Do You Need Makeup Air for a Range Hood? Explained

Makeup air is outdoor air brought into the home to replace the air your range hood exhausts.

A ducted range hood pulls cooking smoke, grease, moisture and odors out of the kitchen and sends that air outside. Every cubic foot it sends out has to be replaced by air coming back in.

In many homes, some of that replacement air simply leaks in through gaps and other openings nobody planned. A makeup air system gives replacement air a planned, controlled way into the house.

When a hood removes air faster than it can get back in, the house can become negatively pressurized, which can hurt hood performance, create drafts and affect the safe venting of some fuel-burning equipment.

Think about drinking through a straw from a sealed container. The harder you pull, the harder it gets until air finds another way in. A house can behave similarly when a powerful exhaust fan is removing air faster than replacement air is entering.

When a strong hood runs in a tight house, a few things can happen:

The hood may move less air than its rated capacity because it is working against negative pressure.

Air can be pulled in through unintended paths, creating drafts or whistling around doors and openings.

Naturally drafting fuel-burning equipment can be affected if the house pressure interferes with its normal venting path.

The tighter the house, the more noticeable the pressure problem can become. That's one reason make-up air gets more attention in newer, tighter homes and in older homes that have been substantially air-sealed.

You'll often hear 400 CFM used as the dividing line, but it is not a universal rule. Check the code that applies to your project and the fuel-burning equipment in your home before you assume either way.

You'll see this stated online as a simple rule: over 400 CFM means you need makeup air. That's not the whole story.

Under the 2021 International Residential Code, Section M1503.6, the trigger applies when at least one gas, liquid or solid-fuel-burning appliance is located inside the home's air barrier and that appliance is neither direct-vent nor mechanically drafted. When that condition is present, an exhaust system capable of more than 400 CFM must be provided with make-up air at approximately the exhaust rate and with an automatic damper.

Massachusetts' current residential code is based on the 2021 IRC with state amendments. Other states and local jurisdictions may amend the model code differently, so your local building department and project team should confirm what applies to your house.

No. Under the 2021 IRC model language, the threshold is more than 400 CFM, and the requirement also depends on the type of fuel-burning equipment inside the home.

Two things are worth separating:

1. The number. The IRC language is "in excess of 400" CFM. A hood rated at exactly 400 CFM sits at the line under that model provision.
2. The condition. If the fuel-burning appliances inside the home's air barrier are direct-vent or mechanically drafted, the model-code provision does not use hood size alone to trigger makeup air. A naturally drafting water heater, boiler, furnace or other qualifying appliance changes that analysis.

That said, "code doesn't require it" is not the same as "the hood will perform well without it." A powerful hood in a tight house can still struggle for replacement air.

The model-code language looks at what the exhaust system is capable of exhausting, not simply the speed you expect to use most often.

Say you buy a hood with a 600 CFM blower but expect to run it on low at 250 CFM most nights. Homeowners naturally ask whether that counts as 600 CFM or 250 CFM.

The IRC language refers to an exhaust system capable of exhausting more than 400 CFM. So don't assume that planning to leave the hood on low changes the permit requirement. If you truly don't need 600 CFM, the better conversation is whether the hood should be sized differently in the first place.

This is also a good reason to read the hood specification sheet carefully. Blower ratings, installed airflow and what the local inspector evaluates are not always the same thing. Ask the HVAC contractor and building department how they will evaluate the specific equipment you are considering.

When the IRC makeup air requirement applies, it calls for replacement air at approximately the same rate as the exhaust airflow, not just the amount above 400 CFM.

This is another place where the 400 CFM number gets misunderstood. It is a trigger in the model-code provision, not an amount you automatically subtract from the hood capacity.

For example, if a 600 CFM exhaust system falls under that requirement, you should not assume that only 200 CFM of makeup air is needed because 200 is the amount above 400. The code language calls for makeup air at a rate approximately equal to the exhaust air rate.

The actual system still needs to be designed for the house and equipment. Duct size, pressure, controls, climate and where the air enters all affect how well the system works, so this is a calculation for the mechanical professional, not a homeowner rule of thumb.

Possibly. A leaky house may behave differently from a tight house, but leakage is not a substitute for checking what code requires or whether combustion equipment can backdraft.

I hear this one a lot in New England: "My house was built in 1890. Air comes in everywhere. Do I really need this?"

It's a fair question. An older house may have more unplanned air paths, but a few things change the picture:

The model-code condition is not based on the age of the house. It looks at exhaust capacity and the type of fuel-burning equipment inside the air barrier.

Renovations can tighten older houses. New windows, spray foam, air sealing and insulation upgrades may change how the house behaves.

Older houses may also have naturally drafting water heaters, boilers, furnaces or fireplaces that deserve particular attention when a powerful exhaust fan is added.

So the practical answer is that an older house may be more forgiving, but house age alone should not decide the question. Have the HVAC contractor and building official evaluate the actual conditions.

Backdrafting is when exhaust from a fuel-burning appliance is pulled back into the house instead of venting safely outdoors.

Some appliances vent by natural draft, meaning hot exhaust rises through a chimney or flue without a fan. That draft can be relatively gentle. If a powerful range hood creates enough negative pressure in the house, it can interfere with that path.

Equipment that may rely on natural draft includes:

• Older gas or oil water heaters
• Some furnaces and boilers
• Wood-burning fireplaces and stoves
• Gas fireplaces that are not direct-vent

Direct-vent, sealed-combustion and mechanically drafted appliances handle venting differently and are less dependent on natural chimney draft. That difference is why the 2021 IRC provision specifically distinguishes them.

