Exhaust is only half of a kitchen ventilation system. Every cubic foot the hood pulls out has to be replaced, and that replacement air, called make-up air, is where many kitchens go wrong: back-drafting appliances, doors that will not open, and cold drafts on the line. These answers explain make-up air units, when they are required, how they are sized and conditioned, and how exhaust and ventilation work together. The final design belongs to a mechanical engineer and your local authority.
Make-up air is the fresh outdoor air brought into a kitchen to replace what the exhaust hood removes. Without it, the fan cannot pull air out efficiently, the room goes strongly negative, and doors, drafts, and appliance back-drafting result. Make-up air is delivered by a dedicated unit and is usually balanced to roughly 80 to 90 percent of exhaust. Learn more in our ventilation and make-up air guide.
If your exhaust moves enough air, yes. Most commercial kitchens with a real cooking hood require dedicated make-up air, and code commonly mandates it once exhaust exceeds a threshold set by your jurisdiction. Without it, the hood underperforms and gas appliances can back-draft. Estimate your need with our make-up air calculator, then confirm with a mechanical engineer and your AHJ.
You need make-up air whenever exhaust would otherwise pull the room too negative, and code typically requires a dedicated system once exhaust passes a set airflow threshold. Practically, any commercial cooking hood needs planned replacement air. Estimate your needs with our make-up air calculator, then let a mechanical engineer confirm the trigger and volume for your jurisdiction.
Make-up air is usually set to replace roughly 80 to 90 percent of the exhaust volume, leaving the kitchen slightly negative relative to the dining room so odors do not migrate. The exact percentage depends on other building airflows and local code. Get a starting figure from our make-up air calculator, then have an engineer finalize it as part of a balanced design.
Many codes require a dedicated make-up air system once exhaust exceeds a defined airflow threshold, but the exact number and how it is applied vary by jurisdiction and code edition. Below that point, infiltration may cover replacement air; above it, you need engineered make-up air. Because the trigger is local, confirm the applicable threshold with your mechanical engineer and AHJ rather than assuming a national figure.
Generally yes. Higher exhaust volumes pull the room strongly negative, and most codes require dedicated make-up air once exhaust passes a set threshold. Even below a code trigger, a high-CFM hood usually needs planned replacement air to capture properly and to keep gas appliances from back-drafting. Check your exhaust total against local requirements with an engineer to know whether make-up air is mandatory.
A make-up air unit (MAU) draws outdoor air, filters it, and often tempers it (heating and sometimes cooling) before delivering it into the kitchen to replace exhausted air. It runs interlocked with the exhaust fans so replacement air is supplied whenever the hood operates. The unit is sized to a target percentage of exhaust and controlled to keep the room's pressure balanced. Read more in our ventilation guide.
A make-up air unit (MAU) is dedicated rooftop or wall equipment that supplies fresh, filtered, and usually tempered outdoor air to replace what the kitchen exhaust removes. Unlike a recirculating HVAC unit, it delivers 100 percent outside air and runs with the hood. It is what keeps the kitchen pressure balanced and the hood capturing correctly. A mechanical engineer sizes and specifies the MAU for your exhaust load.
The MAU is sized to deliver a target percentage of total exhaust, commonly about 80 to 90 percent, adjusted for other supply and exhaust in the building so the kitchen stays slightly negative. Heating and cooling capacity are then set for the local climate. Get a starting airflow figure from our make-up air calculator, but an engineer performs the full sizing including tempering loads.
In cold climates, usually yes. Dumping large volumes of raw winter air onto the cooking line is uncomfortable and can affect food and equipment, so make-up air units commonly include heating to temper the incoming air. Whether and how much heating is required depends on your climate, airflow, and local energy code. A mechanical engineer sizes the tempering to your location and exhaust volume.
It often needs tempering, at minimum heating in cold climates and sometimes cooling or dehumidification in hot, humid ones, so the incoming air does not create drafts or comfort and moisture problems. The level of conditioning depends on climate, local energy code, and how the air is distributed. Fully conditioning 100 percent outside air is costly, so an engineer balances comfort against operating cost for your build.
Many do. Make-up air units are commonly available as direct-fired or indirect-fired gas heated units, and some add electric heat or cooling, to temper the large volume of outdoor air they introduce. Whether yours includes heat depends on climate and code. Cooling and dehumidification are also options in hot, humid regions. The mechanical engineer specifies the heating and cooling capacity for your kitchen.
In a direct-fired unit, the gas burner sits in the airstream, so combustion products mix into the supply air; it is efficient and common for kitchen make-up air where codes allow. In an indirect-fired unit, the burner heats a heat exchanger and combustion is vented separately, keeping products out of the supply air. Choice depends on code, application, and efficiency needs; your engineer selects the right type.
A make-up air unit delivers 100 percent outdoor air to replace exhaust and runs interlocked with the hood, keeping kitchen pressure balanced. An air handling unit (AHU) primarily conditions and recirculates indoor air for comfort, mixing in only a portion of outside air. They serve different jobs; a kitchen typically needs a dedicated MAU for the hood plus separate HVAC for dining comfort. An engineer coordinates both.
Yes. Make-up air units include filters to clean incoming outdoor air before it enters the kitchen, protecting the space and downstream components. Filter type and rating vary by unit and application, and filters must be inspected and changed on a schedule to keep airflow and air quality up. Clogged filters reduce supply airflow and unbalance the kitchen, so include them in routine maintenance.
Often yes. When a make-up air unit delivers heated or cooled air, or when ducts run through unconditioned space, insulation is used to prevent energy loss and condensation. Requirements come from the local energy and mechanical codes and the specific design. Uninsulated tempered-air ducts waste energy and can sweat. Your mechanical engineer specifies insulation where the code and design call for it.
