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Makeup Air

When a balloon loses air or depressurizes, it gets smaller (and makes a funny sound), but houses don’t roll like that 

February 07, 2018

Makeup air is outside air that replaces air exhausted from inside the building. In most houses, there are several appliances that push or pull air out of the house, including clothes dryers, bathroom fans, fireplaces and kitchen range hoods. When even one of those appliances is operating, it depressurizes the interior. Nature abhors a vacuum, so to restore the pressure balance, outside air is sucked into the house.

Unless airways are deliberately provided for that incoming air, it will flow in through the path of least resistance, such as gaps around windows and doors, between framing and the foundation, and at other leaky building components. If the rate at which air is exhausted is high enough, the suction created can be strong enough to reverse the draft in a water heater vent, furnace chimney or fireplace, drawing noxious fumes and deadly carbon monoxide into the house. 

Editor’s Note: Examples of makeup air solutions include: [1] The Residential Makeup Air System (RMAS) from CCB Innovations includes an air flow sensor mounted inside a standard duct coupling, which signals a motorized damper in a second coupling to open when air is flowing and close when air flow stops. [2] Broan’s direct-wire, synchronized Automatic Make-Up Air Dampers work with specific company range hoods, operate automatically when the range hood is activated, and are compatible with whole-house ventilation systems. [3] The automated Makeup Air System (MUAS) from Fantech includes an intake hood, shut-off damper, filter, fan, duct silencer, and optional heater; available models handle air flow up to 2,000 CFM.

Why It Matters

Makeup air is more of an issue these days for two main reasons: (1) houses are getting significantly tighter and (2) range hood fans have been getting significantly more powerful to accommodate a growing trend toward putting large commercial-style ranges in residential kitchens. Large range hoods can draw out air at the rate of 1200 CFM or more—roughly a small roomful of air every 60 seconds—which by itself could be enough to reverse the draft in a furnace or water-heater flue. 

But big range hoods aren’t the only problem. Several smaller fans running at once—for example, a dryer, small range hood, and bath fan—can cause the same level of depressurization. So can a big, open fireplace, in fact, I know of a case in which a roaring fire in a fireplace in a tight house reversed the draft in the secondary flue and filled the basement with noxious fumes.

In addition to the safety concerns, the downside of depressurization includes a big energy penalty from exhausting air that you’ve spent good money to heat or cool, while at the same time drawing in outside air that will cost more money to heat or cool once it gets inside. And unless all this air flow is controlled, it creates drafts, which make people uncomfortable. If the incoming air is humid, it can bring unwanted moisture into the house. In a cooling climate, that moisture can condense against the back side of cool, air-conditioned walls and cause concealed mold and rot. 

Air leaking into the house through an unsealed crawl space also can contain water vapor as well as mold spores, particulate matter and potentially harmful gases. I know of an instance involving a commercial restaurant in which the very powerful kitchen exhaust fans were operating, but the makeup air units weren’t. The depressurization was so powerful that sewer gas bubbled up through the traps in the toilets. The odor chased a lot of unhappy customers out of the dining room.

How To Do It Right

Bubbling toilets aren’t likely to occur in residential construction, but depressurization is still a potential problem. In the past when houses were much leakier, this phenomenon was not as serious an issue because air would just leak in through nearby windows and doors, and even through walls, to make up the pressure difference caused by the exhausted air. People may have been uncomfortable, but at the least the house didn’t fill with noxious fumes. But with tight houses, we can’t rely on air leaks to solve our problem. We have to plan for and provide the solution. 

One place to begin is with the code. To paraphrase Section 1503.4 of the 2012 IRC, an exhaust hood that can draw more than 400 CFM must be supplied with makeup air at a rate that roughly balances the outflow and inflow. Plus, the exhaust and makeup air systems have to be connected. Typically what happens is, as soon as you turn on the range hood, it will send a signal to a damper in the makeup air duct, which will open to allow air to flow in. When the range hood shuts off, the damper will close, blocking infiltration of unconditioned air.

