Skip to Content

How Does a Bathroom Exhaust Fan Work

The purpose of a bathroom fan is to exhaust unwanted odors and moisture out of the house. The fan must be vented because ductless bathroom fans do not remove moisture, and should not be used in a bathroom with a shower.

An exhaust fan works by passing an electric current through the motor. This converts electrical energy to kinetic energy which in turn spins the fan impeller. As the impeller spins it sucks moist air from the bathroom and exhausts it out of the building via an insulated duct.

If you have recently bought a home, and are unsure if the fan actually vents out of the house, keep reading I have an easy tip how you can easily find out where it vents to. This will help prevent any further moisture damage and save you thousands over the long run.

The following is needed for an exhaust fan to work efficiently:

Where Does the Exhausted Air End up

A bathroom fan should ALWAYS be vented to the outside. This can be achieved by either running a duct from the bathroom fan through the ceiling, attic and finally terminating it either through the roof or through the soffit.

Notice I said through everything not into. Not into the attic, not into the soffit.

Venting a bathroom fan through a soffit is quite a controversial topic, some people say it should never be done, and others have done it for years without problems. There are some disadvantages, and advantages to terminating it to the soffit instead of the roof. As long as some basic rules are followed it will cause no issues.

While bathroom fans should be always vented outside, this advice is not always followed. I have seen fans terminating in attics, garages, closed crawl spaces and even neighboring rooms.

If you don’t have access to the entire duct to check where it vents to, there is another way. While it requires some additional equipment, you will know for sure where the moist air is going.

Use a smoke machine(amazon link) to blow smoke* into the exhaust fan. While another person holds the smoke machine in front of the fan, go out and see if smoke comes out from any of the vents. If not, then check the attic, garage and the crawl space for smoke.

If it did not vent outside, there is probably a lot of moisture damage where ever it vented to.

*the smoke from smoke machines is harmless, it is used usually for parties and can be inhaled.

Why Is CFM Important

Bathroom exhaust fan airflow CFM chart

The larger the bathroom, the higher capacity fan you need. The Home Ventilating Institute recommends 8 air changes in an hour for a bathroom. This means the larger the bathroom, the more powerful bathroom fan you need.

The moisture quickly spreads in the room as soon as you turn on the shower. This makes it harder to exhaust the same amount of moisture from a large room compared to a small room. By using the bathroom fan CFM calculator you can check to see if your bathroom fan is correctly sized. +-20% from the calculated value is acceptable. If buying a new fan always get one that is slightly more powerful than the calculator says.

While eventually an undersized bath fan will exhaust all of the air, it will take much longer, and there is a high chance that people don’t keep the fan running long enough.

Quick tip: If you are unsure the bath fan works at all, hold a tissue in front of it. The suction from the fan should easily hold it up. Here are 7 reasons why a bathroom fan won’t remove moisture

Exhaust Duct

Nearly always a bathroom fan is connected to an air duct. While using a duct allows for the fan to be placed anywhere in the bathroom, care should be taken to keep the duct as short and straight as possible.

The size of the duct should be correspond to the housing duct connection on the bathroom fan. Never smaller, even if the fan comes with an adapter. If the duct run is long and with many turns, go up one size. This means, if the fan housing has a 4-inch connection, you should run a 6-inch duct.

There is no downside to a larger duct, but a duct that is undersized will waste energy and make the fan loud and inefficient.

In almost all cases the duct should be insulated to prevent condensation inside the duct. This will also prevent brown stains around the bath fan.

Wall bathroom fans don’t generally have a duct. They vent directly out of the exterior wall.

Makeup Air

Since the bathroom fan vents air out of the bathroom, fresh air must come in somewhere. The fresh air enters the bathroom generally through a gap under the door. In rare cases there could be a vent in the bathroom door for that purpose.

If there is no gap under the door, the fan will have to fight very hard for every cubic feet of air. Since the air can not flow easily under the door it will be sucked through cracks in the door and walls. This creates a vacuum and will make the fan much less efficient.

A 100 CFM bath fan needs a 6-inch duct. This means that the air that enters the bathroom must come from a opening that is as large, or larger than a 6-inch duct. This is very hard to accomplish without a gap under or vent in the door.

By providing sufficient makeup air you could increase the bathroom fan performance up to 70 percent. Depending of course how airtight was the bathroom to begin with.

Now that you know how it works, check out all the benefits of installing an exhaust fan by reading my article on the pros and cons. The pros definitely outweigh the cons!


Bathroom fan works by using an electric motor to spin an impeller blade. This blade sucks air from the bathroom and exhausts it out of the house envelope via air ducts. The duct is generally insulated to prevent condensation from forming in the duct and dripping back down into the bathroom. The entire ventilation system must be designed correctly in order for the bathroom fan to work as efficiently as possible.

I hope you found this article helpful.

Please vote and let me know how I can improve future articles.

Thank you 🙂

Amazon and the Amazon logo are trademarks of, Inc, or its affiliates.