They can, depending on airflow and local code. Larger air-moving units are often required to have duct smoke detectors that shut the unit down on smoke to prevent spreading products through the building. The threshold and details are set by the mechanical and fire codes and your AHJ. Because it varies with unit size and jurisdiction, let your engineer determine whether duct detectors are required for your MAU.
A make-up air damper is a controlled louver that opens to admit replacement air when the exhaust fan runs and closes when it stops, preventing outdoor air, drafts, and energy loss when the kitchen is idle. In simple systems it may be a motorized damper interlocked with the hood; in engineered systems the MAU handles it. Proper damper control is part of keeping the kitchen balanced and efficient.
Because exhaust removes large volumes of air that must be replaced. Without make-up air, the kitchen goes strongly negative: the hood cannot capture properly, exterior doors are hard to open, gas appliances can back-draft dangerous flue gases, and comfort suffers. Code requires it above set exhaust thresholds for exactly these safety and performance reasons. Our ventilation guide explains the pressure balance in detail.
The room becomes strongly negative. The hood starves and stops capturing smoke and grease, exterior doors stick or slam, drafts pull through every gap, pilot lights blow out, and gas appliances can back-draft carbon monoxide indoors. Heating and cooling also suffer as uncontrolled air infiltrates. These are safety and comfort failures, which is why codes require balanced make-up air on real cooking hoods.
An energy recovery ventilator can pre-condition incoming air and help with general ventilation, but it is usually not a substitute for a dedicated kitchen make-up air unit that must supply large, hood-interlocked volumes of tempered air and tolerate grease-laden environments. ERVs are better suited to general building ventilation. Whether one can play any role in your kitchen design is a question for your mechanical engineer.
A make-up air unit is typically set on the roof or an exterior wall, ducted into the kitchen, connected to gas and/or electric power, and interlocked with the exhaust fans and controls so it runs with the hood. It is then balanced against the exhaust. This is a licensed mechanical and electrical job coordinated with the hood installation and inspected before use, not a DIY task.
A make-up air unit is a significant cost that rises with airflow, the amount of heating or cooling, and roof and duct work, and installation adds substantially. A tempered, high-capacity MAU costs far more than a basic untempered one. Because climate, size, and building conditions drive the price, get itemized quotes from mechanical contractors rather than relying on a single number when budgeting your kitchen.
A kitchen exhaust fan is the powered fan, usually an upblast unit on the roof or an inline fan in the duct, that pulls air, smoke, grease, and heat from the hood through the duct and discharges it outdoors. It is the engine of the exhaust system, sized to the required CFM and duct static pressure. For grease hoods it must be a listed commercial grease-rated fan.
The fan creates suction that pulls the hood's captured plume through the grease baffle filters and up the duct, then discharges it outside, typically upward and away from the roof. It must overcome the resistance of the duct and filters, so it is sized to both airflow and static pressure. On grease systems the fan is a listed upblast type with a hinged base for cleaning access.
Yes. Commercial kitchen exhaust, especially grease-laden air from a Type I hood, must be ducted to the outdoors through a dedicated, code-compliant grease duct and discharged safely away from air intakes and combustible surfaces. Venting into an attic, wall cavity, or back into the building is prohibited and dangerous. The termination point and clearances are set by code and confirmed by your AHJ.
No. Discharging grease-laden or even heat-and-moisture exhaust into an attic or any enclosed building space is unsafe and against code; it creates fire risk, grease buildup, moisture damage, and air-quality problems. Commercial kitchen exhaust must be carried in a dedicated duct to an approved outdoor termination. If your current setup vents into an attic, have it corrected by a qualified contractor and inspected.
Commercial kitchen ventilation is the balanced system that removes cooking heat, smoke, grease, and moisture and replaces it with fresh outdoor air. It combines the hood, grease or heat duct, exhaust fan, and a make-up air system, all sized and interlocked to keep the kitchen safe, comfortable, and slightly negative. It is an engineered assembly, not a single appliance, designed together by a mechanical engineer.
Good ventilation removes smoke, grease, heat, steam, and combustion byproducts that would otherwise harm air quality, comfort, and safety, and it feeds gas appliances the air they need to burn cleanly. It also prevents grease buildup that fuels fires and protects staff from heat stress. Balanced exhaust and make-up air together are what make a commercial kitchen safe and workable, which is why the requirements mandate them.
For commercial kitchens with cooking equipment, yes. Codes require hoods, exhaust, and make-up air matched to the cooking duty to remove grease, smoke, heat, and moisture and to supply combustion and replacement air. The specifics depend on your equipment and jurisdiction. Review the general requirements overview, then confirm exactly what your kitchen needs with a mechanical engineer and your local authority.
Start by verifying the system is balanced: exhaust and make-up air measured and set so the room is only slightly negative. Keep baffle filters and ducts clean, confirm the fan meets the required CFM and static pressure, and make sure make-up air is actually being delivered and tempered. Persistent heat, smoke spillage, or door problems signal an unbalanced or undersized system; have a balancing contractor or engineer assess it.
A mechanical engineer starts from the cooking equipment and its duty, sizes hood dimensions and exhaust CFM, designs the grease or heat duct and fan, then sizes make-up air to keep the kitchen balanced, adding tempering for the climate. Everything is interlocked and coordinated with fire suppression and electrical. Our ventilation guide walks through the concepts, but the stamped design comes from an engineer.
A make-up air fan is the supply blower, part of a make-up air unit, that pushes fresh outdoor air into the kitchen to replace exhausted air. It runs interlocked with the exhaust so replacement air arrives whenever the hood operates, and it is sized to a target percentage of exhaust. In tempered units it moves the air across heating or cooling before it enters the space.