Getting hood airflow right is what actually captures smoke, grease, and heat over your cookline. These answers explain capture velocity, CFM per linear foot, how equipment type and hood style change the numbers, duct velocity, and how exhaust and make-up air work together. Codes and listings vary, so verify sizing with your mechanical designer and local authority.
Capture velocity is the speed of air moving into the hood at the cooking surface, fast enough to grab rising smoke, grease, and heat before it escapes into the room. A hood pulls the right CFM so the plume is drawn up and out instead of rolling off the edges. Too little velocity lets effluent spill; too much wastes energy and conditioned air. Design targets vary by hood style and appliance, so let a mechanical designer confirm.
A common rule of thumb sizes a wall-mounted canopy hood at roughly 200-400 CFM per linear foot, with the higher end for heavy-duty cooking like charbroilers. Island and back-shelf hoods need more per foot because the plume is less contained. These are starting figures; the real number depends on appliance duty, hood style, and whether the hood is listed. Confirm the final CFM with a designer and your hood CFM calculator.
Yes, heavily. Appliances are grouped by duty (light, medium, heavy, and extra-heavy), and the hotter and greasier the cooking, the more CFM per foot the hood needs. Ovens and steamers are light-duty; ranges and fryers are medium to heavy; charbroilers are heavy; solid-fuel and wok cooking are extra-heavy. A hood over mixed equipment is sized for its most demanding appliance. Duty categories drive the design, so map your line before sizing.
Canopy hoods are typically built to overhang the cooking equipment on the exposed front and sides, commonly about 6 inches per side, to help capture the plume as it drifts. Some designs and listings call for more overhang, especially for hotter appliances or higher mounting. More overhang improves capture but adds size and cost. Exact overhang requirements come from the hood listing and local code, so verify before fabricating the hood.
A wall-mounted canopy has a wall behind it that helps contain the rising plume, so it needs less airflow. An island (single- or double-island) hood is open on more sides, so the plume can drift and it needs noticeably more CFM per linear foot to capture the same cooking. That is why island layouts are more expensive to ventilate. If you can, placing the cookline against a wall lowers exhaust and make-up air demand.
Almost never. A 600 CFM figure is a residential range-hood number; commercial canopy hoods over real cooking equipment usually move several thousand CFM. Even a short commercial cookline can require 2,000-4,000+ CFM depending on duty. A 600 CFM unit would fail to capture grease and smoke and would not meet code for a Type I application. Size the hood to the equipment and length, not to a household spec sheet.
Yes. A charbroiler is a heavy-duty, high-grease, high-heat appliance, so the hood section over it needs more CFM per linear foot than one over a range, and far more than over an oven or steamer. Solid-fuel charbroilers need even more and usually a dedicated hood and duct. When a charbroiler shares a hood with lighter equipment, the whole hood is often sized up to handle it. Plan extra airflow wherever you charbroil.
They must be balanced. For every cubic foot the hood exhausts, replacement (make-up) air has to come in, or the kitchen goes strongly negative, doors slam, pilots blow out, and the hood stops capturing. Make-up air is typically sized to replace roughly 80-90% of the exhaust, leaving the kitchen slightly negative so odors do not push into the dining room. Size them together; see ventilation and make-up air explained.
Grease exhaust ducts are designed to keep air moving fast enough to carry grease particles instead of letting them settle. Codes commonly set a minimum around 500 feet per minute, and duct is often designed in the 1,500-2,000 feet-per-minute range for good transport without excessive noise or static pressure. The duct size is chosen to hit that velocity at the design CFM. Requirements vary, so verify duct sizing and NFPA 96 details with your designer and AHJ.
The higher a canopy hood is mounted above the cooking surface, the more the plume can spread and cool, so the hood needs more CFM to capture it. Mounting too low interferes with the cook and violates clearances. There is a practical sweet spot, often with the lower edge around 6.5-7 feet off the floor, balancing capture, safety, and headroom. Because height changes the required airflow, set it during design, not after.
Common causes are too little exhaust CFM, not enough make-up air (the kitchen is too negative for the fan to pull), a dirty or blocked grease filter, the hood mounted too high or too small to overhang the equipment, or an unbalanced system after equipment changes. Cross-drafts from doors and fans also spoil capture. Check filters and make-up air first, then have the system rebalanced. Adding hotter equipment under an old hood is a frequent culprit.
It depends on what you cook. Grease- and smoke-producing equipment (fryers, griddles, ranges, charbroilers, woks) needs a Type I grease hood with filters, fire suppression, and welded grease duct. Heat-, steam-, or moisture-producing equipment (dishwashers, steamers, some ovens) needs a Type II hood. Many kitchens need both. Sizing then follows the equipment duty and hood style. Not sure if you even need one? Try the do I need a hood tool.
At a high level, a Type I hood needs enough exhaust CFM to capture the plume, matched make-up air, welded liquid-tight grease ductwork run to the exterior, listed grease filters, fire suppression, and clearances to combustibles, all cleaned per NFPA 96. Type II hoods handle heat and steam with lighter requirements. The specific numbers come from the mechanical code, the hood listing, and your AHJ, so use a designer to size and permit the system.
The exhaust fan is selected to move the hood design CFM against the total static pressure of the ductwork, filters, and any pollution-control unit, not just the raw CFM. An undersized fan will not pull rated airflow through the real duct run. A designer calculates static pressure, then picks a fan (usually an upblast rooftop unit for grease) rated for that CFM at that pressure. Get the fan and duct designed together for the system to perform.
