The first step when sizing for a ventilation fan is to determine the application. Decide whether you are sizing for intermittent or continuous ventilation. If intermittent, determine which application i.e. bathroom, kitchen, or other. Use the following industry recommendations to determine Air Changes per Hour (ACH) for your specific application.

Intermittent (Spot) Ventilation:
The Home Ventilating Institue (HVI) recommends the following Air Changes per Hour (ACH).

• Bathrooms - 8 ACH
• Kitchen - 15 ACH
• Other Rooms - 6 ACH

• House of apartment - 0.35 ACH

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

Both methods, the Sizing Chart Method and the Performance Curve Method, follow industry standards and will give you similar outcomes. The Sizing Chart Method is easier. The Performance Curve Method is often preferred by engineers.

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

Sizing Chart Method

Example: Sizing for an 8 ft x 12 ft x 8 ft ceiling bathroom using 12-foot long, 4 inch diameter aluminum flex duct, one elbow, one wall cap.

1. Determine the ACH required for the given application using the HVI or ASHRAE standards above
   * Bathroom - 8 ACH
   
2. Calculate the Area to be ventilated in Square Feet.
   
   Assuming 8 foot ceiling: room length x width
   * 8 ft x 12 ft = 96 sq. ft.
   
   Guideline for cathedral ceilings:
   i. 10 - 12 ft cathedral = length x width x 1.25
   ii. 12 - 16 ft catherdal = lenth x width x 1.5
   
3. Use the Equivalent Duct Length chart to calculate the duct run.
   i. 12 ft flex aluminum duct x 1.25 = 15 feet EDL
   ii. One elbow jack equals 15 feet EDL
   iii. One roof jack equals 30 feet EDL
   
   * (12 ft length x 1.25 alum. flex) + 15 ft elbow + 30 ft wall cap = 60 ft EDL. This is the equivalent duct length (or resistance) the fan must overcome to move air through the duct to the outside.
   
4. See the Fan Sizing Charts for appropriate Panasonic model.
   i. Select correct ACH chart for the application
   ii. Find the intersect of the Square Feet and EDL to determine the approriate Panasonic model(s). When values are not exact move to next higher value
   
   * 100 sq ft by 60 ft EDL = FV-15VQ3

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 the "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 results of the 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.

Example: Sizing for an 8 ft x 12 ft x 8 ft ceiling bathroom using 12-foot long, 4 inch diameter aluminum flex duct, one elbow, one wall cap.

1. Airflow (CFM) Calculation
   First calculate area in cubic feet (lenth x width x ceiling height). Then divide this number by 60 to get the Cubic Feet per Minute (CFM) required to replenish the entire air volume in one hour. Next, multiply the CFM value by the appropriate ACH value for the given application
   
   i. 8 ft x 12 ft x 8 ft = 768 cubic feet
   ii. 768 / 60 = 12.8 CFM value
   iii. 12.8 x 8 ACH
   
   * 102 CFM for intermittent bathroom ventilation
   
2. Static Pressure (Resistance) Calculation
   The Static Pressure calculation provides EDL and adjusts for airflow resistance caused by duct material, elbows, and termination devices.
   
   i. 12 ft flex aluminum duct x 1.25 = 15 feet EDL
   ii. Once elbew equals 15 feet EDL
   iii. One roof jack equals 30 feet EDL
   
   * 15 + 15 + 30 = 60 feet total EDL
   
   This is the equivalent duct length (or resistance) the fan must over come to move air through the duct to the outside.
   
3. Airflow and Static Pressure on Performance Curve Chart
   The next step is to identify a fan that will provide 102 CFM at 60EDL. Figure B shows the performance curve for a Panasonic model FV-15VQ3. First, find 60 EDL then drop straight dodwn to the horizontal axis to determine the CFM's delivered at 60 EDL.
   
   
   
   Example:
   i. Locate the point on the blue "FAN" curve at 60 ft. In this case, it is a point on the blue fan curve between the yellow "50 ft" and the blue "75 ft" lines.
   ii. From this point on the FAN line move straight down to the horizontal axis to read CFM. In this case, it is close to 100 CFM.
   
   * Therefore, a Panasonic FV-15VQ3 is the appropriate model
   
   iii. Repeat Step 3. You may need to repeat Step 3 on the performance curve for different fan models until you find a model that matches the desired CFM and EDL.
   
   Performance Curves can be found in the product spec sheet link () next to each model.

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The third step is to measure 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:
Static pressure in a typical duct run is caused by the type of duct material, elbows, exterior wall cap, etc. The table below 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 flow resistance caused by the components listed in this chart.

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