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    According to ASHRAE Standard 62.1, an office will require 4-10 air changes per hour depending on the occupancy and size of the office. The illustration above demonstrates how to ventilate more than one area using a single fan. To determine the amount of CFM required for your ventilation equipment, calculate the volume of the room (LxWxH) and divide it by the number of minutes per air change. For example:

    Office A

    18'L x 18'W x 15'H = 4,860 ft3
    4 Air changes per hour: 60 mins. / 4 = 15 minutes
    4,860 ft3 / 15 minutes per air change
    324 CFM

    Office B

    24'L x 24'W x 24'H = 6,912 ft3
    6 Air changes per hour: 60 mins. / 6 = 10 minutes
    6,912 ft3 / 10 minutes per air change
    691 CFM

    TOTAL CFM required for the exhaust fan = 1,015 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 0.40 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, grill, 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.