Make-Up Air: Why Big Range Hoods Need Replacement Air

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A new range hood that roars to life and pulls smoke straight up sounds like a clear upgrade. In a modern, well-sealed house, though, that much exhaust raises a question many remodelers overlook: where does the replacement air come from? Every cubic foot of air pushed outside has to be replaced by air coming back in. If the house is tight and nothing is planned for that replacement, the air will find its own path, sometimes down a water heater flue or a furnace vent. That replacement air is what builders and HVAC contractors call make-up air. This guide explains why exhaust depressurizes a home, when codes commonly require a make-up air system, the backdrafting and carbon monoxide risks, the system types available and what a home inspector will look at in a Front Range house.

Part of our Heating, Cooling & Ventilation guide — start there for the full picture →

What Make-Up Air Is and Why Houses Need It

Make-up air is outdoor air deliberately brought into a house to replace air removed by exhaust equipment. The concept is simple: air out must equal air in. When a fan exhausts air, the house becomes slightly lower in pressure than outdoors, and outdoor air is drawn in through any available opening to balance it.

In older, leaky homes, that balance happens through gaps around windows, doors, sill plates, attic hatches and plumbing penetrations. The house simply breathes through its cracks. Newer homes, and older homes that have been air sealed and weatherized, have far fewer of those leaks. That is good for comfort and energy bills, but it means a powerful exhaust fan has a harder time pulling replacement air in. The fan either moves less air than its rating, or the house depressurizes enough that air comes in through unintended paths.

Several common appliances remove air from a house:

• Kitchen range hoods, often the largest single exhaust device in a home.
• Clothes dryers, which exhaust a steady stream of air while running.
• Bathroom exhaust fans, smaller individually but often several per house.
• Fireplaces and wood stoves, which draw combustion air and send it up the chimney.
• Atmospherically vented water heaters and furnaces, which rely on rising flue gases to vent.
• Whole-house fans and some ventilation systems, depending on their design.

When several of these run at once, especially on a cold Colorado night when the house is shut tight, the combined exhaust can be significant.

Range Hoods and the 400 CFM Threshold

Range hood airflow is rated in cubic feet per minute (CFM). Basic over-the-range microwave fans and small hoods may move a few hundred CFM. Professional-style hoods over large gas ranges can be rated at 600, 900 or more than 1,000 CFM.

Residential codes in many jurisdictions require make-up air for kitchen exhaust hoods above roughly 400 CFM. Under that kind of provision, the make-up air is typically required to be supplied at about the same rate the hood exhausts, and to come through a damper that opens automatically when the hood runs. The exact threshold, exemptions and design rules depend on the code edition and local amendments your jurisdiction has adopted, so confirm locally with the building department before buying a large hood. Front Range municipalities and counties do not all adopt the same edition at the same time.

For anyone shopping for a hood, the practical implication is that choosing a model just above the threshold can add a make-up air system to the project. Some owners select a hood near or below the threshold for that reason, while others choose more capacity and plan for the make-up air from the start. Our range hood installation guide covers sizing and mounting, and our range hood ducting guide explains duct size, length and termination, which also affect real-world airflow. A recirculating unit avoids the exhaust question entirely; our ductless range hood guide covers what those can and cannot do, since they filter grease and some odor but do not remove moisture or combustion gases from the home.

Backdrafting and Carbon Monoxide Risk

The most serious reason make-up air matters is backdrafting. Many Front Range homes still have atmospherically vented gas appliances: a tank water heater with a draft hood, an older furnace or boiler, or a gas fireplace that relies on natural draft. These appliances depend on warm flue gases rising up the vent on their own. That draft is weak. When a large exhaust fan depressurizes the house, it can overpower the natural draft and pull flue gases back down the vent and into the living space.

Flue gases can include carbon monoxide. The EPA lists “back-drafting from furnaces, gas water heaters, wood stoves and fireplaces” among the sources of carbon monoxide indoors, alongside leaking chimneys and furnaces and worn or poorly adjusted combustion devices. EPA’s guidance also recommends having a trained professional inspect, clean and tune up central heating systems, and installing an exhaust fan vented to outdoors over gas stoves.

