Makeup air was something that I had never noticed until it was causing me problems with my home. Still, once I delved into the research, it became apparent why I was having the issues that I was.
They can start off and even remain subtle, but the problems with a lack of makeup air can become quite serious. Understanding makeup air, when it is required, and when it might be helpful are little things that can help you be more in tune with your home and its needs.
Makeup air is required when exhaust systems remove enough air to cause a negative pressure system, which adversely impacts the function of these systems and pulls air into the house through cracks and from gas appliance exhaust vents and drains.
What Is Makeup Air?
Makeup air is outside air that is intentionally brought into a home to replace the air that exits the home via exhaust devices and systems. Such devices and systems are things like bathroom fans or range hoods and the associated ductwork.
Makeup air is meant to prevent the home from entering into negative pressure. Makeup air can be actively brought into a home, but it can also be allowed to seep into the home passively.
Why Makeup Air Is Important
While on the surface one might think that supplying makeup air doesn’t sound like a big deal, it is.
Without makeup air, a negative pressure will build in the home. This means that air will be sucked into the home through every space possible (because air naturally moves from higher pressure areas to lower pressure area).
Negative air pressure:
- Interferes with exhaust systems since the negative pressure (a vacuum) resists losing more air.
- Sucks air in through cracks, which brings unconditioned air into our homes, causing discomfort and overworking the HVAC system.
- Draws sewer gases out of drains to try and even out the negative air pressure.
Combustion air is also impacted by this negative pressure, but that will be discussed briefly later.
- Smoke or steam remaining in the house while the exhaust system is running.
- Gurgling drains.
- Bad odors from the drains.
- Significantly higher or a sudden increase in utility bills.
- Less precise or fluctuating temperature control.
- Overall reduction in air quality.
As you can see, not having makeup air can impact specific systems and also hurt the entire home.
That being said, not all exhaust systems will require a makeup air unit. Whether or not makeup air is needed depends on the cubic feet per minute (CFM) of an exhaust system, the type of system (dryer, range hood, etc., and the presence of the signs listed above.
How Makeup Air Is Provided
If you require deliberately supplied makeup air, i.e., infiltration is not cutting it, then you can install a domestic makeup air unit. These are normally installed on top of a building and are connected to rooms that have large exhaust systems.
The airflow through these systems is controlled by a makeup air damper. It would be no good having an open hole leading between the house and the outside!
Makeup air dampers can be operated by air pressure and gravity or electronically. They are one-way valves that allow air into the home but not out and only open when the corresponding exhaust system is on.
Makeup air units can have entirely independent duct systems or they can be incorporated into the HVAC ductwork. The makeup air ducts don’t have to be insulated. Whether or not your makeup air ducts need to be insulated depends on if the unit tempers the air or just supplies it.
Insulation is good when your unit tempers the air because it maintains the tempered quality of the air better. It wouldn’t make much sense to have a unit that tempers the air but then passes it through uninsulated ducts because the air would lose some if not all of the tempering it received.
The size of makeup air ducts is also important. While not specified by the International Residential Code (IRC), we do know that the total air supplied must equal the exhaust rate of the space being considered for problems with negative pressure to be avoided.
This can then be translated into a guideline for duct size (see the link above).
Acceptable Sources of Makeup Air
You can get makeup air from the attic. This is considered as transfer air by the IRC. However, makeup air is not always ideal because you can run into problems with insulation fibers and dust getting sucked into the home. Additionally, as attics are typically hot, this heat is transferred into your living areas as well.
There are some benefits though like no holes have to be cut in the roof or walls, and there is easier access for repairs if something goes wrong.
Of course, attics are not the only other rooms in the house where makeup air can come from. The condition is that this room has a source of air as well, otherwise, you are just creating the negative air pressure issues in that room.
Makeup air more often comes from outside.
The intake vents need to be a certain distance from exhaust vents as well. Too close, and the makeup air that is supplied may be the air that was exhausted, which can be harmful to one’s health.
Makeup air can be conditioned as well, but it doesn’t have to be. Some makeup air units condition the air, some just temper it, some heat and cool (or just one or the other), some can dehumidify the air, and some rare units add humidity to the air.
Makeup air can also heat or cool the home, but only incidentally. It isn’t the purpose of makeup air to heat or cool, and such features are rarely desirable. Conditioning is simply to prevent the inflowing air from burdening the HVAC system and making you uncomfortable.
