If I’m honest, I used to find gas appliances, like clothes dryers, intimidating. All the dangers of combustion gases and the pressure to provide the proper setup to prevent these from leaking into the home…well, it was easier to just get professionals in and remain ignorant of things like makeup air.
However, when I decided to start researching these appliances in earnest, I found that they function logically, and even makeup air now makes sense to me! Allow me to pass on my knowledge.
Gas clothes dryers require makeup air when the exhaust rate meets or exceeds 200 CFM, when the exhaust rate interferes with the operation of the gas dryer or nearby gas-fuelled appliances, and/or when the gas dryer is installed in a closet.
What Is Makeup Air?
Makeup air is the air entering a house to “make up” for the air that has been lost through the action of exhaust systems.
After a certain point (i.e., when a certain amount of air is lost over x amount of time), makeup air becomes vital to prevent a variety of negative effects resulting from the air pressure changes.
Clothes dryers (not just gas dryers) take air from the room in which they are installed, heat it, and use it to dry the clothes in the drum. This hot, moisture-laden, and lint-filled air is then sent out of the house through the ductwork.
Another round of air is pulled into the dryer, and the cycle continues until the dryer program is complete.
This means that more and more air is pulled from a room and exhausted outside while the dryer operates. In some situations, this loss can be significant enough to necessitate the provision of makeup air.
When Is Makeup Air Required With Dryers
In the International Residential Code (IRC), Section G2439.5 (614.7) (relating to gas-fuelled dryers) states that you need to supply makeup air when:
- Your dryer exhausts 200 CFM (cubic feet per minute) or more of air.
- Your dryer is installed in a closet (makeup air must be provided through openings into another room or directly from the outside).
In addition, Section G2407.4 says that makeup air is required where exhaust systems, including clothes dryers, interfere with the operation of gas-fuelled appliances.
Why Gas Dryers Need Makeup Air
So, makeup air is mostly required where two systems interact: an exhaust system and a gas-fuelled system.
With gas dryers, these two systems exist in the same appliance. Combustion of natural gas (or liquid propane) warms the air that is used for drying clothes.
The Combustion Process Does Not Require Makeup Air
The combustion gases from a gas dryer are exhausted out of the same vent as the moist, lint-carrying air. However, the fact that the appliance is a combustion appliance and these combustion gases exist is not what makes makeup air a factor.
If you have a combustion appliance and there is no exhaust system attached/nearby, no makeup air would be required.
A supply of combustion air serves to replace the air lost to combustion and ensures that the exothermic (heat-producing) reaction produces the safest form of by-products—carbon dioxide as opposed to carbon monoxide.
Still, it is important that these combustion gases are directed out of the house, and makeup air can be essential to securing this goal.
The Exhaust Is What Can Necessitate Makeup Air
The air used to dry the clothes comes from the room, and when it is exhausted, it creates a low-pressure system.
Some dryers don’t exhaust enough air at a time for this to be a problem. The pressure is balanced via the infiltration of air through the gaps and cracks in a house as well as open windows and doors.
Other dryers exhaust enough air that this infiltration becomes problematic (drafts and strain on the HVAC system as it has to repeatedly condition outside air).
Worse still, infiltration might not be sufficient to remedy the pressure imbalance, and backdrafting occurs.
This is the primary reason why exhaust systems need makeup air when combustion appliances are operating nearby or, in the case of gas dryers, when the two systems are combined.
Often, the venting of combustion appliances relies on air pressure to pull the gases into the vent and out of the house.
When there is a low-pressure system in the house, it acts as a vacuum, pulling air into the room from anywhere with a higher pressure, including the combustion vent.
Even if the gas dryer itself is not affected (e.g., the dryer doesn’t dry clothes well), any other gas appliance in the vicinity can be affected, in which case, makeup air is required to make sure that the breathing air does not contain combustion by-products, which are bad for health.
With gas dryers, venting is an active process. A blower is used to push the exhaust (and combustion air) through the ductwork and out of the house.
However, if there is a vacuum in the room, the gas dryer will not be able to pull sufficient amounts of air into the dryer to heat and dry clothes.
Not only can air be pulled back into the dryer by this vacuum, but the combustion gases may not reach the vent in the first place, staying in the machine and being released into the room.
