A question that is not brought up often but is still quite relevant is, can you vent two bathroom exhaust fans together?
Many people that have two bathrooms close to each other wonder if they can be vented together. Many people will tell you that you can’t do it. There are code violations to consider, and a backdraft can push the air of one bathroom into the other.
While all this is true, there are ways in which it can be done that are safe and code compliant.
How It Can Be Done
Building codes prohibit you from circulating air from a bathroom into another bathroom. You must not ventilate air into your attic or garage. It must vent outside.
The issue is if you put two fans and connect them with a Y duct connector, it is easy for one fan to exhaust its air into the other bathroom. This usually happens when one fan is on and the other is off.
There are a few ways to solve this problem. Before attempting these solutions, it is best to check the building codes in your local area as different countries and counties have variations on this building code that will affect this type of work.
One easy solution is to use a backdraft damper. These products were created to keep cold air from blowing inside your home. But backdraft dampers can also be used for our purpose of keeping bathroom air from blowing into the other bathroom.
In addition, backdraft dampers are inexpensive as they can be found for under 20 dollars and are easy to install, requiring a few screws and a screwdriver. By connecting two backdraft dampers, one to each side of your Y connector, you’re allowing air to blow out of each bathroom, but no air can blow back in.
Inline Centrifugal Fan
Another solution to venting two bathroom exhaust fans is not to use two fans but one fan.
An inline centrifugal fan is made for ventilation and is installed inside your ductwork. These centrifugal fans are more expensive and difficult to install than backdraft dampers but can also be more effective.
An inline centrifugal fan should be installed where the two ducts of your Y connector meet into a single duct. This way, the inline centrifugal fan can pull air from both bathrooms and ventilate this air outside.
Drawbacks of Venting Two Bathroom Fans Together
Depending on where you live, building codes will vary. For example, suppose you install two bathroom exhaust fans without proper ventilation. In that case, a building inspector will fail your installation resulting in you having to rebuild the project and fix any issues noted by the inspector. In addition, subsequent failures can result in fines or other disciplinary actions.
Suppose one bathroom exhaust fan vents into another bathroom. Code compliance would not be your only issue. The smells carried in the air of one bathroom can vent into the other bathroom. This can make for some uncomfortable and embarrassing problems.
Improper ventilation cannot only put you in trouble with code compliance it can also cause other problems. If you ventilate your air into the attic, the humid air from your bathroom can result in mold and structural damage in your attic. The same issues can happen if you vent your bathroom exhaust fans into your garage.
Installing a single bathroom exhaust fan can be a moderate amount of work in its own right. By combining two bathroom exhaust fans, you can give yourself additional work and open yourself up to the possibility of more installation issues.
Have Two Separate Exhaust Vents
Though having two bathrooms close together and combining their exhaust fans seems like a more efficient choice, it does come with problems.
Creating two separate exhaust vents will cost you more money and time and has more work involved, but it is a good alternative solution.
By doing the extra work, you eliminate all the issues of having two exhaust fans together. You will have no code compliance issues, odd smells or ventilation, and installation problems.
The Old Fashioned Way
If you have no vent or do not have the money to afford an exhaust fan installation, you can keep your bathroom dry and fresh the old-fashioned way.
You can use a box fan or reciprocating fan to keep your bathroom dry. You can use a dehumidifier and use towels to clean and dry your walls and floors. There are also many air freshener options available to keep your bathroom smelling amazing.
Installing a single bathroom exhaust fan can cost you between 200 to 300 dollars in materials alone. So two exhaust fans would be double the materials cost, so it would likely be between 400 to 600 dollars. Keep in mind these prices are if you’re going the DIY route. Employing a contractor to do this job would cost you much more.
If a code inspector finds you at fault for anything, it is best to simply agree with the inspector’s findings and fix any issues that exist immediately. Fighting a code inspector or not complying can cost you more money in the form of fines, along with the cost of having to tear out and re-do your work.
(International Residential Code Mechanical) IRC M 1507.1