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Steps to Calculating Fan Size Using CFM

General Industrial The Workplace

Whether you’re installing ventilation fans for building safety or comfort, it’s important to estimate the fan calculation as closely as possible. Fan size is based on its cubic feet per minute (CFM). It’s important to match the CFMs to the cubic feet (CF) inside the building.

Industrial Fans Direct has a cubic feet per minute calculator that’s useful for fan sizing, along with a table that shows recommended air changes per minute based on the building’s use. The fan calculator makes calculating CFM’s fast and accurate.

However, if your building has varied ceiling heights, you may need to calculate the space separately to find total CF for the fan calculation.

Variable-Height Ceilings

Instead of measuring the total length and width, separate the building by ceiling plates. Measure wall length and width for each section separately and its ceiling height. Complete the cubic footage measurements for each section separately, and add the cubic footages together.

Pitched Ceilings

To measure the cubic footage of building that has a ceiling open to a pitched roof, use this formula for finding the area inside the unfinished attic area:

Multiply the length and width together, along with the tallest point in the roof (in feet). Divide the product of these three by two to find the CF inside the topmost area of the space. Add the CF of the lower portion and the upper portion to find total CF.

Selecting the Fan

Choose the fan or fans that most closely matches the number of air changes per minute you need to adequately ventilate the building. A 15,000 cubic foot area that needs an air change every 3 minutes would need a fan with a CFM of 5,000. The fan's product specifications will list its CFM on its packaging or data sheet. 

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