How Do Bathroom Fans Work? (2024 Guide)

Do you want to learn how bathroom fans work?

Bathroom fans remove moisture, stale air, and foul odors from bathrooms so you don't have to open a window.

In this guide, I will go over...

  • Bathroom fan ducting
  • CFM or fan strength
  • Fan noise measured in sones
  • And more...

Let's get started with this guide!

how do bathroom exhaust fans work


Sometimes when I am inspecting a house, I discover that one or all of the bathrooms have missing bathroom exhaust fans. When I break it to my clients, they are usually either surprised that they didn't notice, or they ask me if it is needed.

Bathroom fans play an important part in a healthy home.

Bathroom exhaust fans work by removing moisture from bathrooms after showers, and they also remove stale and smelly air. Bathroom fans are typically installed on the ceilings of bathrooms, but they are also sometimes installed vertically on a wall.

Read Also: What Are Bathroom Fan Venting Options?

Bathroom Fan Ducting

The enclosed fan sucks in the air from the bathroom, and it sends it to the exterior through a duct.

The exhaust duct is usually made of flexible aluminum, and typically has a 3-6 inch circumference. The bathroom duct may only be 1-2 feet long, but some ducts can be over 20 feet.

Duct Insulation

It can sometimes be very important to insulate the bathroom duct if it goes into an attic, garage, or crawl space.

With the temperature difference between the bathroom duct air, and the uninsulated room, condensation or water beads can form on the duct. This condensation can lead to wood damage or even mold growth.

Read Also: What Are The Best Ducts For Bathroom Fans?

What Is An Exterior Hood?

On the outside, there needs to be a hood that has some type of flapper.

The flapper will either be a circular flap or it will be three louvers. The flapper or grille allows air to move to the outside, but when the exhaust fan has stopped, then these flaps will close to prevent exterior air and bugs from entering the duct.

Another common 'pest' that can play havoc on a bathroom duct (or dryer duct) are birds.

Bird Nests

Birds just love to make nests in these flexible ducts because they are secure and sometimes warm places. It's really no different than making a nest in a tree hole.

When the bird makes the nest, it can completely block the air flow and making your bathroom fan useless.

It can even be a fire hazard if it causes the fan motor to overheat. In fact, birds nests in dryer vents are a very common fire hazard because the heat builds up in the duct due to the dryer.

To prevent bird nests, there should just be a flapper, but also some type of screen. But you don't want a screen with small holes though because these can get easily clogged with debris. A screen with centimeter sized holes is the right size. In the big box stores, they call these screens 'pest barricades'.

The CFM Rating (Fan Power)

Bathroom fan strength are rated in CFMs which stands for cubic feet per minute.

The CFM rating basically means how quickly the bathroom fan can exhaust the air in your bathroom on a per minute basis.

The lowest CFM bathroom fans are in the 50-cfm range, but household bathroom fans can go as high as 180-cfm. Anything higher than around 180-cfm is usually reserved for commercial properties.

For bathrooms with 100-sf or less, the general rule is 1-cfm for every square foot but with a minimum of 50-cfm.

Read Also: How To Size A Bathroom Exhaust Fan? (CFM Guide)

Fan Noise

One common complaint of bathroom fans is their noise. Some bathroom fans are just noisy, and you can hear them down the hallway. Other bathroom fans are so quiet that they have indicator lights just so you know that they are on.

The sound rating of bathroom fans is called sones.

Anything that is 1.0 sone or less is considered very quiet. Anything in the 2-3 sone range would be average. And bathroom fans in the 4+ sone range would be considered loud.

Sone isn't a physical measurement, but it is a rating based on how humans perceive loudness. When you double the sone rating, you double the perceived loudness, so a 4-sone is twice as loud as a 2-sone fan.

Read Also: What Are Bathroom Fan Sones?

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How To Control A Bathroom Exhaust Fan?

The most common way to control a bathroom fan is with a simple wall switch.

You flick the switch when you are taking a shower, and then you flip it off when you are done. However, you are supposed to leave the bathroom fan for 15min-30min after you take a shower so it cna remove all of the moisture.

Timer Switches

So as a home inspector, I always recommend installing a timer switch.

With a timer switch, you can pick between 5min to 60min so that the fan will turn off automatically when the time has elapsed.

Read Also: What Are The Best Timer Switches For Bathroom Fans?

Humidity Sensor

Besides using a timer switch, you can also buy a bathroom fan that has a humidity sensor. The humidity sensor turns the fan on automatically when the humidity level rises, and then it shuts off by itself when the humidity drops.

This is also a great option if you have tenants or children in the home who fail to use the bathroom fan. In addition to the fan itself having a humidity sensor, you can also buy a wall switch that has a humidity sensor so you don't have to buy an entire new fan.

Read Also: What Are The Best Bathroom Fans With Humidity Sensors?

Motion Sensor

There are also bathroom fans that have motion sensors (usually working in tandem with a humidity sensor) so that the exhaust fan will turn on if someone walks into the bathroom.

And when they leave the room, the motion sensor will turn the fan off after a period of no movement.

You can even set the motion sensor to work together with a humidity sensor. You could set the bathroom fan to turn on when the motion sensor detects movement, but it only turns off when the humidity level drops below a set level.

Final Thoughts

Bathroom fans work by sucking in the moist bathroom air and sending to the exterior through a duct.

Bathroom fans have a wide range in power, all the way from 50-cfm and even as high as 180-cfm. Some fans are also loud (4-sones) while others are whisper quiet (1-sone or less).

And there are a wide variety of control options for bathroom fans. You can use a simple wall switch, or you can use an advanced timer switch, humidity sensor, or even a motion sensor.

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