Ventilation Central - How To Size

Measure the Equivalent Duct Length (EDL)

Properly sized ventilation in airtight homes and apartments helps to assure healthy indoor air quality. Both intermittent (spot) ventilation and continuous (whole house) ventilation should be considered.Intermittent ventilation is used to exhaust sources of moisture and odors, while continuous ventilation is used to remove accumulated indoor air pollutants. Ventilating fans should be located near the source of moisture and indoor air pollutants in bathrooms, laundry rooms, kitchens, hobby rooms and smoking rooms.

Calculate the Area Being Ventilated

The second step is to calculate the area being ventilated. Calculate square feet or cubic feet depending on which sizing method you choose.

The following method for selecting the proper fans follows industry standards and will give you the recommended fan for your application. The Sizing Chart Method is easier. The Performance Curve Method is often preferred by engineers.

Note: You should check with your local building inspector to confirm that these methods are accepted in your area.

Sizing Chart Method

Performance Curve Method

Sizing Chart Method:
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:
See the Fan Sizing Charts Use the Equivalent Duct Length chart below to calculate duct run.

Equivalent Duct Length
Duct Diameter
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
Material	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

4a.
4b.
4c.
•

12 ft aluminum flex duct x 1.25 = 15 ft
FOne elbow = 15 ft EDL
One roof jack = 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:
Select a properly sized fan using the sizing chart below.

5a.
5b.
5c.
•

Identify the column with 60 ft EDL
Identify the color with 80 CFM
Identify the proper models with their duct size
FV-08VKS1 4" duct
FV-08VKM1 4" duct
FV-08VK1 4" duct
FV-08VKL1 4" duct
FV-11VQ3 4" duct
FV-11VQL4 4" duct
FV-11VF2 4" duct
FV-11VFL2 4" duct
FV-11VQD2 4" duct
FV-11VH1 4" duct
FV-11VHL1 4" duct

Sizing and Selecting a Ceiling Mounted Fan Steps to Select A Fan:

1. Calculate the CFM needed for your application
2. Calculate the Equivalent Duct Length (EDL) for your installation

Maximum Airﬂows at various Equivalent Duct Lengths (EDL) Maximum CFM Table Using 3", 4" and 6"
Model	Duct size	40 feet	60 feet	80 feet	100 feet
FV-05VQ3	4"	50 CFM	50 CFM	40 CFM	40 CFM
FV-05VF2	4"	50 CFM	40 CFM	40 CFM	40 CFM
FV-05VF2	3"	40 CFM	30 CFM	30 CFM	30 CFM
FV-05VFL2	4"	50 CFM	40 CFM	40 CFM	40 CFM
FV-05VFL2	3"	40 CFM	30 CFM	30 CFM	30 CFM
FV-05VK1	4"	50 CFM	50 CFM	50 CFM	50 CFM
FV-08VKS1	4"	80 CFM	80 CFM	70 CFM	70 CFM
FV-08VKM1	4"	80 CFM	80 CFM	70 CFM	70 CFM
FV-08VK1	4"	80 CFM	80 CFM	70 CFM	70 CFM
FV-08VKL1	4"	80 CFM	80 CFM	70 CFM	70 CFM
FV-08VQ3	4"	70 CFM	60 CFM	60 CFM	50 CFM
FV-08VQL4	4"	70 CFM	60 CFM	60 CFM	50 CFM
FV-08VF2	4"	70 CFM	60 CFM	60 CFM	50 CFM
FV-08VF2	3"	50 CFM	40 CFM	30 CFM	30 CFM
FV-08VFL2	4"	70 CFM	60 CFM	60 CFM	50 CFM
FV-08VFL2	3"	50 CFM	40 CFM	30 CFM	30 CFM
FV-11VQ3	4"	90 CFM	80 CFM	70 CFM	70 CFM
FV-11VQL4	4"	90 CFM	80 CFM	70 CFM	70 CFM
FV-11VF2	4"	90 CFM	80 CFM	70 CFM	70 CFM
FV-11VF2	3"	60 CFM	50 CFM	40 CFM	40 CFM
FV-11VFL2	4"	90 CFM	80 CFM	70 CFM	70 CFM
FV-11VFL2	3"	60 CFM	50 CFM	40 CFM	40 CFM
FV-11VQD2	4"	90 CFM	80 CFM	70 CFM	70 CFM
FV-11VH1	4"	90 CFM	80 CFM	70 CFM	70 CFM
FV-11VHL1	4"	90 CFM	80 CFM	70 CFM	70 CFM
FV-15VQ4	6"	150 CFM	150 CFM	140 CFM	140 CFM
FV-15VQL4	6"	150 CFM	150 CFM	140 CFM	140 CFM
FV-20VQ3	6"	190 CFM	170 CFM	160 CFM	160 CFM
FV-30VQ3	6"	260 CFM	240 CFM	230 CFM	220 CFM
FV-40VQ3	6"	340 CFM	320 CFM	300 CFM	280 CFM