The CPSC describes carbon monoxide as a “colorless, odorless, poisonous gas” and recommends installing battery-operated CO alarms or alarms with battery backup “on every level of the home and outside sleeping areas,” noting that interconnected alarms are best. CPSC lists symptoms of CO poisoning as “headache, dizziness, weakness, nausea, vomiting, sleepiness, and confusion” and advises getting to fresh air immediately and calling 911 if poisoning is suspected. Our carbon monoxide detector installation guide covers alarm placement and testing.

Signs that backdrafting may be happening include:

• Scorch marks, melted plastic or rust around a water heater draft hood.
• Condensation on windows or walls near combustion appliances.
• Smoke from a fireplace drifting into the room when the range hood runs.
• Stale or exhaust-like odors near the furnace or water heater.
• A CO alarm sounding when large exhaust fans are running.

Any CO alarm activation should be treated as an emergency first and a ventilation puzzle second.

Passive Versus Powered and Tempered Make-Up Air

Make-up air systems range from a simple opening to a fully conditioned supply. The choice depends on the hood size, the climate, local code and the budget.

Passive make-up air

A passive system is a duct from outdoors into the house, usually with a motorized damper that opens when the hood runs and closes when it stops. The hood’s own suction pulls air in through the duct. Passive systems are the simplest and least expensive, but the incoming air arrives at outdoor temperature, and the hood must still overcome the duct’s resistance.

Powered make-up air

A powered system adds a supply fan that pushes outdoor air in at a rate matched to the hood. This takes load off the hood and keeps house pressure closer to neutral. Powered systems usually include a motorized damper and an interlock so the supply fan runs only when the hood runs.

Tempered make-up air

Tempered systems heat, and sometimes cool, the incoming air before it reaches the kitchen. That can be done with an electric duct heater, a hydronic coil or by delivering the air into the return side of the furnace so it mixes with house air. The goal is to avoid blasting below-freezing air at the cook on a January evening.

Interlocked dampers

Whatever the type, the make-up air path typically needs a damper that stays closed when the hood is off. Otherwise the opening becomes a permanent hole in the building envelope, letting in cold air, dust and moisture. An interlock, either a relay wired to the hood or a current or pressure sensor, opens the damper and starts any supply fan whenever the hood runs.

Colorado Winters and the Cost of Tempering

Along the Front Range, winter nights regularly drop well below freezing, and cold snaps can push temperatures far lower. Bringing a few hundred CFM of that air directly into a kitchen while cooking is uncomfortable and can make the hood seem like a draft machine. That is why many contractors recommend tempering on larger systems.

Tempering has its own operating cost. Heating incoming air takes energy, and the bill depends on how often and how long the hood runs, the outdoor temperature, the heating fuel and the efficiency of the tempering method. For a household that runs a large hood for short periods, the added cost may be modest. For heavy, daily use at high fan speed, it can be noticeable. An HVAC contractor can estimate the load for your specific system. Delivering make-up air near the range, rather than across the room, can also reduce how much air the hood needs to move to capture cooking fumes.

Some practical points for cold-climate installations:

• Insulate make-up air ducts where they pass through conditioned space to reduce condensation.
• Use dampers designed to seal tightly when closed.
• Locate the outdoor intake away from dryer vents, flues, the garage and other exhaust outlets.
• Provide a filter or screen at the intake and plan for periodic cleaning.
• Confirm that the interlock works every time the hood runs at any speed.

How Blower Door Testing and Other Fans Fit In

How much make-up air a house needs depends partly on how tight the house is. A blower door test measures air leakage by depressurizing the house with a calibrated fan and recording how much air flows through the envelope. Energy auditors and HVAC contractors can use the results, and related combustion safety testing, to judge whether exhaust equipment is likely to cause backdrafting. Worst-case depressurization testing, where the auditor runs all exhaust fans and checks draft at each combustion appliance, is a common approach in weatherization work.

