How to Choose the Right Exhaust Fan

Ventilating a building simply replaces stale or foul air with clean, fresh air. Although the ventilation process is required for many different applications, the airflow fundamentals never change: Undesired air out, fresh air in. The key variables that do change depending on applications are the fan model and the air volume flow rate (CFM). Other considerations include the resistance to airflow (static pressure or SP) and sound produced by the fan (Sones). Sometimes you need a exhaust fan to perform a particular function, but it’s not clear which model to use or even what CFM is needed. If this is the case, you’ll need to do some fan specification work. Fan specification is not an exact science, but it can be done confidently when the fan application is understood.

Based on the application, four parameters need to be determined. They are:

1. Fan Model
2. CFM
3. Static Pressure (SP), and
4. Loudness Limit (Sones)

1. Fan Model

Fans all perform the basic function of moving air from one space to another. But the great diversity of fan applications creates the need for manufacturers to develop many different models. Each model has benefits for certain applications, providing the most economical means of performing the air movement function. The trick for most users is sorting through all of the models available to find one that is suitable for their needs. Here are some guidelines.

• Direct Drive vs. Belt Drive
  Direct drive fans are economical for low volume (2,000 CFM or less) and low static pressure (0.50” or less). They require little maintenance and most direct drive motors can be used with a speed control to adjust the CFM. Belt drive fans are better suited for air volumes above 2,000 CFM or static pressures above 0.50.” Adjustable pulleys allow fan speed and CFM to be adjusted by about 25%. High temperature fans above 50ºc (above 122º F) are almost always belt driven.
• Propeller vs. Centrifugal
  Wheel propeller fans provide an economical method to move large air volumes (5,000+ CFM) at low static pressures (0.50” or less). Motors are typically mounted in the airstream which limits applications to relatively clean air at maximum temperatures of 40ºc (104º F). Centrifugal fans are more efficient at higher static pressures and are quieter than propeller fans. Many centrifugal fan models are designed with motors mounted out of the airstream to ventilate contaminated and high temperature air.
• Fan Location
  Fan models are designed to be mounted in three common locations: on a roof, in a wall, or in a duct. Whatever the location, the basic fan components do not change. Only the fan housing changes to make installation as easy as possible. Determining the best location for a fan depends on the airflow pattern desired and the physical characteristics of the building. By surveying the building structure and visualizing how the air should flow, the place to locate the fan usually becomes evident.

2. Cubic Feet per Minute (CFM)

Once the fan type is known, the volume of air exchanged must be determined. Your local building codes should contain information pertaining to the suggested air changes for proper ventilation. The ranges specified will adequately ventilate the corresponding areas in most cases. However, extreme conditions may require “Minutes per Change” outside of the specified range. To determine the actual number needed within a range, consider the geographic location and average duty level of the area. For hot climates and heavier-than-normal area usage, select a lower number in the range to change the air more quickly. For moderate climates with lighter usage, select a higher number in the range. Use the following formula to calculate the CFM needed to adequately ventilate an area.
Room volume = LxWxH CFM = Room Volume ÷ Min/Change

Here’s a chart showing some suggested air changes for a range of room sizes.

3. Static Pressure

An accurate measurement of 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” SP. Static pressure in fan systems is typically less than 2” SP, or 0.072 Psi. The Exhaust Fan drawing below illustrates how static pressures are measured in ductwork with a manometer . A pressure differential between the duct and the atmosphere will cause the water level in the manometer legs to rest at different levels. This difference is the static pressure measured in inches of water gauge. In the case of the exhaust fan in the drawing, the air is being drawn upward through the ductwork because the fan is producing a low-pressure region at the top of the duct. This is the same principle that enables beverages to be sipped through a straw. 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 below can be used for estimating static pressure.

4. Sound Levels (Sones)

The sound generated by a fan must be considered. For the fan industry, a common unit for expressing sound pressure level is the sone. In practical terms, the loudness of one sone is equivalent to the sound of a quiet refrigerator heard from five feet away in an acoustically average room. Sones are a linear measurement of sound pressure levels. For example, a sound level of 10 sones is twice as loud as 5 sones. Refer to the Suggested Limits for Room Loudness chart below to determine the acceptable sone range for the application. As a general guideline, choose a fan that has a sone rating within the range specified.

Once the exhaust fan application is known, keep in mind these four parameters to choose the right one for the job. For more thorough information on this topic, view Ventilation Fundamentals (PDF).

The information contained in this article is intended for general information purposes only and is based on information available as of the initial date of publication. No representation is made that the information or references are complete or remain current. This article is not a substitute for review of current applicable government regulations, industry standards, or other standards specific to your business and/or activities and should not be construed as legal advice or opinion. Readers with specific questions should refer to the applicable standards or consult with an attorney.