Can Bathroom Fan and Dryer Share a Vent

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When installing a bathroom fan and a dryer, both require adequate vents to expel exhausted air. Duct installations can be a challenge, especially if you have limited room through which to duct. Initially, it seems like a logical solution for the bathroom fan and dryer to share a vent. However, if you look a little further into the matter, you will find that it is not as cut and dry as this.

A bathroom fan and a dryer should never share the same vent. International building code states that dryer vent exhaust systems must be separate from all other systems and must expel moisture out of the house through a dedicated vent. This is because of the potential hazards associated with improper dryer venting.

While bathroom fans and dryers are both useful appliances for a home, incorrect installation of the ventilation systems introduces certain hazards, such as fires. Let’s more closely examine why bathroom fans and dryers cannot share a vent and what some of the consequences would be if they did.

Why a Bathroom Fan and Dryer Can’t Share a Vent

The exhaust systems of bathroom fans and built-in dryers are crucial, and they must be installed correctly to make them as safe as possible and to adhere to international building codes and guidelines (which are in place to ensure the primary goal of safety).

There are many reasons why these appliances cannot share a vent, and it is important to understand them before attempting to install them.

Here are the most important reasons why bathroom fans and dryers can’t share a vent:

It is Not Legal to Combine Bathroom Fan and Dryer Ducts

According to the International Residential Code (IRC), it is illegal to combine dryer ducts with any other exhaust systems within a house. 

Section M1502.2 of the IRC: Independent Exhaust Systems states

“Dryer exhaust systems shall be independent of all systems and shall convey the moisture to the outdoors. Exception: this section shall not apply to listed and labeled condensing (ductless) dryers.”

This section of the IRC clearly prohibits drying ducts from being combined with any other ventilation ducts or exhaust systems. This means that it is illegal for bathroom fans and dryer ducts to have a combined ventilation system.

Dryer Ductwork Must Be Cleanable

Ducting for dryers and bathroom fans have different requirements. For example, flexi duct can be used for bathroom fans, but is prohibited in dryer ventilation systems. One of the reasons for this is that dryers have stricter cleaning requirements.

For a dryer duct to efficiently vent exhausted air outside of the house, it must be thoroughly cleaned regularly. Keeping dryer ducts clean is essential to prevent damage to the dryer and to keep the system running safely and efficiently.

Cleaning out these ducts is essential because dryer ducts tend to gather lint which can develop mold, which is a health concern, and if this lint dries out, it becomes a fire hazard.

If the drying duct is combined with a fan duct, more dirt and debris are likely to accumulate within the dryer duct, making cleaning even more vital. Simultaneously, combining the two exhausts makes cleaning the ducts far more difficult.

Combining a bathroom fan vent and a dryer duct may cause both systems to function less efficiently and makes maintaining both systems a challenge

Keeping the duct and exhaust system of a dryer as a stand-alone system allows it to be easily cleaned and properly maintained.

Moist Air Can Be Released Back Into the Bathroom

The primary function of a built-in dryer is to remove moisture from clothing and expel it outside the house. This moisture is expelled by way of the ventilation system of the dryer.

If the dryer and bathroom fan vent share the same exhaust system, moist air is likely to be released back into the bathroom if the dryer is on while the fan is off.

When moisture that is extracted by the dryer is pumped through a shared ventilation system with a bathroom fan, the moisture from the air that is being expelled may be pushed toward the fan rather than out through a vent.

The dampers that are used in bathroom fans are not completely airtight, and some are less effective than others. Thus, any moisture that collects near the fan as a result of air pushed from the dryer exhaust can leak through the fan. This may damage the fan, and water may leak directly into the bathroom.

If water or moisture from the dryer exhaust is let into the bathroom through a fan that shares the same ventilation system, it can cause problems that may be hazardous.

These problems include:

  • Mold Growth – this is a common moisture-related issue, and it can threaten the health of the occupants of the house and can damage paint and finishes in the bathroom.
  • Mildew – the growth of mildew in a bathroom is also a direct result of too much moisture, such as may occur when the bathroom fan and dryer share a vent.
  • Slip Hazards – if the bathroom fan leaks water into the bathroom or lets in too much moisture, surfaces within the bathroom, such as a tiled floor, will become unexpectedly slippery, which can be very dangerous.
  • Damp – excess moisture can accumulate as damp, which may cause significant damage to walls and ceilings, especially in the bathroom. Damp also has the potential to cause structural issues within a building, not just limited to bathroom.

Lint Can Be Pushed Into the Bathroom

One of the biggest problems with a bathroom fan and a dryer sharing a vent is the accumulation of lint.

