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Gas Dryer | Does It Need to Be Vented? (Outdoors)

Combustion appliances require venting for the combustion gases. Exhaust systems require venting for the exhaust. Gas dryers are a unique combination of both a combustion appliance and an exhaust system. This means contaminated exhaust and dangerous combustion gases both need to be dealt with.

Gas dryer design tackles this problem in a pretty simple way. Let’s look at what this is. We’ll also cover what the rules say and why.


Gas dryers release lint, water vapor, and combustion gases. This comprises the exhaust. Outdoor venting of this exhaust is required for code compliance. Proper venting prevents fires, mold and mildew growth, and the buildup of harmful gases in the home.

Shared Combustion and Exhaust Venting

Instead of creating two separate venting systems, the exhaust air and combustion air are vented together through the exhaust vent.

So, when looking at how to vent a gas dryer, we have to look at the general dryer venting rules, and, arguably, venting correctly is more critical to health and safety.

Outdoor Venting Required for Code Compliance

Section G2439.1 International Residential Code (IRC) says that ventilation for gas dryers must be in accordance with the manufacturer’s instructions, so check your appliance’s manual for any specific directions.

The IRC is comfortable in deferring the final say on this subject to individual companies for a couple of reasons:

  • Appliance testing: During quality and safety tests for a dryer, companies measure combustion gas emissions and assess the proper ventilation requirements for their product.
    • Appliances must be tested against a standard before they are allowed on the market, so you can trust the manufacturer’s installation instructions to give you correct information on how to safely ventilate your dryer.
Three technicians checking and testing the dryers and washing machine.jpg
  • Varied dryer design: Different dryers will release different levels of heat and combustion gases. The IRC can only include more general regulations for gas dryers, while a dryer’s manual can give detailed information about your specific appliance.
    • As dryer design evolves, manufacturer instructions may be more up-to-date on what safety requirements are needed for their product than the IRC is.

In the absence of sufficient manufacturer instructions, Section G2439.1 and Section G2439.3 of the IRC both state that dryers must be vented outdoors. This means that you can’t vent indoors just because your manufacturer did not explicitly say that you cannot.

Section G2439.3 also says that the outdoor termination vent must be equipped with a backdraft damper.

A backdraft damper allows for air to flow in only one direction. When installed at the end of a dryer vent, hot air from the dryer can be expelled without allowing air from outside in.

neverest - RKI 4" - Professional Backdraft Damper Inline Spring Loaded for Dryer Duct

This furthers the argument that it is important to ensure that dryer exhaust is not vented anywhere but outside.

Codes Are Legally Binding

Chapter 1 of the International Building Code states that it is intended to be used as a legally enforceable document.

In other words, violating building codes, such as the IBC and IRC, also violates the law.

Your local building department handles the enforcement of these laws.

While you are not going to be put on criminal trial for an improperly vented dryer, local code enforcement officers have the right to assign fees to residents with non-compliant dryer ventilation setups.

They may also require that corrections be made to the system to resolve safety issues. These corrections will obviously be costly, and you won’t get a choice on the timeline of when it must/can be done.

Before even starting to run your dryer ventilation system, consider if you need to hire a plumber and check with your local building codes to see if you need a permit to install, alter, or extend dryer ventilation in your home. 

Many countries and states (Wisconsin, for example) require you to have a permit because properly installed ventilation is so important to your health and safety.

Venting Gas Dryers Indoors Is Dangerous

Venting gas dryers outdoors is required for a reason. It is necessary for the health and safety of inhabitants. Indoor ventilation can lead to exposure to harmful gases, mold growth, and fire.

Combustion Gases Released in Exhaust

As mentioned, unlike other appliances that have separate ventilation systems for combustion gases and air exhaust, gas dryers combine the two into one vent.

This vent must carry the air outside in order to prevent the redistribution of harmful combustion by-products in your home.

Dryer with duct connected to a vent leading outside.jpg

But what exactly are these combustion by-products?

Complete combustion, which is supposed to occur in gas dryers because they are required to have combustion air supplies, produces carbon dioxide (CO2).

Although we breathe in a percentage of CO2 from the air around us, high levels of this gas from an improperly vented dryer can cause health complications. High levels of CO2 in the air limit the amount of available oxygen for a person to breathe.