Makeup air differs from fresh air in that makeup air is brought in to replace exhausted air, and fresh air is allowed in to improve air quality or to adjust the temperature.
Gas Appliances and Makeup Air
When it comes to gas appliances, the subject of makeup air can become rather confusing. A lack of makeup air can negatively impact the function of a gas appliance, like a gas fireplace, but they don’t need makeup air to function.
Gas appliances need a combustion air supply. Makeup air is only used in the context of exhaust systems, not fuel-burning appliances.
Combustion air differs from makeup air in that combustion air provides fuel-burning appliances with enough oxygen for flames to ignite and be sustained. Similar to makeup air, combustion air can come from outside the home or from the inside.
If exhaust systems are removing too much air, then it can cause backdrafting from the gas appliance. This means that all of the bad gases that are supposed to be removed from the house are pulled back in.
Additionally, depending on how the combustion air is supplied, it may be taken away from the gas appliance and used to counter the negative pressure system.
We come across a unique case in this matter: gas dryers. Gas dryers may require makeup air. However, this is not because they are gas-burning appliances. It’s because they are exhaust systems.
Laundry Room Makeup Air
We’ve just mentioned makeup air and dryers, but it bears going into a little more detail.
Laundry rooms don’t always require makeup air, but there are certain criteria that, if met, will mean that it is needed.
Laundry rooms only need makeup air when there is a gas appliance present and the dryer is rated above 200 CFM. This is the rating at which the IRC deems problems would likely start to show themselves.
Dryers with lower CFM ratings can also need makeup air if it is installed in a closet or if there are obvious signs of developing negative pressure systems.
A dryer in a closet needs makeup air because closets are so small. They enter negative pressure very quickly, so they need makeup air (although the room they are in does not require makeup air, just the closet).
So, a laundry room will require makeup air only when the dryer that is in it requires makeup air. And this is not just gas dryers. Electric dryers can also need makeup air. However, it will not be required for ventless dryers.
Kitchen Makeup Air
Makeup air is not always required in kitchens, but the need for them in domestic kitchens is becoming increasingly likely.
Some reasons for this include the presence of high CFM range hoods, more weathertight houses, and the resurgence of the popularity of gas ranges.
Range hoods are the most noteworthy feature of a kitchen that makes makeup air so important. Range hoods rated 400 CFM or greater require makeup air when they are in the same area as a fuel-burning appliance that lacks direct-venting or mechanical draft venting.
Makeup air may still be required, if the range hood operates at lower than 400 CFM. It all depends on the presence of nearby exhaust systems that may compound the loss of air by the range hood or signs of a negative air pressure system developing.
Furnace Makeup Air
The furnace is another important appliance to keep in mind when debating about where to supply makeup air. Makeup air is not required for furnaces. The air that furnaces use and exhaust is supplied deliberately as combustion air.
Furnaces aren’t exhausting large amounts of the general air from the room, but, if there is an exhaust system in the room with the furnace, then makeup air might be needed so that backdrafting doesn’t occur.
In addition, combustion air might be taken and used to balance the negative pressure created in the room by the exhaust system.
This would impede the function of the furnace, and is just another reason that makeup air is necessary in such a scenario.
Bathroom Makeup Air
Bathrooms and bathroom fans are also relevant to the makeup air discussion. While makeup air is typically not necessary for a bathroom (because the bathroom fan doesn’t exhaust enough air), it can still be helpful.
Makeup air should seriously be considered when the bathroom fan doesn’t function well, there are gas appliances in the bathroom, or if the drains gurgle and smell. These are all evidence that there is negative pressure in the bathroom.
If the bathroom fan that is used is more powerful than what the bathroom needs, then you might think that makeup air is needed, but all you really need is a smaller fan.
How Much Makeup Air Is Required?
Knowing the amount of makeup air to supply is very important because it impacts how effectively the makeup air system is working.
While there is no makeup air sizing guide, the IRC offers some guidelines in that it says you should provide as much air as is removed.
In cases where your home is not weathertight and there is infiltration (air seeping through cracks in the home), a makeup air amount equalling the amount removed may end up being too much, so infiltration must also be taken into account.
Knowing the amount of air entering from infiltration requires a professional.