When Is Make-Up Air Required? (Answered)

If you’re in the middle of looking for a new range hood, chances are you’ve also heard of the term ‘make-up air’. But what exactly is make-up air, and when is make-up air required?

A make-up air unit is required if your range hood is above 400 CFM. But even if it’s not over 400 CFM, a make-up air unit is highly recommended because, without it, negative air pressure will hinder your range hood’s performance. 

But there’s a lot more to it than that. So, in this article, you will learn what make-up air is, why it’s important, and how to determine if having one is required for your kitchen.

Editor’s note: This article was updated on December 29, 2021, to include additional information regarding make-up air and HVAC.

What is make-up air?

Make-up air, as its name indicates, “makes-up” the air that is exhausted from your home via your range hood. When you operate your range hood, it removes filthy air from the kitchen. Clean air must be reintroduced to the area once the dirty air has been removed. So it essentially brings fresh air in from the outside.

Why is make-up air important

With so many cooking options out there, it’s possible that you’ll need more than one exhaust system in your kitchen to handle the many types of cooking equipment you’ll be using (such as dishwashing equipment and baking ovens).

Without a make-up air unit for your kitchen, your exhaust hood won’t accomplish its job as effectively. That’s because the absence of supply air creates a rise in static pressure that decreases the cubic feet per minute (CFM) of air the equipment can remove from your kitchen.

When your exhaust hood doesn’t operate correctly, you’ll notice more grease piling up on the surfaces of your kitchen, and you will have to spend more time cleaning. And respiratory difficulties might arise as a result of prolonged exposure to smoke and toxins. Finally, when your kitchen lacks replacement air, your heating and cooling conditions will be uneven.

Image of the view from inside of a galvanized steel air duct on the exhaust fan.
Image of the view from inside of a galvanized steel air duct on the exhaust fan.

When is make-up air required?

Make-up air units may be required in the following situations:

  • Your is very small kitchens with little space
  • Your kitchen has poor ventilation kitchens with poor ventilation
  • Your kitchen requires it per building code
  • You use your range hood often

Small Kitchens

Because there are few cubic feet of air in smaller kitchens, make-up air devices can be beneficial. You may notice difficulties with negative air pressure if you operate your range hoods in a compact area or if you don’t have a well-performing range hood.

Negative air pressure means that the pressure within your house is lower than the outside air. This pressure differential might allow outside air to leak into your house. 

Positive air pressure is required for your range hood or exhaust fan to function effectively. If your pressure is negative, the range hood will have a tough time extracting air and performing at its best.

You may have negative air pressure if you see inward-facing doors opening rapidly or if your doors are difficult to open. Whistling or crackling noises may also be heard. Keep an eye out for growing energy expenditures, a strained HAV system and drafts. These are all signals that a cosmetic air unit might be beneficial in your kitchen.

Poor ventilation

Make-up air units are recommended for households that need additional ventilation to enhance air quality. You should constantly circulate clean air back into your house when utilizing a range hood or exhaust fan. If you don’t have a make-up unit and have a ventilation problem, air will be drawn from other sources such as your HVAC system, water heater, or chimney. These sources aren’t suited for this purpose and won’t generate enough circulation.

Your kitchen range hood will not perform at its best if you have negative pressure. When the outer pressure exceeds the inside pressure, the interior air pushes back, preventing the exterior air from entering. As a consequence, the range hood’s ability to extract air is reduced. The range hood, however, can no longer transport air out when the pressure differential between the inside and outside expands. Negative air pressure and fluctuating temperatures are also possible in your house. These problems may be alleviated with the use of a make-up air unit.

Building code regulations

The building industry in the United States has long recognized the need for enough make-up air for exhaust systems. Since 2009, the International Residential Code (IRC) has mandated the provision of make-up air for kitchen hood exhaust systems with a capacity of 400 CFM or larger.

However, you should still check your local codes in case this information is outdated or has changed since the time of publishing.

Long term use of a range hood

If you wish to leave your range hood on for hours at a time, you’ll almost certainly require a make-up air unit, regardless of its CFM. In general, the longer your range hood runs, the more air is expelled from your kitchen. If you’re cooking a lot or at high temps, you may leave it on for a long time. Whatever the cause, when the range hood is turned on for an extended period of time, the air within the kitchen is evacuated rather than restored.

If you want to further understand how make-up air works and when you might need it, take a look at this video called “All About Make-up Air” from the This Old House YouTube Channel.

Video called “All About Make-up Air” from the This Old House YouTube Channel.

Frequently Asked Questions (FAQ)

Why can’t I just open an outside window or door?

This is technically possible, but you’d need moderate and dry conditions outdoors for it to work. Even if you don’t use air conditioning or heaters, keeping doors and windows open in the winter and summer may put a strain on your budget. Is there any use in designing and building efficiency if that’s the solution? It is not possible to control what enters via that opening. Contaminants might also blow in.

Do make-up air units regulate temperature?

A good make-up air unit will give you the ability to regulate the temperature so that the air coming inside is at a similar temperature as the air inside the house.

How do I buy a make-up air unit?

You can purchase a make-up air unit online or at a local shop. However, there are many things you should consider before doing so. Take a look at our guide on how to choose a make-up air unit here.


Knowing when you need a make-up air unit isn’t complicated. If your ranger is running under 400 CFM, odds are you won’t need a make-up air unit. However, if it’s over 400. You’re going to need one.

In this article, we covered what make-up air is, why it’s important, and how to determine if having one is required for your kitchen. Here are some key takeaways:

Key takeaways

  • Make-up air units replenish the air that is lost one running a range hood.
  • They help prevent negative airflow in your home.
  • They are often required by building codes.
  • Anyone with a range hood over 400 CFM should have a make-up air unit.

So, do you have a dedicated make-up air unit, or do you just crack open a window? Let us know in the comments below (we read and reply to every comment). If you found this article helpful, check out our full blog for more tips and tricks on home and kitchen maintenance.

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