So, you have two or more bathrooms and only one duct. There are many ways to vent multiple bathrooms through one common vent. We will discuss all the possible solutions below.
Bathroom fans can share a common vent. Both fans must be of similar capacity and have back-draft dampers installed. The duct must be over-sized by one inch for every additional fan added to the system.
Let’s dig a little deeper on how to install two bathroom fans in a common vent.
How to Vent Multiple Bathroom Fans Through a Common Vent
There are a couple of things you need to keep in mind when doing this, otherwise it will end up not being a very efficient system or will blow moist air into the other bathroom.
Related Article: Are Bathroom Fans Necessary
Install a Damper For Each Bathroom Fan
A back-draft damper is a simple flap that is installed inline in the system. Normally it prevents cold air from the outside from blowing back to the bathroom. But in this case, it will also prevent one bathroom fan from blowing air into the other bathroom.
Some bathroom fans come with a damper installed, but if yours lacks a damper make sure to install one. I recommend spring-loaded dampers by AC infinity(Amazon link). AC infinity dampers are very airtight and can be installed both horizontally and vertically.
Spring-loaded dampers are quiet when it is windy outside. A gravity-operated damper can start flapping when it is very windy outside.
The back-draft damper is probably the most important part of the whole system. If you skip this then bathroom fan no. 1 will blow some of the moist air into bathroom no. 2
Here is an helpful guide if you would like to learn more about ventilation dampers.
Choose Similar CFM Bathroom Fans
Both bathroom fans should be more or less same capacity.
For example, if the first bathroom has a 100 CFM fan installed then the other fan should be 90-110 CFM.
CFM = Cubic feet per minute (This means how much air the fan will move) Learn more about how bathroom fans are rated.
If one of the fans is much higher capacity then there is a chance of air moving from one bathroom to another.
Although this is a problem only when both fans are running it is still worth considering.
Common Vent Duct Must Be Over-sized
The common duct should be at least 1 size larger than the individual vents.
If your bathroom fans both call for a 4-inch duct then the common vent line should be at a minimum of 5 inches in diameter.
The Other Option – Install a Remote Inline Fan
This is a great option if you have easy attic access. Most cases it will be quieter as-well since the motor is further away from the bathroom.
This means that you don’t have individual fans in each of the bathrooms, instead, there will be one larger inline fan in the attic. See the sketch below/
The only downside to using this configuration is that when you turn on the fan, both bathrooms will be vented.
Although this is not such a big deal it is worth considering.
There are kits available with inline fan and vent covers. However, I am sure that when buying individual parts you can put together a better system for less money.
Often the back-draft dampers included in the sets are of low quality.
Here are the individual parts that I recommend.
Adjust the CFM and duct size to your bathroom size. The items below are sized for two average-sized bathrooms.