What Is a Bathroom Exhaust Fan and How Does It Work?

Updated: May 17, 2024

This little-known appliance keeps your bathroom dry and odor-free. Models are available that mount in the ceiling, a wall or the window.

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Wondering what bathroom fans are for? Having a bathroom exhaust fan helps keep that space odor-free. While that perk is important for people using the loo, it’s eclipsed by the good things an exhaust fan does for the bathroom itself. It ventilates the space and dries out the air, the importance of which can’t be overstated.

When moist air lingers, it makes paint peel, promotes mold growth and rusts metal fixtures. It can even damage wall and ceiling framing. Bathroom ventilation is so important that residential building codes require at least one openable window in each bathroom. Many local codes also require a bathroom exhaust fan.

What Is a Bathroom Exhaust Fan?

A bathroom exhaust fan sucks air out of the bathroom and directs it outside. It creates air circulation in the space, with replacement air usually coming through the door or an open window. The fan blows most of the moisture out, and the air movement prevents what moisture remains from condensing onto bathroom surfaces.

An exhaust fan must have a clear pathway to the outdoors, typically supplied by a duct that passes through a side wall or through the roof. The fan is usually controlled by a wall switch and draws power from the 20-amp circuit that powers the bathroom lights and receptacles. If it has an attached heat lamp, that may need its own 20-amp circuit.

Types of Bathroom Exhaust Fans

Depending on your bathroom configuration, one of four basic types might work best for you:

Ceiling mount

Probably the most common type, a ceiling mount fan exhausts through a duct that passes through the attic to a vent outlet mounted on the roof. This type of fan can be centrally located in the bathroom to handle the shower, bathtub and toilet.

Inline exhaust fan

An inline fan is mounted in a remote location (often the attic) and connects to an intake duct that terminates in a grille mounted on the bathroom ceiling. The output vent extends through a wall or the roof. Besides being quieter than a ceiling mount fan, an inline fan can serve more than one bathroom at the same time.

Wall- or window-mount fan

If the bathroom has an exterior wall, you can mount an exhaust fan on it, eliminating the need for lengthy ducts. It’s also possible to mount an exhaust fan in the window in the same way as an air conditioner.

What Size Bathroom Exhaust Fan Do You Need?

The Home Ventilation Institute (HVI) recommends a ventilation rate of eight air exchanges per hour for bathrooms.

Fan capacity is measured in cubic feet per minute (CFM) and has a one-to-one relationship with the square footage of the bathroom, up to 100 sq. ft. A 70-square-foot bathroom needs a fan with at least 70 CFM, while a 50-square-foot bathroom needs a 50 CFM fan. The minimum is generally 50 CFM, so that’s the smallest one available.

Bathrooms larger than 100 square feet need more ventilation than required by the square footage. The HVI recommends upping the capacity as follows:

  • 50 CFM per toilet;
  • 50 CFM for a shower;
  • 100 CFM for a jetted bathtub;
  • 50 CFM for a regular bathtub.

If you undersize your exhaust fan, the bathroom won’t be properly ventilated. But there’s no problem with over-sizing it. If you can’t find a fan that’s an exact match, it’s best to opt for a model with a larger capacity. Experts recommend oversizing if the ceiling is higher than eight feet.

Bathroom Exhaust Fan Installation

Vent fan installed in the ceiling of a domestic bathroom, Lafayette, California, January 20, 2022. Gado/Getty Images

A window fan that plugs into a wall receptacle needs hardly any installation and anyone can do it, though it’s important for the receptacle to be GFCI protected.

Installing a wall fan is more challenging because it involves cutting a hole in the wall and sealing it after securing the fan. It may call for some electrical wiring if it doesn’t have a plug. Ceiling-mount fan installation is even more complex and involves these steps:

Plan the duct route

Ideally, the duct should be as straight and short as possible. It usually passes through the attic to the roof or sidewall. If there’s no attic, it may have to pass between the ceiling joists, which may call for some drywall removal.

Cut the holes

The installer needs to cut a hole for the fan in the ceiling and another one for the exhaust vent in the roof or sidewall.

Install the fan and exhaust vent

The fan is typically secured to ceiling joists with braces. The exhaust vent attaches to the roof deck or siding along with the appropriate flashing.

Connect the duct

Flexible duct is suitable for exhaust fans. It’s clamped to the fan and the exhaust vent.

Hook up the fan to power

Wire the fan to a wall switch. Then it’s connected to the electrical system by tying into an existing circuit or creating a new one with its own circuit breaker. For all but the most electrical savvy homeowners, this is a job for a professional electrician.