Makeup air is actually very simple to understand if you know a little fundamental physics, namely, that the universe hates vacuums, and when you remove air from a room, you create a vacuum. Vacuums (low-pressure systems) can create a variety of problems in your home. Enter—quite literally—makeup air.
However, it’s not always required. While all vacuums are a deficit of air, the extent of the vacuum depends on how much air is removed and needs replacing. The International Residential Code (IRC) has guidelines for makeup air provision, but even outside of these, makeup air can be helpful.
As per the IRC, range hoods rated 400+ CFM need makeup air when they are in the same area as fuel-burning appliances that don’t use direct-venting or mechanical draft venting. Makeup air may be advisable for range hoods in airtight houses, if other exhaust systems are close, or if the hood causes drafts.
When Makeup Air Is Required (IRC)
According to Section M1503.6 of the IRC, makeup air is required when a kitchen contains one or more appliances that burn gas, liquid, or solid fuel in addition to an exhaust system.
Ovens and stoves, especially gas-burning ones, use the process of combustion, which emits a variety of combustion pollutants, including carbon monoxide and nitrogen dioxide.
These combustion pollutants can cause headaches, queasiness, trouble breathing, etc. At high concentrations for an extended period of time, combustion pollutants can cause brain and heart damage and even death.
Because certain appliances, such as ovens, use combustion processes and produce combustion products, areas like the kitchen must have proper ventilation for the safety of its inhabitants.
The negative pressure produced by exhaust systems can threaten this by causing backdrafting.
The exception is if the fuel-burning appliances use a direct-vent or mechanical draft venting system.
- With direct-vent systems, the combustion gases are pushed outside directly through a closed duct leading from the appliance to the outdoors. So, the combustion air and gases are separate from the air in the room, and negative pressure cannot cause backdrafting.
- Mechanical draft venting systems actively pull fresh air in for combustion and push combustion gases out. They don’t rely on passive processes that are affected by the pressure in the room (like chimneys do), and negative pressure cannot cause backdrafting.
Additionally, the exhaust system used must exhaust more than 400 cubic feet per minute (CFM).
This is the determined average threshold at which the air supplied by natural infiltration is not enough to balance the vacuum created by exhausted air, and the house will start displaying signs of needing makeup air.
Determining the CFM Rating of the Hood
In order to determine whether or not you need makeup air introduced into your kitchen, it is important that you know the CFM of your existing hood to see if it is rated at 400 CFM or higher.
The CFM of your range hood can be found in a variety of places.
- One location may be on the box. It could be near the name of the product or in the description of the product.
- Additionally, the user manual will most likely have information pertaining to the CFM.
- If your range hood was bought online, the site you bought it from is likely to list the CFM.
Why Makeup Air Is Not Required for Lower CFMs
A low CFM basically means that less air is being removed each minute, meaning that overall, less air is removed during the time that your range hood is on compared to if it was being removed at a faster rate.
The slower the rate of air removal and the less air that is being removed means that the air can be replaced more easily and without issue.
Although the infiltration of air that does occur with lower-CFM range hoods could undermine your HVAC’s work, the volume of inflowing air should be small enough and should be entering slowly enough to make any additional energy costs negligible.
The vacuum is also not likely to be strong enough to cause structural damage, backdrafting, or any of the other problems associated with a lack of makeup air.
How Much Makeup Air Is Necessary?
Section M1503.6 states that makeup air must be introduced at the same rate that it is removed.
If air is added to the home at the same rate as it is removed, it prevents the formation of the vacuum that leads to low-pressure systems.
However, infiltration of air through gaps, cracks, and open windows and doors must be accounted for since it reduces the amount of purposefully supplied makeup air required to replace the exhausted air.
So, a simple equation for how much makeup air it is necessary to introduce looks like this:
Makeup air supply rate = exhaust rate – infiltration rate
The exhaust rate can be found in your range hood’s product information (on the hood, on the packaging, in the owner’s manual, and/or online). It will be a CFM rating.
The infiltration rate will need to be determined by performing a test. Unfortunately, this test cannot be done by the homeowner. It requires a building performance professional.
The test is called a blower door test, and it determines the amount of airflow necessary to pressurize a building to a certain standard pressure—most often 50 Pascals (Pa).
A blower door test’s findings are given in CFM50s, which is the airflow rating in CFM when the air pressure is at 50 Pa.
Let’s say your range hood is rated at 500 CFM and the infiltration is determined to be 100 CFM50. Subtracting 100 from 500 gives you 400 CFM, which is how much makeup air your makeup air unit will need to provide.
Other Conditions in Which Makeup Air Is Helpful
Well-Sealed Houses (Particularly in Winter)
Well-sealed houses are desirable because they protect our conditioned and purified indoor air environment and ensure maximum comfort.
However, this level of weathertightness is not without drawbacks. Not only is it detrimental to the quality of the air (the air is trapped and readily becomes stagnant), but it can also make the range hood exhaust system ineffective.
As you know, when a low-pressure system occurs due to a lack of makeup air, a vacuum is created. A lack of air infiltration from the outside due to a well-sealed house will make it much harder for the low-pressure system to be eliminated without the deliberate supply of air through a makeup air unit.
The result of this setup is decreased effectiveness of the range hood itself.
Even an active exhaust system will find it difficult to draw air from a room with lower air pressure than the outside.
Not only will it be difficult to remove all the steam, smoke, and grease from the air in the kitchen, but the air that was already vented can be pulled out of the ducts and back into the kitchen.
Overall, a vacuum preventing a range hood from exhausting air will decrease the quality of air in the kitchen.
This is especially true in the winter when windows are kept closed to reduce the escape of warm air, and we take all manner of other steps to ensure that fresh air has less access to the home.
When Bathrooms or Dryers Are Near the Kitchen
Additionally, if you have your kitchen next to a bathroom or a laundry room, then the loss of air from an area in the home through the range hood is compounded by the loss through the bathroom fan or dryer.
Bathroom fans do not often exhaust enough air to create pressure systems that require makeup air.
However, the exhaust rate of the range hood, in addition to that of the bathroom fan, can become high enough to make makeup air necessary for the continued function of both systems and the prevention of unwanted side effects.
The same applies to laundry rooms containing vented dryers. Dryers rated at 200+ CFM require makeup air. But dryers with ratings lower than this, in conjunction with nearby range hoods (even those with ratings lower than 400 CFM), can warrant the provision of makeup air.
The provision of makeup air in such circumstances may not be required for code compliance, but it can be required to stave off negative effects like sewer gas being pulled from drains.
Drafts When Range Hood Is On
If every time you turned your range hood on, you were treated to noticeable drafts coming in from around your window and door frames, under your baseboards, through any cracks and gaps in wood panels and masonry, then regardless of the CFM rating of your range hood, you would benefit from makeup air.
Even if the infiltration of air is enough to counter the negative pressure system, preventing things like gurgling or smelly drains, the solution is not pleasant.
These drafts can make your home even chillier on a freezing day or cause doors to slam, papers to fly away, or other disturbances.
The drafts are uncomfortable, but they also have serious ramifications when it comes to keeping HVAC costs down. The more unconditioned air is pulled into the home, the harder your HVAC has to work to keep the environment matching the thermostat.
Introducing makeup air can introduce new air into the home without it having to sneak into the home through cracks.