The EPA’s ventilation guidance cites ASHRAE Standard 62.2, noting that ASHRAE also suggests intermittent exhaust capacities for kitchens and bathrooms, and that “dwellings with tight enclosures may require supplemental ventilation supply for fuel-burning appliances, including fireplaces and mechanically exhausted appliances.” That is a concise statement of the make-up air problem.

Smaller fans add up. A bathroom exhaust fan moves far less air than a big range hood, but two or three running alongside a dryer and a hood can depressurize a tight house more than any single fan. When planning a remodel, it helps to total the exhaust capacity of everything that might run at the same time.

Planning Make-Up Air During a Kitchen Remodel

Make-up air is far easier and cheaper to plan before cabinets, drywall and tile go in than to retrofit afterward. A sensible planning sequence looks like this:

1. Pick the cooking equipment first. A large gas range or cooktop with high-output burners generally calls for more capture than an induction or electric unit, which affects hood sizing.
2. Size the hood for the cooking, not the showroom. Hood width, mounting height and capture area often matter as much as the CFM number. A well-designed hood at moderate airflow can outperform an oversized fan in a poor position.
3. Check the local threshold. Ask the building department which code edition applies and at what airflow make-up air becomes required.
4. Inventory combustion appliances. Note any atmospherically vented water heater, furnace, boiler or fireplace. Their presence raises the stakes for depressurization.
5. Choose the make-up air type. Decide between passive, powered and tempered approaches with the HVAC contractor, and plan the intake location, duct route and controls.
6. Coordinate trades. The electrician, HVAC contractor and hood installer all touch the interlock, so one of them should own the final testing.
7. Test at completion. Run the hood at every speed and confirm the damper opens, any supply fan starts and combustion appliances still draft properly.

Common Make-Up Air Mistakes

Several problems show up again and again in remodeled kitchens:

• No plan at all. A large hood is installed without any make-up air, and the owner discovers the issue when the fan underperforms or a fireplace smokes.
• Damper without interlock. A motorized damper is installed but never wired, so it stays shut or stays open permanently.
• Intake near exhaust outlets. Outdoor air is drawn in next to a dryer vent, a flue terminal or a garage door, bringing contaminants inside.
• Air delivered in the wrong place. Make-up air released across the room can create drafts that pull cooking fumes away from the hood.
• Ignoring maintenance. Intake screens clog with debris and cottonwood fluff, and dampers stick after a few seasons without anyone noticing.

What a Home Inspector Notes

The ASHI Standard of Practice directs inspectors to inspect kitchen, bathroom, laundry and similar exhaust systems, along with installed heating equipment and its vent systems. The same standard states that the inspector is not required to determine “the adequacy of combustion air components.” That means a general inspection is not a combustion safety test or a make-up air design review.

Within those limits, an inspector may note:

• A large range hood with no visible make-up air provision, flagged for evaluation.
• Signs of spillage at a water heater or furnace draft hood.
• Missing CO alarms or alarms in poor locations.
• A make-up air damper that is stuck, disconnected or lacks an interlock.
• Exhaust fans that terminate in attics instead of outdoors.

When a hood seems oversized for the house, or combustion appliances share space with strong exhaust, an HVAC contractor or energy auditor can perform draft testing. Our hiring process guide explains how to scope an inspection, and the heating and cooling hiring hub covers when to add an HVAC specialist.

References

• Carbon Monoxide’s Impact on Indoor Air Quality — U.S. Environmental Protection Agency
• How Much Ventilation Do I Need in My Home to Improve Indoor Air Quality? — U.S. Environmental Protection Agency
• Carbon Monoxide Information Center — U.S. Consumer Product Safety Commission
• ASHI Standard of Practice for Home Inspections — American Society of Home Inspectors

Remodeling a kitchen or buying a home with a big range hood and gas appliances? Talk with us about finding an inspector who will look at venting and combustion safety.