Not usually. Type II hoods handle heat, steam, and moisture rather than grease, and are sized to remove that load, which is often less demanding than capturing a greasy, smoky plume. A condensate hood over a dish machine, for example, is sized to the moisture it must carry away. Type I hoods over heavy cooking generally move more air. Either way, size to the specific appliance load rather than assuming a number.
A balancing technician measures airflow at the hood, usually by taking velocity readings across the filter face or in the duct with an anemometer or flow hood, then compares the result to the design CFM and adjusts fan speed or dampers. This is part of test-and-balance during commissioning and is worth repeating if capture problems appear. Measured airflow, not the fan nameplate, tells you whether the system is actually performing.
Yes. Over-exhausting wastes energy, dumps heated or cooled conditioned air outside, and can pull so much air that make-up air cannot keep up, making the kitchen negative and hurting capture. It also raises operating costs year-round. The goal is enough airflow to capture the plume reliably, not the biggest fan possible. Right-sizing (and adding demand-control ventilation that varies fan speed with cooking) saves money while keeping the line clear.
Deck and pizza ovens are generally lighter-duty for grease, so their hood sections need less CFM per foot than fryers or charbroilers, but high-heat and any grease from toppings still matter. A conveyor or deck-oven line is often ventilated with a canopy sized to the oven openings and heat load. Wood- or coal-fired ovens are extra-heavy-duty and need much more airflow plus a dedicated duct. Size to the specific oven and fuel.
Fryers are medium- to heavy-duty for grease, so a canopy over a fryer battery is sized toward the higher end of the CFM-per-foot range, often several hundred CFM per linear foot depending on the hood style and listing. Because fryers throw a lot of grease-laden vapor, filters and a properly sized grease duct matter as much as raw CFM. Size the section to the fryers and any adjacent heavy equipment.
An undersized hood cannot capture the full plume, so grease and smoke roll into the kitchen, coating surfaces, setting off alarms, and creating a fire and health hazard. You get poor air quality, hot working conditions, and inspection problems. Adding hotter equipment under an existing hood is a common way to end up undersized. The fix is rebalancing or, often, a larger hood and fan, so size correctly from the start.
Largely, yes. The hood must be long enough to cover and overhang the entire lineup of cooking equipment, so the cookline length sets the hood length, and the hood length times the CFM-per-foot for the equipment duty sets the total exhaust CFM. A longer line or hotter equipment means more airflow and a bigger fan and duct. Lay out the full cookline before sizing so the hood covers everything with proper overhang.
A hood tested and listed to UL 710 can carry a manufacturer-certified exhaust rate that is often lower than the prescriptive value a field-built hood would require, because it has been proven to capture at that airflow. Using a listed hood can cut exhaust and make-up air CFM, saving energy and equipment cost. The listing specifies the minimum airflow, so follow the manufacturer data and confirm acceptance with your AHJ.
There is no single national number; the minimum comes from the mechanical code method for your appliance duty and hood style, or from a listed hood's certified rate. Codes set minimum duct velocities and capture requirements rather than one CFM figure. The practical minimum is whatever reliably captures your plume and satisfies the code path your designer uses. Because it varies by jurisdiction and equipment, have the CFM engineered rather than guessed.
Overhang extends the hood beyond the edges of the equipment so the plume, which drifts outward as it rises, still lands under the hood and gets pulled in. Front and side overhangs matter most because that is where the plume escapes; a wall behind the hood substitutes for a back overhang. More overhang lets a hood capture reliably at a given CFM. Listings specify overhang, so build it in rather than trimming the hood to fit.
Yes. Wok ranges and other high-heat cooking produce a fast, hot plume that is hard to capture, so they are treated as heavy- to extra-heavy-duty and need more CFM plus often a specialized wok hood with a water-wash or extended capture design. Standard canopy CFM sized for a range will spill smoke off a wok line. Tell your designer you are cooking on woks so the hood and airflow are sized for it.
Make-up air is essential to capture. If the exhaust fan pulls out more air than can flow back in, the kitchen goes negative and the fan cannot move its rated CFM, so the plume spills even with a big hood. Balanced make-up air (replacing most of the exhaust) lets the hood work as designed and keeps doors from slamming. Whenever capture is poor, check make-up air along with the exhaust side.
Solid-fuel cooking (wood, charcoal, or a wood-fired oven) is extra-heavy-duty and typically requires its own hood and dedicated exhaust duct, higher airflow than gas or electric equipment, and added features like a spark arrestor and often a separate ash and grease strategy. It cannot simply share a standard grease hood. The airflow is set case by case, so have solid-fuel equipment ventilated by a designer experienced with NFPA 96 solid-fuel rules.
Start by listing your cooking equipment and its duty (light through extra-heavy), measure the total cookline length the hood must cover with overhang, and multiply the hood length by an appropriate CFM-per-linear-foot value for the heaviest duty and hood style. That gives a working estimate to compare hoods and plan make-up air, which a designer then refines to code. Our hood CFM calculator walks through this quickly.
It can, because a taller ceiling usually means the hood is mounted higher above the cooking surface, and higher mounting lets the plume spread, requiring more CFM to capture. If headroom forces a high mount, expect to increase airflow or use a hood designed for the height. Keeping the hood at the recommended height over the equipment, even under a tall ceiling, keeps CFM (and energy cost) down. Set mounting height during design.
Usually the system is out of balance: not enough make-up air, an undersized or dirty hood, a fan not pulling rated CFM, or hotter equipment added under an old hood. Heat also builds when make-up air is not cooled or when the hood captures grease but not radiant heat. Check filters and make-up air, then have the system tested and balanced. If the load grew, the hood or fan may simply be undersized now.