CFMs are color coded

Performance Curve Method: 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. This method requires two calculations and plotting on a graph. First is the Airflow (CFM) calculation, which calculates cubic feet and the required CFM for the area to be ventilated. The second is the Static Pressure (Resistance) calculation, which calculates the Equivalent Duct Length (EDL) of a straight duct run from the inlet to the termination (outside) point of the building. Third, the result of airflow and static pressure calculations are then plotted on the performance curve of ventilating fan models to find the ideal model for a given application and duct run. Sizing and selecting an In-Line Fan and Installation Kit
Step 1: Determine the total square footage (sq ft) of either the bath or baths that require ventilation or the whole house, depending on the desired application.
Step 2: Determine the number of vents (intake pick-ups) to be used in the ventilated area. This is the equivalent duct length (or resistance) the fan must overcome to move air through the duct to the outside.
Step 3: Estimate the length of ducting from the farthest vent (intake pick-up) to the outside exhaust point.
Step 4: Download this pdf to select fan models Download this pdf to see Performance Curves per model.
Step 5: Download this pdf to choose an installation kit
Example: Desire to ventilate two bathrooms. The total square footage is 200 sq ft. Each room will have a single vent (intake pick-up). Therefore, there are a total of two vents (intake pick-ups). The estimated length of ducting from the farthest vent (intake pick-up) to the outside exhaust point is 50 ft.
Step 1: 200 sq ft = 200 CFM.
Step 2: 2 vents (intake pick-ups)
Step 3: 50 ft
Step 4: Download chart Moving along the top of the chart from left to right using the data in steps 1-3, the correct fan would be FV-20NLF1.

Measure the Equivalent Duct Length (EDL)

The third step is to calculate the Equivalent Duct Length of the planned duct run. This requires a basic understanding of static pressure caused by a duct run design and its components.

Static Pressure and Duct Run:
A ventilating fan must overcome resistance when pushing air from the inlet, through the duct, to the outside of the building. This resistance is known as static pressure. The amount of static pressure depends on the duct length, type of duct, elbows and the roof jack or wall cap.

Equivalent Duct Length (EDL):
Static pressure in a typical duct run is caused by the type of duct material, elbows, exterior wall cap, etc. The Equivalent Duct Length Table (Figure B) shows the standard values for duct components. The EDL chart allows you to calculate the equivalent straight duct length in order to overcome static pressure caused by each component in a duct run. The EDL helps assure the fan performs as expected under the airfl ow resistance caused by the components listed in this chart.

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 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 airfl ow. In reality, an installed fan will be somewhere in between these two points.

Equivalent Duct Length
Duct Diameter
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
Material	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

WhisperGreen™ 80 CFM Premium Ceiling Mounted Continuous and Spot Ventilation Fan with SmartAction™ Motion Sensor

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