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.
1. Determine the Application
The first step when sizing for a ventilating 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 Institute (HVI) recommends the following Air Changes per
Hour (ACH). (See HVI)
I. Bathrooms - 8 ACH
II. Kitchens - 15 ACH
III. Other Rooms - 6 ACH
Continuous (Whole House) Ventilation:
Many building codes have adopted the American Society of Heating Refrigerating
and Air Conditioning Engineers (ASHRAE) Standard 62, as shown below.
I. House or apartment - 0.35 ACH
2. 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.
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 your 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.
Step 1. Determine the ACH required for the given application using the
HVI and ASHRAE standards above.
• Bathroom – 8 ACH
Step 2. Calculate the Area to be ventilated in Square Feet.
2a. Assuming 8 foot ceiling: room length x width.
• 8 ft x 12 ft = 96 sq ft.
2b. Guideline for cathedral ceilings:
i. 10 – 12 ft cathedral = length x width x 1.25
ii. 12 – 16 ft cathedral = length x width x 1.5
Step 3. Use the Equivalent Duct Length chart above to calculate the duct
run.
3a. 12 ft flex aluminum duct x 1.25 = 15 feet EDL
3b. One elbow equals 15 feet EDL
3c. 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.
Step 4:
4a. Select the correct ACH chart for the application
4b. Find the intersect of the Square Feet and EDL to determine the
appropriate 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 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.
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.
Step 1: Airflow (CFM) Calculation
First calculate area in cubic feet (length 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.
1a. 8 ft x 12 ft x 8 ft = 768 cubic feet
1b. 768/60 = 12.8 CFM value
1c. 12.8 x 8 ACH
• 102 CFM for intermittent bathroom ventilation
Step 2. Static Pressure (Resistance) Calculation
The Static Pressure calculation provides EDL and adjusts for airflow resistance
caused by duct material, elbows, and terminations devices.
2a. 12 ft flex aluminum duct x 1.25 = 15 feet EDL
2b. One elbow equals 15 feet EDL
2c. 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 overcome to move
air through the duct to the outside.
Step 3. Airflow and Static Pressure on a Performance Curve Chart:
The next step is to identify a fan that will provide 102 CFM at 60 EDL. Figure B
shows the performance curve for a Panasonic model FV-15VQ3. First, find 60 EDL
and then drop straight down to the horizontal axis to determine the CFM's
delivered at 60 EDL.
Example:
3a. 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 blue "75 ft"
lines.
3b. 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
Repeat Step 3. You may need to repeat Step 3 on the performance curve
different fan models until you find a model that matches the desired CFM with
EDL.
3. Measure the Equivalent
Duct Length (EDL)
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 (EDL):
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 airflow
resistance caused by the components listed in this chart.
|
