Bathroom fans are sized in cfm (cubic feet per minute) or L/s (Liters per second). These terms both describe the volume of air the fan can move in a period of time. The model 2012 IRC specifies a minimum of:
• 50 cubic feet per minute (23.6 L/s) intermittent exhaust capacity, or
• 20 cubic feet per minute (9.4 L/s) continuous exhaust capacity.
However, these IRC requirements are minimums and do not consider the size of the room, the amount of moisture produced, or the efficiency of the installation.
A more effective way to consider fan size is to look at ventilation needs and then size the fan accordingly. The Home Ventilating Institute recommends that the fan system should provide eight air changes per hour (ACH). This means that in one hour, the fan must be able to exhaust a volume equal to eight times the volume of the room. To determine what size fan is needed, follow this formula:
Step 1—Determine the volume of the room in cubic feet:
Length x width x height = volume of room
Step 2—Multiply the volume of the room by 8 ACH Step 3—Divide by 6Q minutes in an hour to get cfm
Example A: A bathroom is 8 feet 6 inches (8.5 feet) by 10 feet, with 8-foot ceilings
Step 1—8.5 x 1Q x 8 = 68Q cubic feet
Step 2—68Q x 8 = 544Q cubic feet per hour
Step 3—544Q/6Q = 9Q.67 cubic feet per minute (cfm)
Example B (a metric example): A bathroom is 3 meters by 3.5 meters with a 2.5 meter ceiling.
Step 1—3 x 3.5 x 2.5 = 26.25 cubic meters Step 2—26.25 x 8 = 210 cubic meters per hour Step 3—210 x 1000 = 210,000 liters/hour Step 4—210,000/60 = 3500 liters/minute Step 5—3500/60 = 58.3 liters/second
A short-cut to this formula is to take the volume of the room (length x width x height) and divide by 7.5.
The fan size determined by the air change per hour method is effective fan capacity—or how much air the fan can actually move. Effective fan capacity is not the same as the mechanical size of the fan. The effective fan capacity will depend on a number of factors including:
• Length of duct runs from the intake vent to the exhaust vent. Generally, if the duct run is more than about 5 feet (1.5 meters), the size of the fan should be increased to compensate for the resistance of a longer duct run.
• Elbows or bends in the ducts. Generally, if there is more than one elbow or bend in the duct, the size of the fan should be increased to compensate for the greater resistance.