Can you run two bathroom exhaust fans to one vent?

Having proper ventilation in a bathroom is important to remove moisture, odors, and contaminants from the air. Some bathrooms have more than one exhaust fan, which raises the question of whether you can connect multiple bathroom exhaust fans to a single vent outside. There are a few considerations when deciding if running two bathroom exhaust fans to one vent is a good idea.

Key Factors to Consider

There are three main factors to think about when deciding if you can tie two bathroom exhaust fans to the same duct and external vent:

• Airflow requirements
• Duct sizing
• External vent sizing

Getting all three of these right is crucial for proper ventilation and avoiding problems down the road.

Airflow Requirements

Bathroom exhaust fans need to move a certain amount of air, measured in cubic feet per minute (CFM), to effectively remove moisture and odors. Most residential bathroom fans range from 50-110 CFM. The total CFM needed depends on the size of the bathroom.

Generally, you need:

• 50 CFM for bathrooms up to 50 square feet
• 60-80 CFM for bathrooms 50 – 100 square feet
• 80-110+ CFM for bathrooms over 100 square feet

If you have two fans, the total CFM ratings of the fans needs to meet or exceed the recommended airflow for the total bathroom space.

Duct Sizing

The ductwork running from the fans to the external vent needs to be sized properly to handle the combined airflow. If the duct is too small, it can restrict airflow and reduce the fans’ effectiveness.

For bathrooms up to 100 square feet, 4-inch duct is usually sufficient for up to 100 CFM of exhaust. For larger bathrooms or higher airflow, 5-inch or larger duct may be required.

External Vent Sizing

Like the ductwork, the external vent needs to be large enough to handle the exhaust from two fans. For up to 150 CFM of exhaust, a 4-inch vent is common. Over 150 CFM may require a 5-inch or larger vent.

The vent should also have a damper to prevent outdoor air from entering the ductwork when the fans are off.

Ways to Connect Two Fans to One Vent

If the airflow, duct sizing, and vent sizing requirements are met, there are a couple ways two bathroom exhaust fans can be connected to a single external vent:

Joining Ducts

One option is to run separate ducts from each fan and join them together via a Y-fitting into a common duct before going through the external vent. This allows each fan to be controlled separately while still exhausting through the same duct and vent.

Manifolded Fans

Another choice is to use manifolded or tandem fans. These units have two fans housed in the same casing with a combined duct outlet. The fans can run together or separately. This provides convenient control with a single exterior vent.

Considerations for Multiple Fans to One Vent

Here are a few other things to keep in mind when planning to connect two bathroom exhaust fans to a single vent:

• Make sure both fans are GFCI protected for safety.
• Isolate the ducts if they run through unconditioned spaces to minimize condensation.
• Consider installing humidistat or timer controls for automatic fan operation when needed.
• Select high-quality, quiet fans to minimize noise.
• Route ducts as straight and smooth as possible to maximize airflow.

Can You Run Bath Fans and Dryer to Same Vent?

While connecting multiple bathroom exhaust fans to one external vent is generally okay if done properly, sharing a vent between bathroom fans and a dryer is not recommended. This is because:

• Dryers produce much more moisture than bath fans. Most dryers need at least 150 CFM of exhaust capacity.
• Dryer lint can gradually clog bath fan ductwork, reducing ventilation.
• Makeup air requirements are different. Bath fans just need ceiling vents for makeup air. Dryers need controlled makeup air ducted from outside.
• Joint vents could allow odors from dryer vent to enter bathrooms.

For these reasons, it’s best to have completely separate venting systems for dryers and bathroom exhaust fans even when located close together.

Conclusion

Connecting two bathroom exhaust fans to a single external vent is permissible provided some key guidelines are followed:

• The fans must have a combined CFM rating appropriate for the total bathroom space.
• Ductwork and external vent must be sized adequately to handle the total airflow.
• Fans should be good quality, quiet models with appropriate controls.
• Ducts should be straight, smooth, and well-insulated.

However, bathroom fans should never share ducting or venting with dryers, as the airflow and ventilation requirements are completely different.

When installed properly using the right approach, two bath fans can work well on one duct and vent assembly. This can provide needed airflow and flexibility in controlling moisture removal from the bathroom. But careful planning of the layout and sizing is crucial.

Frequently Asked Questions

Can you use the same vent for two bathroom fans?

Yes, two bathroom exhaust fans can share the same vent to the outside, provided the ductwork is sized appropriately to handle the combined CFM and the external vent is large enough for the total airflow.

How do I connect two exhaust fans to one vent?

You can connect two bath fans to one vent either by joining separate ducts together with a Y-fitting into a common duct, or by using a manifolded fan unit with two fans in one housing that combines into a single duct.

What size duct do I need for two bathroom fans?

For bathrooms up to 100 square feet, 4-inch duct is often adequate for up to 100 CFM of total exhaust. For larger bathrooms or higher airflow, 5-inch or larger may be needed.

Can I use the same vent for bathroom and kitchen?

It’s not recommended. Kitchen and bathroom exhaust fans should have separate venting. Kitchen fans produce more grease and moisture than bath fans, requiring their own ductwork and venting.

What happens if I use an undersized vent for multiple fans?

An undersized vent can restrict airflow, reducing exhaust capacity and leading to moisture buildup or humid conditions. It can also put strain on the fans causing premature failure.

How big should the external vent be for 2 bath fans?

For up to 150 CFM of total exhaust, a 4-inch vent is usually sufficient. For higher combined airflow above 150 CFM, a 5-inch or larger external vent will be needed.

Fan CFM	Minimum Duct Size	Minimum Vent Size
Up to 100 CFM	4-inch	4-inch
100 – 150 CFM	4-inch	4-inch
150+ CFM	5-inch	5-inch

Should I get 4-inch or 6-inch ducts for two bath fans?

For most residential bathrooms, 4-inch duct is sufficient for up to around 100 CFM of exhaust. Only go with 6-inch duct if your bathroom is large or the combined CFM is over 150.

Can I run two 80 CFM bath fans on one 4-inch duct?

Yes, running two 80 CFM bathroom exhaust fans on a single 4-inch duct is fine, since the total CFM is 160, which is within the capacity of 4-inch ductwork.

What is the minimum CFM for a bathroom exhaust fan?

The current building code minimum is 50 CFM for bathrooms up to 50 square feet. Larger bathrooms require higher minimum airflow, usually around 1 CFM per square foot of space.

Should bath fans run continuous?

Bathroom fans don’t need to run continuously. They should be run for at least 20-30 minutes after showering to remove moisture. Timer or humidistat controls can provide automatic operation when needed.

How many CFM do I need to vent two showers?

For a bathroom with two showers, you typically need at least 100-120 CFM of total exhaust capacity to properly vent moisture. Size the fans and ductwork to handle this higher airflow.

Can I install an exhaust fan outside?

Bathroom exhaust fans are designed for indoor installation and wouldn’t hold up to outdoor elements. You can install the ductwork to vent outside, but the fan itself should be mounted inside the bathroom ceiling or wall as intended.

In Closing

Installing two bathroom exhaust fans to vent through the same duct and exterior vent is a common way to provide sufficient airflow and moisture removal in larger bathrooms. But it’s important to follow good practices for sizing fans, ducts, and vents to ensure proper ventilation. With the right approach, you can reap the benefits of multiple fans ventilating through a single, shared pathway outside.