Lack of oxygen causes issues such as:

  • Rapid breathing and heart rate
  • Fatigue
  • Clumsiness
  • Nausea
  • Unconsciousness 
  • Permanent damage to the brain and heart

Incomplete combustion should not occur in gas dryers, but it is a possibility, especially in old appliances that may have faulty combustion air supply systems. It can also be more of a problem if there is a lack of makeup air.

Incomplete combustion causes the release of carbon monoxide (CO). Although this gas is colorless and odorless, its effects can be life-threatening. 

Exposure to CO often causes “flu-like” symptoms such as:

  • Dizziness
  • Chest pain
  • Fatigue
  • Nausea
  • Unconsciousness 
  • Death 

Even if your gas dryer is properly ventilated, it is recommended that you install a CO alarm in your home.

Vents can become obstructed, and it is important to know about dangerous levels of CO accumulating in your home before you start feeling the effects of it.

Gas concentration is measured in ppm, or “parts per million.” Normal levels of CO2 in the air measure anywhere from 300-900 ppm. At 40,000 ppm (4% of air concentration), the negative effects of CO2 are immediately apparent. 

Since dryers emit an average of 1.8 kg of CO2, improperly vented dryers in smaller areas will increase air concentration of CO2 more rapidly than those in larger rooms.

CO is harmful at much smaller concentrations. According to the CPSC, symptoms become noticeable at 70 ppm. At just 150-200 ppm, people may experience unconsciousness and death!

Especially in a small space such as a laundry room, proper ventilation of combustion gases is necessary. Just a few drying cycles could lead to health issues if the dryer is not vented properly.

Lint, Moisture, and Heat Also Pose a Risk

Combustion gases are not your only concern when thinking about ventilation for gas dryers. The lint, moisture, and heat expelled by a gas dryer can also pose a risk to inhabitants.

Vent full of lint and dust

According to the National Fire Protection Association (NFPA), U.S. fire departments responded to about 15,970 fires that were started by washers or dryers each year. 92% of these fires were caused by dryers.

Lint buildup can clog dryer vents and increase the chances of harmful gas exposure. When combined with heat, lint becomes a fire hazard. 

Lint fibers from clothes are released during the drying and tumbling process. Depending on what material your clothes are made of, these fibers can be highly flammable.

Natural fibers (cotton, linen, and silk) burn readily. Synthetic fabrics (polyester, nylon, and spandex) melt and can cause tough-to-remove buildup in vents.

Dryer vents, even ones that lead to the outdoors, should be cleaned regularly to remove lint buildup.

The air expelled by dryers has high levels of moisture in it since it removes water from wet clothes. When vented outdoors, this moisture is not a problem. But if the dryer is vented indoors, excess moisture remains within the building.

Excess moisture, especially when combined with warm air, provides the ideal environment for bacteria and mold growth.

Sources

https://codes.iccsafe.org/content/IBC2018/effective-use-of-the-international-building-code

https://codes.iccsafe.org/content/IRC2021P2/chapter-24-fuel-gas#IRC2021P2_Pt06_Ch24_SecG2439.1

https://localhousingsolutions.org/housing-policy-library/code-enforcement/

https://www.cdc.gov/co/faqs.htm#:~:text=The%20most%20common%20symptoms%20of,pass%20out%20or%20kill%20you.

https://www.cpsc.gov/Safety-Education/Safety-Education-Centers/Carbon-Monoxide-Information-Center/Carbon-Monoxide-Questions-and-Answers#:~:text=As%20CO%20levels%20increase%20and,unconsciousness%2C%20and%20death%20are%20possible.

https://www.phoenix.gov/fire/safety-information/home/fabrics

https://www.nfpa.org/News-and-Research/Data-research-and-tools/US-Fire-Problem/Home-fires-involving-clothes-dryers-and-washing-machines

https://www.ci.superior.wi.us/613/Permits—Needed-or-Not

https://www.ccohs.ca/oshanswers/chemicals/chem_profiles/carbon_dioxide.html https://www.fsis.usda.gov/sites/default/files/media_file/2020-08/Carbon-Dioxide.pdf

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