Contractors who think this is an overly complex and expensive solution often note that the simplest makeup air strategy is to open the window next to the range hood. This will work, but only in certain, very specific climate zones where you don’t need to heat or cool incoming air at any time of year. Codewise, however, it’s a manual solution, and not an automatic connection.

More typically, the solution involves ductwork. In a forced air heating system, outdoor air can be brought into an air handler on the return side, then distributed through the ductwork. In milder climates, the makeup air may not need to be conditioned directly because it is tempered as it moves through the ductwork. But in cold or hot climates, it may be necessary to heat or cool the incoming air directly to prevent comfort issues.

Range hood manufacturers are starting to provide interlocked makeup air damper kits, and several third-party “universal solutions” also are available. Both types of systems typically consist of a short length of ductwork with a motorized damper inside that operates by means of a pressure switch located in the exhaust duct. Some models are “interlocked” with “slave” dampers so that the exhaust and intake are synchronized, and some also are designed to work with whole-house ventilation systems.  

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About the Author

About the Author

Jonathan Smegal is senior project manager at RDH Building Science Laboratories, in Waterloo, Ontario. This article was adapted from “Vapor Permeance: 7 Minutes of BS,” a building science podcast series at ProTradeCraft.com.



I'm installing a kitchen island vent fan unit. I know I need to have a makeup air system per code. How does the make up air system deliver make up air from outside ducted to the return duct in my AC system? Isn't the pressure in the return duct still positive so how can outside air get into the return duct without a blower to drive it? and how do I determine how much makeup air from outside air is allowed to be delivered to the return air duct, i.e., how much would overload the AC system particularly in summer hot weather (Houston).

I am looking at adding a make up air system for my 100 year old semi-detached home. Over the past 20 years I have been insulating my home. Now I am finding that I am drawing air from my attached neighbour which is a problem for me because they smoke inside their home in the winter. I want to create a relative positive pressure inside my home so that the air leaks out instead of in. Do you have any tips?

Hi Barbara,
If you have an exhaust-only system, such as a few bath fans and a range hood, your building will most likely be under negative pressure. You're sucking the air out, so replacement air is being pulled in from your neighbors. Exhaust-only ventilation can also cause back-drafting if you have combustion appliances, which can be deadly.

If you have a supply-only system, then you're pulling air into the house but not exhausting it, therefore pushing it into the framing. The reason for the point-load exhaust systems in the previous paragraph is to exhaust moist air, so this supply-only system will push the moisture into your framing and the framing may rot.

It is best to have a balanced system, where the air that is removed with the exhaust fans is replaced (purposefully) with clean, fresh air.
One way to do this is to add a heat recovery ventilator or energy recovery ventilator (depending on your climate).

Another way is with a passive air intake, which amounts to a 'smart' hole in your wall that lets air in when other air is being drawn out. this can be either chilly or humid, depending on where you live.

A third way is to add a duct running from outside the building to the supply air side of your air handler. This will bring in fresh (unconditioned) air and mix it with the conditioned air in the air handler, which is a little smarter energy-wise than a hole in the wall.

There are a lot of variables depending on your exact situation, but those are the basics. If you're interested in learning a little more about supply, exhaust, and balanced ventilation you can read this article on ProTradeCraft.com:

I hope that helps,

Great feedback. I am in the same boat at I have radiators for heating and central air (only air registers on 1st floor and air handler on the 3rd floor). I am adding 1200cfm hood to capture the heat from my 48” dual fuel Viking range. Since my kitchen faces an exterior wall it appears that the best scenario seems to add an 8” hole with a Brian MUA kit with a 8” vent in the ceiling (10 feet from my range hood). I can get any closer as add the all the other areas are blocked with a chimney and relocated radiator pipes. I also live in Wisconsin so understand that the air will be cold in the winter, which doesn’t bother me as I usually crack a window when I run the hood at a height speed (blacken fish in February). It also seems that the code is to add MUA after 600cfm. Do I need to run the MUA closer to the range?

Hi Rob,

Looking at a similar situation here in NJ. Curious to know if you cam up with a solution.

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