While the combustion air supply is supposed to be separate, if it is not directly fed to the dryer, then this can also be affected by the vacuum.
Instead of entering the machine, the combustion air is redirected to balance the low-pressure system, and incomplete combustion can take place. Incomplete combustion is dangerous because it is this process that produces carbon monoxide as opposed to carbon dioxide.
So, as you can see, gas dryers require makeup air to ensure that the process of exhausting dryer air does not pull combustion gases back into the house and does not interfere with the combustion process itself.
There are issues besides backdrafting that can be caused by a lack of makeup air. You can read more about these in Makeup Air | Here’s What Happens If You Don’t Have It.
Why Only 200 CFM Dryers?
While the above explanation may make it seem like makeup air is always required, you should remember that the problems associated with low-pressure system formation typically only occur at a certain point.
The IRC puts this point at 200 CFM.
CFM is a unit used to express the rate (quantity per time) at which air moves. It’s used for bathroom fans, range hoods, ceiling fans, vented dryers, and more.
A gas dryer rated at 200 CFM will exhaust 200 cubic feet of air every minute that it is operating. The IRC deems this as the rate at which most households will start to experience problems.
Likely, this number was chosen based on statistical and experimental data.
An example of the statistical data that may have influenced the designation of 200 CFM as the point at which makeup air is required is the infiltration rate of the average house.
Knowing how weathertight a house is means that you can estimate how much exhausted air will be replaced by the natural inflow of air through the gaps and cracks in the house.
The point at which this infiltration becomes problematic, causing unpleasant drafts or overwhelming the HVAC system, will also have been taken into consideration.
An example of experimental data that may have influenced the IRC’s decision would be at what CFM rating the average house started developing a low-pressure system.
While widely applicable, it stands to reason that 200 CFM may not be universally applicable, and some houses may experience problems at lower ratings while others wouldn’t see the negative effects until more than 200 CFM was exhausted from the house.
The second scenario doesn’t matter. As soon as your gas dryer exhausts more than 200 CFM, you need makeup air to be code-compliant, even if you don’t need it to correct the air pressure.
The first scenario is also accounted for in the code. Remember where it says that clothes dryers cannot interfere with the operation of gas appliances?
Well, this regulation means that if your gas dryer is rated below 200 CFM but your house shows the need for makeup air, you have to supply this air to remain compliant and safe.
Why Dryers in Closets Need Makeup Air
If you put a gas dryer into a closet, you change the rules a bit. The reason the rules change is because the air available to be exhausted is all of a sudden much more limited. This would be especially true if the closet was closed off completely from the larger room in which it is located.
The gas dryer pulling in air from around it (air from the closet) to be used to dry your clothes should not interfere with the combustion air supply as this is separate.
However, it will quickly upset the air pressure balance in the closet because the total air volume is much smaller and removing x amount means that you are removing a greater percentage of the total than if the dryer were in an open room.
To account for this, the IRC (Section G2439.5.1 (614.7.1)) says that you have to supply makeup air. This is irrespective of your dryer’s exhaust rating and whether or not there are any signs of the need for makeup air.
You are, however, given a choice when it comes to how this makeup air is supplied.
If you choose to provide it by creating an opening between the closet and the main space, then you have to make sure that this opening is 100+ square inches.
The IRC does not specify what dimensions a space needs to have to be considered a closet. However, according to the IRC definitions, a closet is defined as:
“A small room or chamber used for storage.”
There are also often minimum airspace requirements in dryer installation manuals when they are installed in closets. Remember that the IRC defers to the installation instructions given by the manufacturer.
While these airspace requirements do not relate to makeup air requirements but rather clearances and combustion air provision, it does indicate at what point a space is considered to be a closet large enough to house a gas dryer and at what point the makeup air requirements for closets come into play.
For example, minimum space requirements for closet installations are specified for Kenmore electric or gas dryers and Whirlpool gas dryers.
Makeup Air Sizing Guide and Makeup Air | Simple Homeowner’s Guide
Sources
https://pediaa.com/difference-between-complete-combustion-and-incomplete-combustion/
https://homedetoxing.com/how-many-cfm-does-a-dryer-exhaust/
https://www.cambridgeair.com/make-up-air
https://manuals.plus/kenmore/electric-or-gas-dryer-manual#ixzz7kXCHYb3K