The exhaust fan and ductwork move everything the hood captures safely out of the building. These answers cover how commercial exhaust fans and grease ducts work, sizing, installation, venting, electrical draw, cost, cleaning, and lifespan. Requirements vary by jurisdiction and NFPA 96, so confirm specifics with your installer and local authority.
A commercial kitchen exhaust system is the whole path that removes cooking effluent: the hood captures smoke, grease, and heat, grease filters knock out droplets, the exhaust fan pulls the air through welded ductwork, and it discharges outside, usually above the roof. A matched make-up air system replaces the air removed. On Type I systems, fire suppression and NFPA 96 cleaning are part of it. Every piece must be sized and installed to work as one system.
The fan creates the suction that pulls air from the hood through the ductwork and out of the building. For grease (Type I) applications it is typically an upblast rooftop fan that throws grease-laden air up and away from the roof, with a grease-collection cup and a hinged base for cleaning. The fan must be sized to move the hood CFM against the system static pressure. It runs whenever the cookline is in use.
Most commercial grease exhaust uses an upblast fan mounted on the roof at the end of a vertical duct, which discharges up and away and simplifies grease containment. Wall-mounted (sidewall) fans exist and are used where roof access is impractical, but they must discharge away from openings, walkways, and air intakes at code-required distances. Roof mounting is the common default. Placement and clearances are code-driven, so let your designer and AHJ confirm.
No. A general-purpose or residential fan is not built for grease-laden, high-temperature air and does not meet code for a Type I system. Commercial grease exhaust needs a listed fan (typically UL 762 rated) with the right construction, a grease drain, and cleanable design, matched to the duct and hood. Using the wrong fan is a fire hazard and a failed inspection. Use equipment listed for commercial kitchen grease exhaust.
Yes, that is much of its job: pulling out cooking odors, smoke, grease vapor, and heat so the kitchen stays workable and the smells do not build up or drift into the dining room. It only works when make-up air replaces what it removes and when filters are clean. It will not scrub odor for neighbors, though; some jurisdictions require pollution-control or odor-mitigation units on the discharge, so check local rules.
You can, and sometimes it helps clear residual heat and fumes, but it also pulls conditioned air out all night and runs up energy costs, and running exhaust without make-up air makes the building negative. Best practice is to run it whenever cooking equipment is hot and shut it down with the line, or use interlocks and demand controls. Never leave cooking equipment on unattended just to justify running the fan.
Commercial grease exhaust should not be dumped into a soffit; it must run in listed grease duct and discharge outdoors at the code-required height and distance from openings, walkways, and air intakes, usually above the roof. Venting greasy air into a soffit or eave creates a fire hazard and a code violation. Only certain low-grease Type II or residential-style exhaust might use short wall or soffit terminations, so confirm with your AHJ.
Grease-laden air leaves the hood through welded, liquid-tight steel ductwork that runs, ideally vertically, up through the building to a rooftop upblast fan, with required clearances to combustibles or a listed grease-duct wrap or enclosure. The duct has cleanout access panels, slopes so grease drains back or to a collection point, and no dips that trap grease. Because this is fire-critical work, it is designed to NFPA 96 and built by qualified installers.
A qualified contractor fabricates and welds the grease duct liquid-tight, routes it (preferably straight up) to the fan with minimum bends, maintains clearance to combustibles or wraps it with a listed enclosure, adds access panels for cleaning, and slopes it to drain grease. Joints are welded, not taped or screwed. Everything follows NFPA 96 and the mechanical code, and it is inspected before being concealed. This is specialized, fire-critical work, not general ducting.
In broad terms, grease duct must be welded liquid-tight steel, sloped to drain, run with minimum bends toward an exterior discharge, kept at required clearance to combustibles or protected by a listed wrap or enclosure, and fitted with access panels for cleaning. Duct velocity must keep grease moving (often a code minimum around 500 feet per minute). It is all governed by NFPA 96 and the mechanical code, so verify the specifics with your installer and AHJ.
Size the fan to move the hood design CFM against the total static pressure of the duct run, filters, and any pollution-control unit, not just the CFM. A designer calculates static pressure from duct length, size, bends, and accessories, then selects a fan rated for that airflow at that pressure. An undersized fan will not pull rated CFM through real ductwork. Fan and duct are sized together so the system actually captures at the hood.
Duct size is chosen so the design CFM moves at the required velocity, commonly keeping grease-transport velocity above the code minimum (around 500 feet per minute) while staying in a practical range to limit noise and static pressure. Bigger CFM needs a bigger duct; too large a duct lets grease settle, too small raises resistance. The size is calculated from your CFM and layout, so have it engineered rather than guessed.
The upblast fan is mounted on a roof curb over the duct opening, bolted down and sealed, wired to a disconnect and controls, and fitted with its grease-collection cup and a hinge kit so it can tilt up for cleaning. It must be set at the code distance from intakes, walkways, and property lines. Electrical is done by a licensed electrician. It is then tested and balanced with the hood and make-up air.
For a commercial Type I grease system, no, this is licensed work: the grease duct must be welded and inspected, the fan wired by a licensed electrician, and the whole system permitted, tested, and balanced to code and NFPA 96. DIY installation is a fire hazard and will fail inspection. Owners can handle cleaning access and routine upkeep, but design, ductwork, fan mounting, and wiring belong to qualified contractors.
