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Gas Dryer | How Does It Vent Carbon Monoxide?

According to the Center for Disease Control and Prevention, ERs in the US annually treat approximately 50,000 cases of accidental carbon monoxide poisoning. Although this figure isn’t referring specifically to carbon monoxide released from gas dryers, it does show how dangerous exposure to this gas can be. 

The toxic nature of carbon monoxide has deterred a lot of people from purchasing gas dryers, but they wouldn’t be sold as widely as they are if a significant risk existed. However, you do have to comply with the installation and venting rules; otherwise, the dryers can become a greater risk.

Carbon monoxide forms in the gas burner assembly and is pushed into the drum along with the heated air. It also follows the same exit path as the hot, moist air, i.e., it leaves the dryer through the vent at the back, enters the duct system, and is released outside.

Carbon Monoxide Is Released Into the Exhaust Vent

Besides their method of generating the heat used in drying clothes, another key difference between electric and gas dryers is that while electric dryers come in vented and ventless models, gas dryers only come in vented models.

One of the reasons for this is that the vent in gas dryers is also required to expel combustion gases, including carbon monoxide (CO), in addition to moist air and lint that is typically exhausted by dryers. 

This single vent serves for exhausting both the used dryer air and the combustion gases.

How the Exhaust Is Vented

In gas dryers, the air that is heated and pushed through the drum to dry your clothes also contains the combustion exhaust gases since these are formed in the burner assembly over which the air passes to be heated.

Typically, the combustion gas is carbon dioxide (CO2), but in some cases, some CO will form, particularly if there is an issue with the supply of combustion air.

Incomplete combustion produces carbon monoxide, how do carbon monoxide forms

All of the hot, moist exhaust, which can also include lint and CO, is drawn out of the drum by the internal fan and pushed into the vent located at the back of the dryer.

More air is then pulled into the dryer and passed through the burner assembly, where heat and more combustion gases are picked up, before being sent into the drum, then out through the vent.

This repeats until the cycle is finished.

Proper installation requires this exit vent to be connected to a ducting system, which leads to the outside.

This Makes Venting Indoors Dangerous

The risk associated with venting a dryer indoors is higher in gas dryers.

Besides the mold and condensation problems—the primary concerns with discharging an electric dryer indoors—with gas units, you also expose yourself to CO2 and potentially CO. 

The tricky thing about CO is that it is colorless and odorless, so you might have high levels of the gas in your home without realizing it if you do not own a carbon monoxide detector (amazon link)

Kidde Carbon Monoxide Detector, AC Plug-In with Battery Backup, CO Alarm with Replacement Alert

As you very well know, the body requires oxygen to function. The presence of CO in the body results in oxygen shortage. 

This is because CO displaces the oxygen in the blood. It does this by bonding with hemoglobin (the red pigment in your blood cells) to form carboxyhemoglobin (COHb).

When this happens, there are no places for oxygen to bind, preventing it from reaching the brain, heart, and other body organs/tissues, resulting in a range of health challenges. 

Symptoms of CO exposure include: 

  • Fatigue
  • Headache
  • Irritability
  • Dizziness
  • Tightness in the chest
  • Nausea

Prolonged exposure or exposure to a high amount of the gas can lead to more serious issues like: 

  • Vomiting
  • Breathing difficulties
  • Confusion
  • Muscle weakness
  • Loss of consciousness
  • Heart problems
  • Brain damage
  • Loss of pregnancy

Symptoms of CO exposure vary from one person to the next and because the symptoms mimic other illnesses, it is often misdiagnosed. 

The risk of health challenges is worse in children, the elderly, people with already elevated COHb levels (e.g., smokers), and individuals with heart or lung disease. 

So, rather than risk the consequences of CO exposure, it’s essential that you vent your gas dryer outdoors.

If it’s impossible to access an exterior wall or the roof, I suggest that you get an electric ventless dryer or seek out alternative ways to dry your clothes. 

Terminal Vent Location Is Important

The point of venting dryers outdoors is to ensure that moist air and combustion gases are not deposited in your home or released into your “breathing zone”.  

To fully eliminate the possibility of CO re-entering your home, you need to ensure that it cannot just blow back into your house once it leaves the terminal vent. This is ensured through correct placement of the vent.

Section M1502.3 of the International Residential Code (IRC) provides guidelines on the ideal termination location for air exhausts.

According to the code, the dryer exhaust vent should be placed not less than 3’ from operable windows and doors.

the dryer exhaust vent should be placed not less than 3’ from operable windows and doors   according to section M1502.3 of the International Residential Code (IRC)

Although it is against the IRC principles, it is a common practice for people without access to an exterior wall to vent dryers through a window.

Obviously, this practice violates the 3′ rule, so you should not listen to anyone who tells you that this is an appropriate practice.

Another issue covered in Section M1502.3 is the distance between the vent and air intakes, like the combustion air or makeup air supply vents because these actively pull air in from outside and you don’t want it to pull in air that could contain CO.

The recommended distance listed by the IRC is a minimum of 10’ between the dryer vent opening and the air intake opening.  

Carbon Monoxide Normally Released in Small Amounts

Before a gas dryer can produce heat, combustion must take place. Combustion is basically a chemical reaction between a fuel (in this case natural gas or liquid propane) and an oxidant (oxygen). 

As mentioned briefly earlier, carbon monoxide isn’t the primary byproduct of combustion. The two major byproducts are water (H2O) and carbon dioxide (CO2). These elements are produced whenever there is complete combustion. 

For complete combustion to take place, there has to be an adequate supply of oxygen to the dryer. 

Whenever there is a shortage of oxygen, there would be partial oxidation of gas, resulting in incomplete combustion. So, rather than water and CO2, CO and sometimes just carbon (in the form of soot) is released. 

Here are some factors that contribute to the shortage of oxygen in gas dryers: 

  • Improper installation of dryer components. For instance, if the burner isn’t correctly installed it would influence the oxidation process.
  • Faulty dryer components can slow down oxidation. This mostly occurs whenever the heat exchanger is defective.
  • The combustion air supplied to the dryer is insufficient.
  • Excess gas flow into the dryer would cause the gas level to be higher than the available oxygen (it can also make your dryer emit a blowtorch sound).
  • A clogged vent would limit the passage of air from the dryer and can cause CO to flow back into the dryer and your laundry room and this would interfere with the combustion process. 
Clogged vent limiting the passage of air from dryer

How much CO the dryer releases would depend on the severity of the issue. So, if your dryer is in good condition, it will not produce a significant amount of the gas. 

Routine maintenance checks and regular vent cleaning are ways of ensuring that dryer components function properly and reduce the level of CO emission. 


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