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    A commercial restroom will require air to be changed 8-12 times an hour depending on the size of the facility and your local code. The illustration above demonstrates the use of a ceiling exhaust fan mounted in the Restroom exhausting the stale air. This exhaust air can be routed either to the roof using ductwork and a roof vent or to the side of your building using ductwork and a hooded wall vent. To calculate the amount of CFM you need to exhaust, first determine the volume of your room (LxWxH) and then divide your room volume by the number of minutes per air change needed. For example:

    10'L x 8'W x 10'H = 800 ft3
    10 air changes per hour: 60 mins. / 10 = 6 minutes
    800 ft3 / 6 minutes per air change
    133 CFM

    The pressure generated by fans in ductwork is very small. Yet, accurately estimating the static pressure is critical to proper fan selection. Fan static pressure is measured in inches of water gauge. One pound per square inch is equivalent to 27.7 in. of water gauge. Static pressure in fan systems are typically less than 2 in. of water gauge, or 0.072 Psi.

    The amount of static pressure that the fan must overcome depends on the air velocity in the ductwork, the number of duct turns (and other resistive elements), and the duct length. For properly designed systems with sufficient make-up air, the guidelines in the table can be used for estimating static pressure.

    Static Pressure Guidelines

    Non-Ducted:0.05 inches to 0.20 inches
    Ducted:0.2 inches to 0.4 inches per 100 feet of duct (assuming duct air velocity falls within 1,000-1,800 feet per minute)
    Fittings:0.08 inches per fitting (elbow, register, grille, damper, etc.)
    Important: Static pressure requirements are significantly affected by the amount of make-up air supplied to an area. Insufficient make-up air will increase static pressure and reduce the amount of air that will be exhausted. Remember, for each cubic foot of air exhausted, one cubic foot of air must be supplied.