pressure (Resistance)