How do I determine which kind of ventilation fan to use?
First, settle for no less than a quiet fan so that noise is not objectionable when the unit is operating. Selecting an ENERGY STAR fan may help with that.
Second, determine what application the mechanical ventilation fan will be used for, i.e., bathroom, kitchen or whole-house.
Third, determine if additional features are required, i.e., wall versus ceiling installation, light fixture (night-light), or auxiliary heater.
Fourth, calculate the require cubic feet per minute CFM of air flow that is recommended by industry standards for the application and size of room or house and duct run.
To calculate required ventilation for a bathroom: 8 ft by 10 ft with an 8 ft ceiling:
Calculate the cubic feet of your bathroom by multiplying length x width x ceiling height 8 x 10 x 8 = 640
Divide that number by 60 (the number of hours in a minute) 640/60 = 10.7
Multiply the result by 8 HVI (Home Ventilation Institute) recommendation for bathroom air changes per hour ACH). 10.7 x 8 = 86 CFM or greater
HVI recommend a minimum 86 CFM for this bathroom.
The next step is to calculate the duct run to ensure the selected fan can move 86 CFM of air through the entire duct length to exit the home. Please refer to the Equivalent Duct Length EDL chart located in the sizing section of this website for more details on duct sizing and applications other than bathrooms.
For this example:
12 ft of aluminum flex duct
1 elbow (90 degree bend)
1 termination device (exterior hood)
Length = 12 aluminum flex x 1.25 = 15 ft
Elbow = 15 ft
Termination hood = 30 ft
15 + 15 + 30 = 60 ft EDL