What Does an Inline Duct Fan Do?


Thorough and efficient circulation of air is crucial to a successful home HVAC system. Not only does it contribute to efficient distribution of the temperature across the living space, but it also helps control other factors such as humidity and improve general air quality.

Inline duct fans are often part of the HVAC system, but exactly does an inline duct fan do? An inline duct fan is a fan that’s connected with the duct work. Inline duct fans are usually installed near the end of a long stretch of duct. The increased airflow from the inline fan assists the air conditioning fan motor to push air further into distant rooms. It’s usually plugged into a sensor/controller device on the duct, which then automatically switches on the inline duct fan whenever necessary. Most are powered by a 120-volt power.


The Difference Between Inline Duct Fans and Duct Boosters

The key difference between an inline duct fan and duct booster boils down to the CFM capacity of the fans.

CFM is simlply a measure of how many cubic feet of air per minute a fan can blow. The higher the CFM, the further and faster air can flow in a space.

An inline duct fan can provide upwards of 1500 cfm for a 12-inch model whereas the same size duct boosters are usually limited to about 800 cfm for the same size fan. This difference directly translates to a difference in power consumption of the fans. Where a typical inline fan consumes around 250 watts for the above-mentioned capacity, the duct booster fan consumes just 100 watts.

These differences determine the application scenarios for either of these fans. An inline duct fan’s primary application is to ventilate large volumes of space. On the other hand, a duct booster is employed where the existing HVAC system fails to deliver and distribute air uniformly, usually in-home applications.

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Advantages of an Inline Duct Fan

Inline duct fans provide several advantages over the standard duct boosters, listed as follows.

  • The inline duct fan can be strategically installed anywhere along the duct work, whereas the duct boosters have limited placement options owing to the fact that they have to be installed on preexisting outlets along the walls or on the roof vents.
  • An inline system provides the option to branch out the duct work to supply ventilation to multiple spaces such as the bathroom without much hassle.
  • An inline duct fan can be easily employed where the duct losses are high and duct work is just too long for efficient exhausting. 
  • Inline duct fans provide efficient and powerful exhausting for moisture and humidity
  • Generally, the inline duct fans are well reputed for having high quality motors that last a long time compared to duct boosters.
  • For large spaces and future expansion inline duct fans offer a generally better solution.
  • Inline duct fans produce less noise.


Types of Inline Duct Fans

Typically, inline duct fans can be categorized into two types, Tube axial belt drive duct fans and tube axial direct drive fans. The housing for both fans is tube shaped with the difference being that one turns by taking power from belt driven by a motor while the other is driven directly by motor.

The belt driven variants require occasional maintenance to check for belt tension or for replacing the belt. Meanwhile their direct driven counterparts require little to no maintenance. Belt driven fans are ideal for a relatively large duct network with a lot of bends and curves or where a high static pressure is required, usually in large spaces. The direct drive fans provide an ideal solution for smaller domestic spaces. 

Moreover, in spaces where square ducts are used, inline duct fans are available with square inlet and outlet openings in both belt driven and direct driven variants.


Why Chose Inline Duct Fans

One of the main reason people opt for an inline fan is for cosmetic reasons. Since it is installed inline with the duct work it remains hidden in the attic, unlike duct boosters that are visible and often not so visually appealing. 

Often times in an urban setting space is premium, and duct boosters require some space for installation. On the contrary most inline fans require no extra space since the can be spliced directly into the duct work and remain hidden with it. This is especially beneficial for small spaces such as bathrooms where ventilation is necessary to remove odors and moisture.

Occasionally, in some spaces where there is no direct access to exterior walls the user is left with no other option than to use an inline duct fan in conjunction with the existing duct network to exhaust the air form the space.

Since the duct fans are hidden away with the duct work they don’t make much noticeable noise compared to duct fan boosters. 


Choosing the Right Inline Duct Fan

Duct fans come in a variety of sizes and capacities ranging from 4 to 14-inch inlet outlet sizes and from 100 to 1500 CFM capacities. The sizing of the fan depends directly on the duct size used in your space.

As far as the capacity is concerned, there is a general rule of thumb that you need 1 CFM per Square foot of the room. This rule. However, doesn’t consider the losses in the ductwork due the convolutions and turns.

But for small domestic application this rule should suffice. So, to choose the right inline duct fan for your needs you first need to know the size of the duct that ventilates the room and the actual area in square foot to reasonably size the duct fan.

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Related Questions

What size do inline duct fans come in?

Typically, you’ll find them in 4-, 6-, 8-, 10-, or 12-inch sizes. They’re usually matched up to fit the same size as the duct work, and you’d consider the volume of a room/space and how many CFM you need as well. The larger inline duct fans are, the more CFM they offer, but they’re also louder. But compared to the same size and capacity booster fans, they’re usually quieter.