Lint from the dryer is expelled through the ventilation system along with the air and moisture, and if the bathroom fan shares that system, some of the lint will be pushed into the bathroom and onto the fan itself.

Over time, the bathroom fan dampers will be covered with lint, and this may severely hinder the performance of the fan and may eventually cause the fan to stop functioning altogether.

What is Lint

Lint is defined as short strands and fragments that have separated from the material, cloth, or clothing while cleaning, processing, or using these items.

These small fibers are very light and can travel easily through ventilation systems. Dryers produce copious amounts of lint during the drying process.

Lint Is a Fire Hazard

The lint that is produced from the process of drying clothes and other items is very thin, porous, light, and dries very quickly.

These attributes of lint make it potentially flammable. This is a serious problem if it is not disposed of correctly or if it accumulates in ventilation systems.

If a ventilation system from a dryer is shared with a bathroom fan, lint is likely to accumulate in the system, and this creates a severe fire hazard. 

Ventilation and exhaust systems produce heat, which is likely to set accumulated lint on fire if it collects in the wrong areas of the system.

Bathroom Fan Pushes Moist Air to the Dryer When the Dryer Is Not Running

A problem that may not be considered when combining the ventilation systems of a bathroom fan and a dryer is moist air being pushed into the dryer by the fan when the dryer is not in use.

The combined ventilation system affects both appliances, and moist air from the bathroom is pushed into the dryer by the bathroom fan it can cause issues for the dryer.

Moist air from the bathroom will prolong the drying process within the dryer simply because there is more moisture to extract from the appliance.

Additionally, this extra moisture in the dryer from the bathroom fan may cause the build-up of undesirable odors within the dryer that can be transferred to the clean clothing that is placed into it. These odors are caused by damp and mold that accumulated from the moist air pushed into the dryer by the bathroom fan.

Won’t Pass a Home Inspection When It Comes Time to Sell

For every homeowner, it is important to consider the condition of the house in case the time comes to sell the home. 

A simple, overlooked mistake, such as combining the vents of a bathroom fan and a dryer, is enough to cause a home to fail the health and safety inspection required in order for the home to be sold.

This mistake could cost the sale of the house, and rectifying it will cost valuable time in the selling process.

Insurance Could Deny Your Claim in Case of a Fire

Another important consideration for every homeowner is insurance. Insurance claims can be thrown out due to minor details, and a shared ventilation system between a dryer and a bathroom fan is one such detail.

In the case of a fire, the shared ventilation system between the two appliances could cause an insurance claim to be denied due to the build-up of lint and other flammable materials within the system and because this setup is an IRC violation.

Dryer Venting Checklist

The ventilation system of a built-in dryer has specific requirements to be up to code, safe, and to perform at optimum levels. 

This is a checklist of the important aspects of dryer venting to follow when installing a domestic internal dryer:

  • Dryer exhaust should be vented according to manufacturers’ instructions.
  • Dryer exhaust systems must be separate from all other ventilation systems.
  • Dryer exhaust must not be placed within or near insulation.
  • A verticle riser must be included for cleaning out the system.
  • All dryer ventilation ducts must lead outdoors.
  • Makeup air must be provided if more than 0.09 m3/s of air is extracted by the dryer in one room.
  • All dryer ducts must be made from metal with a smooth surface on the interior.
  • Dryer ducts may be no longer than 25 ft.

All domestic dryer systems must be built to these specifications to comply with IRC Section M1502 and to be safely used within a home.

Related article: Can a Gas Dryer Be Installed in a Bathroom

Can Two Dryers Share One Vent?

We have established that a dryer cannot share a vent with a bathroom fan, but can two dryers share one vent?

Two dryers can share one vent, but it is better if they have separate ventilation systems. If one dryer is operated without the other, some of the air is pushed into the room through the unused dryer. If two dryers share one vent, be sure that the duct is large enough to accommodate both dryers.

Can Two Bathroom fans Share One Vent?

If you are trying to work out ducting space for your various ventilation systems, then vent bathroom fans together, instead of a fan and dryer.

Two bathroom fans can share one vent, but there are some specifications required to make it possible.

For two bathroom fans to share one common vent, the fans must be of a similar capacity, or one fan will overpower the other and cause a backdraft within the system.

Both fans must have backdraft dampers installed to optimize airflow and keep all of the air moving in the right direction.

The size of the exhaust vent that the fans use must be increased by one inch for every fan that shares the same vent.

This article is a great resource for more information about bathroom fans sharing an exhaust vent.

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