Can You Connect Two Bathroom Fans to One Vent?

Do you have two bathrooms next to each other? Have you decided you want to save time and energy? If so, you may ask, “Can you connect two bathroom fans to one vent?” Let me tell you if it’s a good idea, and how to do it if you decide you must!

It’s Not Recommended

Code inspectors will rarely approve two bathroom fans connected to one vent. You can connect two fans, but you’ll send the moisture from one bathroom to the other bathroom. This means you won’t be venting to the exterior, or both will vent at the same time. 

Dampness diverted to any enclosed space in the house will produce mold. It will also hasten the aging of any wooden surfaces. That’s why it won’t pass the National Electrical Code (NEC) or Heating, Ventilation, and Air Conditioning (HVAC) code inspections.

Older Homes

If you have an older model house, I always say you should get an inspection. You may need to update the existing venting. Today’s houses are well insulated. They have air-tight construction that keeps moisture on the inside if not vented.

Older home construction would often use the window for ventilation purposes.

Window ventilation was inefficient, especially in colder temperatures. Using the window as a vent produced condensation on windows, mirrors, and walls. 

Cold air mixing with warm air was not an effective way to remove odors or moisture. Moisture not vented to the outside generates mold spores. These spores create allergens and respiratory ailments.

Updating or correcting ventilation has health as well as aesthetic purposes. Here are some things to focus on when wondering, “Can you connect two bathroom fans to one vent?”

The Right Fan

Choosing the right fan for your bathroom may seem challenging. The amount of air vented determines fan size. The size rule is one cubic foot per minute for square feet of the bathroom. Multiplying the square foot by 1.1 will give you the size fan you need.

A larger fan is also practical in a high-ceilinged or heavily used bathroom. So, purchasing a larger fan is more cost-effective and efficient.

Fans have ratings which refer to the noise level they produce. For example, a rating of one is best as opposed to a three or four. Plus, some fans include humidistats, lights, and heating. 

There are many types of fans available at your local hardware store. When deciding which will work for you, rely on the advice of an HVAC and electrician.

In older houses, bathrooms usually aren’t next to an exterior wall or have a direct route to the exterior for ventilation. An HVAC technician can advise you on whether you can get through the wall, the ceiling, or the floor. Having the advice of an HVAC expert before you purchase the fan and ductwork can prevent future problems.

HVAC contractors must meet requirements specific to each state. Most states require a level of experience as a journeyman before obtaining a license. Contractors are aware of code requirements and the installation process. I suggest hiring one before beginning the project.

Electricians are licensed and must meet specific state requirements.

Attic Installation

You can vent two bathrooms with one fan through the attic. You’ll need one in-line centrifugal fan that vents through the roof to meet code requirements. A grill and ducts in each bathroom attach to a “Y” connector, drawing air into the ducts simultaneously while venting it to the outside.  

When the fan is on, it will remove the moisture and vent it through the roof, eliminating any dampness while being quieter because of its location.

Combining both bathrooms into a single “Y” also prevents the need for a second vent through the roof. A switch in each bathroom connects to a single in-line fan.

Venting into two bathrooms has multiple considerations. First, if one bathroom is rarely used, conditioned air is wasted. Second, if the bathrooms are far apart, the cost saving with one vent is spent on ducting materials. Third is the limited, expensive fan options.

Exterior Wall Fans

There are two basic types of exterior wall fans. First, a low-profile exhaust fan has a slimmer profile, so it cannot mount inside an interior wall cavity. Second, through-the-wall exhaust fans are the other option and often come with the exterior hood and piping.

Through-the-wall fans are not designed for the bathroom. They will still vent moisture and odors. These fans install directly into the cavity between the bathroom wall and outside. This eliminates the need for ductwork to the roof if it’s next to an exterior wall.

The Project

A professional will tell you whether you can vent two bathrooms with one fan.

Five existing considerations are:

  1. The proximity of bathrooms.
  2. Waste of conditioned air from an unused toilet.
  3. When the fan is on, both bathrooms ventilate.
  4. Distance from the bathroom to the roof.
  5. Local code restrictions.

Here are a few more considerations to take.

Electricity and Wiring

The NEC specifies that a bathroom with both lighting and a fan must have at least a 20 amp circuit. Most older bathrooms have a single circuit of 15 amperes

If the fan also has a heat lamp, it must have a separate circuit just for the fan and heat lamp. So, you will need to update to two 20 amperage circuits if you add a heater or a heat lamp.

If you add a new switch, it has to have a green grounding screw.

Laying out the system and locating the power supply to the fans is integral to safe installation. Finding the circuit breaker to an existing fan and turning it off is the first step in the process. 

Compare the amperage of the breaker and fan. This will tell you whether you need the services of an electrician. Most breakers will have enough amperage to run a fan. You can connect them to the existing light switch.

If the amperage is inadequate, check the local building codes. You can determine if you need a licensed contractor to upgrade the wiring. Remember that local regulations supersede national codes.

Due Diligence

An HVAC contractor can tell you if you need an electrician for wiring. If your home has older construction, I suggest you get the advice of an electrician. Poor wiring is dangerous, so don’t take chances. 

Cutting holes into walls, ceilings, and floors requires specialized tools. Most of us don’t have this equipment on hand. It’s hard to maintain the integrity of the construction and add to the equity in your home. Let professionals handle it.

When Hiring Contractors

First, check their references and don’t go by reviews on their website. Perform cost comparison and only hire licensed and insured professionals. Ensure you check with your local homebuilder’s association on their craftspersonship. 

A contractor will make recommendations for specific equipment. Give them a list of features you wish to have, with alternatives included. Keep in mind that added accessories will add to the installation cost. 

Professionals often showcase complex do-it-yourself projects on the internet. These projects seem easy until you cut the hole in the ceiling and those uncapped wires fall out. 

Or, the wall has mildewed to the point where it crumbles instead of cutting. You are in the attic and find that the air conditioning ducts run on top of the bathroom. An experienced, qualified professional has seen and dealt with these problems. They can save you money, time, and effort in the long run.

So, Can You Connect Two Bathroom Fans to One Vent?

In restricted circumstances, yes. Connecting two bathrooms to one fan is possible, but only in limited circumstances. Individual vents and fans are the most cost-effective and efficient. The services of an HVAC expert and an electrician are essential. They’ll keep you from wasting time, effort, and money.

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