If you do not know what type of equipment you have, that is a perfectly normal question to ask your HVAC contractor during planning. And every home with fuel-burning equipment should have working carbon monoxide alarms regardless of the range hood.

Not necessarily. The hood still removes air, and the fuel-burning appliance that matters for the code may be somewhere else in the house.

This one surprises people. "We're switching from gas to induction, so makeup air doesn't apply, right?"

The range type does not change the basic fact that the hood is exhausting air from the house. Under the model-code provision, the relevant fuel-burning appliance could be a naturally drafting water heater, furnace, boiler or fireplace elsewhere inside the home's air barrier.

Induction eliminates the combustion byproducts created by a gas burner, but cooking still produces grease, moisture, smoke, particles and odors that need to be captured. Switching to induction may affect the ventilation capacity you choose, but it does not eliminate the need for good kitchen ventilation.

Opening a window demonstrates the concept and may improve hood performance, but you should not assume it satisfies a code-required makeup air system.

Try it sometime: run the hood on high, then crack a nearby window. In some homes you may hear the hood get quieter or notice it pulling more effectively because you've given replacement air an easier path into the house.

Where makeup air is required under the IRC provision, the system includes a damper that opens automatically when the exhaust operates. It is not supposed to depend on someone remembering to open a window. Picture January in Massachusetts. Nobody's opening the window to sear a steak.

Usually it includes an outdoor air intake, ductwork, a damper and controls tied to the hood. Some systems also use a fan or temper the incoming air.

The exact configuration varies, but you may hear your contractor discuss:

An exterior air intake located to meet required clearances from exhaust outlets, combustion vents and other potential contaminants

Ductwork that brings outdoor air into the house

A damper that stays closed when the system is not operating and opens when needed

Controls that coordinate the damper, and any fan, with the range hood

Sometimes a fan that actively brings replacement air in

Sometimes equipment that heats, cools or otherwise tempers the incoming air

The right configuration depends on the hood, the house, the climate and the mechanical system. That is an HVAC or mechanical design decision. Your job as the homeowner is to make sure someone is clearly responsible for making it.

Under the IRC model provision, kitchen makeup air is delivered to the room with the exhaust system or to an adjoining room or duct system that communicates with it through permanent openings.

That sounds simple, but location has a big effect on how the kitchen feels and functions.

This is where the designer in me gets involved. A technically compliant solution that drops freezing outdoor air beside the island may not feel like a very good solution in a Massachusetts kitchen. Where the supply lands can affect:

Comfort for whoever is standing at the range or island

Which wall, toe kick, floor or ceiling space has to accommodate a grille or duct

Cabinet layout, soffits and the path the ductwork can take

How visible or noisy the supply becomes

These are all reasons to talk about makeup air while the kitchen is still on paper.

Not in every installation, but in a cold or very hot climate you should ask how the incoming air will affect comfort and the home's HVAC system.

Tempering means heating, cooling or otherwise conditioning incoming air before it reaches the occupied space. In a New England winter, several hundred CFM of very cold outdoor air can make a kitchen uncomfortable quickly.

Possible approaches depend on the mechanical design and may include introducing outdoor air through an HVAC system or using dedicated equipment to temper it. Each approach has tradeoffs in cost, energy use, controls and installation complexity.

The residential model-code provision does not make tempering a universal requirement for every make-up-air installation, and local requirements can differ. Ask the HVAC contractor what is appropriate for your house and climate, and why.

Cost varies widely because a simple intake and damper is very different from a retrofit that needs long duct runs, powered airflow, tempering and new electrical controls.

Rather than rely on a national average that may have little to do with your house, look at the things that drive the price up or down:

Whether the system can use a relatively simple intake and damper arrangement

How much new ductwork is needed and how difficult the route is

Whether a fan is needed to provide the required airflow

Whether the incoming air needs to be tempered

Electrical work and controls needed to coordinate the system with the hood

Whether the work happens during open framing or has to be retrofitted through finished walls and ceilings

My advice is simple: get make-up air priced while you are pricing the hood, not after the appliances and cabinetry are ordered. A hood decision can quietly become a much bigger mechanical decision.

Usually an HVAC or mechanical contractor designs and installs the system, coordinating with the hood installer and electrician while the general contractor manages the overall scope.

The problem is rarely that nobody can do it. It's that everyone assumes someone else is.

Don't let this become one of those items where the appliance salesperson assumes the contractor is handling it, the contractor assumes HVAC is handling it, and HVAC doesn't hear about it until rough-ins are finished.

A few simple questions keep it on track:

☐ Who is confirming whether make-up air is required for this hood and this house?

☐ Who is designing it, and where will the supply air enter?

☐ Is the makeup air work included in the HVAC scope and price, in writing?

☐ Has the hood specification sheet been given to the HVAC contractor and electrician?

Before the hood, cabinetry and mechanical rough-ins are finalized.

The hood selection drives the makeup air question. The makeup air answer can affect duct routes, wall and ceiling space, cabinet layout, the HVAC scope, electrical work and the budget. All of that is easier to adjust on paper and more expensive to change after walls are closed or cabinetry is ordered.

If you haven't chosen your hood yet, start with What Size Range Hood Do You Need? Explained. Then come back here once you know the hood capacity you are considering.