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Roof Exhaust Fan: Tips and Tricks

2021-09-04 hqt

Roof Exhaust Fan: What You Need To Know

Proper ventilation is essential to replace foul or stale air with fresh, clean air. The process of ventilation is critical for various applications, but the fundamentals of airflow never change. Therefore, the purpose of installing a roof exhaust fan is to keep the fresh air inside and undesired air outside of the building.

The things that do change according to the application type are the CFM (air-volume-flow-rate), fan model, and Sones (sound that a fan produces). Therefore, it is important to choose the exhaust fan according to your application type. In this guide, we will share some useful tips and information that will help you understand how to select a suitable exhaust fan for your building.

roof exhaust fan

How Would You Choose The Right Roof Exhaust Fan?

Sometimes, you require an exhaust fan for performing a specific function. But unfortunately, you have no idea about the CFM you need and which fan model to choose. In such a situation, you will have to learn about the fan specification. Specification of a fan isn’t a complete science. However, you can do it confidently after understanding the application of the fan. According to your application type, you need to determine the following four parameters:

  • Fan model
  • Sones (loudness limit)
  • SP (static pressure)
  • CFM

Once you understand the above-listed parameters, the selection of the right roof exhaust fan will become a straightforward process. Let us show you how you can choose the most appropriate parameters for your application.

1. Tips on Choosing the Fan Model

All fan models you find in the market perform the same function; helping the air pass through one place to another. But, fan applications can be greatly diverse for which manufacturers develop different fan models. Remember that all fan models come with unique benefits for specific applications. The trick is to find a fan model that is ideal for your needs. Read the following information to choose the most suitable fan model for your application.

Belt Drive vs. Direct Drive

The Direct Drive Fan would be a suitable choice if you need a roof exhaust fan for low-static pressure (up to 0.50”) and low-volume (up to 2,000 CFM). These fans are an economical choice for low-static pressure and low volume applications. You need to put less effort into the maintenance of direct drive fans. Moreover, you can use the motors of a direct drive fan with speed control for adjusting the SFM.

On the other hand, a Belt Drive exhaust fan is perfect for above 0.50” static pressures or over 2000 CFM air volumes. Adjustable pulleys help CFM and fan speed to adjust by approximately 25%. If you are looking for an exhaust fan that will work above 122F (above 50C), then it must be a Belt Driven Exhaust Fan.

Centrifugal vs. Propeller

A propeller fan is the most economical choice for moving larger volumes of air (over 5000 CFM) at low-static pressures (up to 0.50”). Typically, the motors of the fan mount in an airstream. It limits the application to clean air under temperature conditions of 104F (40C). On contrary, a centrifugal fan proves highly efficient at greater static pressures.

Moreover, they also produce much less sound than a propeller fan. Most centrifugal models have their motors outside the airstream for venting high temperatures and contaminated air.

Fan Location

Different fan models are available that you can mount on different locations, such as on the roof, in the duct, or in the wall. The components of the fan never change regardless of the location.

The only thing that changes is the housing of the fan to ensure easy installation. To determine the most ideal fan location, you should keep in mind the desired pattern of the airflow along with the building’s physical characteristics.

2. CFM

After knowing the right fan model, your next job is to determine the exchange of air volume. The building codes in your area contain information about the recommended air changes to ensure proper ventilation.

Usually, this information is adequate for proper ventilation. However, in extreme conditions, you may need to consider (Minutes-per-Change”). You can consider your geographic location along with the area’s average work level for determining the apt number required inside the range. For heavier areas and hot climates, you should opt for lower numbers for changing the air quickly.

3. Static Pressure

You must also measure the static pressure accurately to identify the most appropriate roof exhaust fan for your application. The measurement of the static pressure of fans is done in water gauge inches. One pound/square inch equals 27.7” static pressure. The fan system static pressure is usually below 0.072 Psi, or 2” SP. If there is a difference between the atmosphere and the duct, it will result in water level resting at various levels in manometer legs. The static pressure amount that fans need to overcome subjects to:

  • The ductwork air velocity
  • Duct turns number
  • The length of the duct

This information holds critical importance in the selection of the right exhaust fan model for your application.

4. Sones (Sound Levels)

When buying an exhaust fan, you must also pay special attention to the level of sound that it will generate. Sone is the common unit in fan industries for expressing the level of sound pressure. Practically, 1-sone loudness equals the sound that a quiet refrigerator produces and heard at a distance of five feet. The greater the sone number is the loudest the fan would produce sound. You would never like your exhaust fan producing high sounds, so you must check the sone number before your purchase.

By keeping the above parameters in your mind while buying a roof exhaust fan, you would be able to choose the most appropriate model. However, one main player that may affect your buying decision is your buying power.

roof exhaust fan

If your budget is limited, you may have to compromise over a few factors. However, in any case, you must choose the right type of exhaust fan according to your application type. Additionally, make sure that you are buying a high-quality exhaust fan that can work flawlessly in the long run.

You can also talk to our experts for more information or suggestions on exhaust fans.