If you have a Type I hood over grease-producing equipment, then yes, a listed exhaust fan moving the air outside is part of the required system. Type II applications also need mechanical exhaust for heat and moisture. Codes require the exhaust to actually remove the effluent and discharge it safely outdoors. Whether your equipment triggers a hood at all depends on what you cook and local code, so confirm with your AHJ.
Nearly every kitchen with cooking equipment needs mechanical exhaust, but the type depends on the menu. Grease- and smoke-producing equipment needs a Type I hood and grease exhaust fan; heat- or steam-producing equipment needs Type II exhaust. A few very limited operations using only low-heat or ventless equipment may avoid a traditional hood. Because it hinges on your specific equipment and local code, confirm your requirement rather than assuming you are exempt.
It varies with fan size and motor horsepower, but many commercial exhaust fan motors draw roughly 3-15 amps, with larger fans more. The exact figure is on the motor nameplate and drives the circuit and disconnect sizing. Fans are wired on their own circuit with a disconnect near the unit. Use the nameplate amps, not a rule of thumb, when your electrician sizes the wiring.
A commercial exhaust fan motor is often in the range of a fraction of a horsepower up to a few horsepower, so power draw commonly runs from a few hundred watts to a couple thousand watts or more for large systems. Multiply the running watts by daily hours to estimate energy use. Variable-speed or demand-control ventilation cuts this by slowing the fan when cooking is light. Check the motor nameplate for exact figures.
Operating cost depends on motor size, run hours, and local electric rates: a small fan may cost a few dollars a day, while a large system running long hours costs more, and it also exhausts conditioned air you paid to heat or cool, which adds indirect cost. Demand-control ventilation that varies fan speed with cooking can cut both. See how to lower restaurant energy bills for ways to trim it.
An upblast rooftop grease fan itself commonly runs about $700-$3,000+ depending on CFM and features, with larger or specialty fans more. That is just the fan; the full exhaust system (hood, welded duct, fan, curb, make-up air, fire suppression, wiring, and balancing) is far more, often tens of thousands installed. Replacement of just a failed fan is much cheaper than a new system. Get local bids for your specific CFM and roof.
Common repairs like a new motor, belt, bearings, or capacitor typically run a few hundred dollars in parts plus labor, while a full fan replacement is more. Rooftop access, hardwiring, and after-hours service raise the bill. Regular cleaning and belt checks prevent many failures. Because a down exhaust fan can shut your kitchen, keep a service relationship so repairs happen fast. Get a quote once the technician diagnoses the specific fault.
A well-maintained commercial exhaust fan often lasts 10-20 years, with the motor, belt, and bearings being the parts that wear and get replaced along the way. Grease buildup, weather, and running without cleaning shorten life. Keeping the fan cleaned during hood service, checking belts, and replacing the grease cup extend it. When the housing corrodes or repairs stack up, replacing the fan is usually cheaper than continued fixes.
Match the fan to your system: the required CFM at the calculated static pressure, a listing for grease exhaust (UL 762 for Type I), an upblast rooftop design for grease with a grease cup and hinge kit, and a motor and electrical rating your service can support. Consider variable speed for energy savings. The fan is selected as part of the engineered system, so let your designer specify it to the duct and hood.
The exhaust fan pulls dirty air out; the make-up air fan (in a make-up air unit) pushes fresh outdoor air in to replace it, often tempered or heated so the kitchen does not get cold or negative. They work as a pair and are usually interlocked so make-up air runs whenever exhaust does. Without the make-up air side, the exhaust fan cannot pull its rated CFM. See ventilation and make-up air explained.
The fan is cleaned as part of the hood and duct cleaning that NFPA 96 schedules by cooking volume, commonly monthly for high-volume or solid-fuel operations, quarterly for moderate, and semiannually for light cooking. The cleaner tilts the fan up, degreases the blades and housing, empties the grease cup, and checks the belt and bearings. Regular service prevents grease fires, keeps airflow up, and is an inspection expectation, so keep the service records.
During scheduled cleaning, a technician shuts off and locks out the fan, tilts it up on its hinge kit, scrapes and degreases the blades, housing, and the base, empties and replaces the grease-collection cup, and checks the belt and bearings before lowering and restarting it. Rooftop grease containment (a cup or absorbent pad) keeps grease off the roof between cleanings. Because it is hot, greasy, rooftop work, most operators leave it to a certified hood-cleaning company.
Record the fan's CFM and static-pressure rating, motor horsepower and voltage, and the duct or roof-curb dimensions it mounts on, plus the model number if legible. The replacement must move the same CFM at the system static pressure and fit the existing curb, or a new curb adapter is needed. Matching just the physical size is not enough; the performance rating matters most. Have your service company confirm the spec before ordering.
If the fan is not clearing the hood, suspect too little make-up air (the building is too negative for the fan to pull), a dirty fan or grease-clogged filters and duct, a slipping or broken belt, or a fan that is simply undersized for equipment added later. Cross-drafts also spoil capture. Clean the filters and fan, verify make-up air, and check the belt first, then have the system tested and balanced.