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What Is Make-up Air? And When Is Make-Up Air Required? (Answered)

What exactly is make-up air? In this article, you will learn what make-up air is, why it’s essential, and how to determine if you need it in your kitchen.

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If you’re 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?

In this article, you will learn what a make-up unit kit is, why it’s important, and how to determine if having one is required for your kitchen.

A make-up air unit “makes up” the air exhausted from your home via your range hood. 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.

What is make-up air?

Make-up air, as its name indicates, “makes up” the air exhausted by 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?

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’ll have to spend more time cleaning. And respiratory difficulties might arise as a result of prolonged exposure to smoke and toxins. And when your kitchen lacks replacement air, your heating and cooling conditions will be uneven.

When is make-up air required?

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

  • You have a very small kitchen with little space.
  • Your kitchen has poor ventilation.
  • Your kitchen requires it per the building code.
  • You use your range hood often.

Small Kitchens

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 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.

Poor ventilation

Your kitchen range hood will not perform at its best if you have poor ventilation. Make-up air units are recommended for households that need additional ventilation. Using a range hood or exhaust fan, you should constantly circulate clean air back into your house.

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.

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 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)

Do you still have questions? Below are some of the most commonly asked questions about make-up air.

Can 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 strain your budget, and 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.


Knowing when you need a make-up air unit isn’t complicated. If your hood is 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.

This article covered what a make-up unit 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 lost air when running a range hood.
  • They help prevent negative airflow in your home.
  • Building codes often require them.
  • 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 (I 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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Written By Roger Harris

Hey there, My name is Bradley, and I've been riding fixed for over ten years. I love all the joy and pain that comes with this unique style of cycling and the passionate community that drives it. If you love fixed gear and single-speed bikes, this is the place for you.

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