Kitchen exhaust and grease duct field guide (NFPA 96)

The kitchen exhaust system and the fire it carries

A commercial kitchen exhaust system is the path that takes grease, smoke, and heat off the cookline and throws it out over the roof. It is three parts working as one: the hood that captures the plume over the appliances, the grease duct that carries it up through the building, and the exhaust fan on the roof that pulls the whole train. Every fryer, griddle, range, and charbroiler under that hood sheds grease into the airstream, and the airstream deposits it on the inside of the system the entire way to the fan.

That grease is the hazard. It is fuel, it is everywhere the air touches, and a flare-up on the cookline can light it. A grease fire that starts at a fryer can run up the duct in seconds and find the building structure around it. This is the failure NFPA 96 is written to prevent, and it is why the kitchen exhaust system is treated as fire-protection equipment, not just ventilation.

Two related systems live in the same hood and get covered in their own guides. The wet-chemical suppression system that fights the fire when it starts is its own scope, and the makeup air that replaces what the fan exhausts is another. This guide is the exhaust and the grease duct: the steel that carries the grease, the clearances that keep the fire off the building, and the cleaning that keeps the fuel from building up in the first place.

What is NFPA 96?

NFPA 96 is the standard for ventilation control and fire protection of commercial cooking operations. It is the document that governs the whole grease side of a commercial kitchen: the hood, the grease removal devices, the welded duct, the clearances to combustibles, the exhaust fan, the fire-extinguishing system over the cooking equipment, and the periodic cleaning that keeps grease from accumulating. If it touches grease-laden vapor, NFPA 96 has something to say about it.

It is a standard, not a law on its own. It becomes enforceable when a jurisdiction adopts it, usually through the fire code and the mechanical code, and the authority having jurisdiction, the AHJ, enforces the adopted edition with any local amendments. The mechanical code, commonly the International Mechanical Code, carries its own commercial kitchen exhaust requirements that run alongside NFPA 96, and the two are written to agree on the major points.

The standard is revised on a cycle, so section numbers and some specifics shift between editions. Cite NFPA 96 for the requirement and confirm the exact figure against the edition the jurisdiction actually adopted before you put it on a submittal or a report. When this guide gives a number, treat it as the common requirement and verify it locally, because the AHJ is the one who signs off on the kitchen.

The hood: capture and containment over the cookline

The hood is the first piece, and its job is capture and containment: catch the grease-laden plume rising off the appliances and hold it under the canopy until the exhaust pulls it into the duct. A hood that captures keeps the smoke and grease out of the cook's face and off the kitchen ceiling. A hood that does not capture spills the plume into the room no matter how good the duct and fan behind it are.

Capture depends on the hood style, the overhang past the front and sides of the appliances, the distance from the cooking surface to the hood lip, and the exhaust rate moving through it. A heavy-duty charbroiler throws a hot, fast plume that takes more exhaust and more overhang to contain than a light oven does. The hood is sized and listed for the appliances it covers, so a hood that was right for the original lineup can fail to capture once the kitchen swaps in higher-duty equipment.

The hood is also where the grease starts coming out of the air. It collects in the filters, in the plenum behind them, and in the trough and cup the design provides to catch the runoff. From there the part that escapes the filters goes up the duct as a film. Everything downstream is built around the fact that the hood never catches all of it.

What is a Type I hood, and how does it differ from Type II?

A Type I hood is a grease hood. It is built for cooking that produces grease and smoke, the fryers, griddles, ranges, charbroilers, and woks, and it carries the full system behind it: listed grease filters, a welded liquid-tight grease duct, the clearances and fire protection NFPA 96 requires, and a fire-extinguishing system over the appliances. If the appliance makes grease-laden vapor, it lives under a Type I hood, full stop.

A Type II hood is a heat-and-steam hood. It handles the water vapor and heat off a dishwasher, a steam kettle, or a similar non-grease appliance, and it is a far simpler animal. No grease, so no grease filters, no welded grease duct, no grease-duct clearances, and no suppression over it. Its duct only has to handle moisture and heat, which is why the construction rules are lighter.

Getting the type wrong is a real failure, and it goes one direction. A Type II hood hung over a grease appliance has none of the protection that appliance needs: the duct is not built or cleaned for grease, and there is no suppression. The plume loads an unlined, un-welded duct with fuel and nobody is treating it as a fire system. When you see a steam-style hood over a fryer, that is a finding, not a detail.

Grease filters and the grease cup

The grease filters in the hood are the first stage of grease removal, and NFPA 96 requires listed grease filters or baffles. The trade ran on mesh filters for years, but mesh is not tested or listed for this duty and it carries a fire-spread problem: grease loads up in the mesh and the mesh itself can carry flame. The standard moved the trade to listed baffle filters, and a mesh filter in a commercial hood is a finding to correct.

A baffle filter works by turning the air sharply through metal blades. The air makes the turn, the heavier grease droplets cannot, and they strike the baffle and drain down rather than passing through. That is the whole trick: inertia separates the grease from the air. Because the grease drains instead of caking in a screen, the baffle keeps working and it does not become a flame path the way a loaded mesh screen does.

The grease the baffles knock out has to go somewhere, so the hood has a trough at the bottom of the filter bank and a removable grease cup or drain that catches it. The staff empties the cup on a routine, because an overflowing cup puts liquid grease back where you do not want it. The filters come out and get washed on a schedule too, since a clogged filter loses capture and pushes more grease past it into the duct. Clean filters are the cheapest grease control in the whole system, and the first one people let slide.

Why must a grease duct be welded?

Because the duct has to be liquid-tight and it has to survive a fire inside it, and only a continuous weld gives you both. The grease that escapes the filters condenses on the inside of the duct as a flammable film. If the duct has seams, screws, or unwelded joints, liquid grease finds them and weeps out into the building cavity, and now you have fuel sitting against structure where nobody can see it or clean it.

NFPA 96 requires a grease duct to be built of welded steel with a continuous liquid-tight external weld on all seams and joints. No screws through the duct wall, no riveted seams, no sealant doing the job a weld should do. The point is a single sealed channel from the hood to the fan, with the grease staying inside it where it can be cleaned and where a fire is contained, not leaking into the space around it.

The fire is the other half. A grease fire inside the duct can run hot enough to fail a joint that was only screwed or sealed. A continuous weld holds the duct together as a sealed enclosure under that heat, so the fire stays in the steel instead of breaking out into the building. The welded liquid-tight duct is the rule a grease system lives or dies by, and an un-welded or leaking duct is one of the more dangerous things you can find on a kitchen.

How NFPA 96 builds the grease duct

Beyond the weld, NFPA 96 sets the metal and the geometry. The field-built grease duct is commonly 16-gauge carbon steel or 18-gauge stainless steel, welded liquid-tight, or a factory-built grease duct assembly listed for the purpose. Galvanized steel, aluminum, and fiber-reinforced plastic are not allowed. Galvanizing is out because the zinc coating volatilizes at fire temperatures and fails, so the duct that has to hold a fire is built of bare carbon or stainless steel.

The duct is pitched, not run dead level. The standard requires a slope back toward the hood or to a listed drain so liquid grease runs to a low point that can be cleaned, instead of pooling in a flat section where it collects and bakes. Horizontal runs are kept to the minimum the building allows for the same reason, since every flat foot is a place grease wants to settle.

The grease duct carries grease and nothing else. No other system shares it, no other duct ties into it, no dampers sit inside it unless they are listed for grease duct service. It runs as a dedicated, sealed path from the hood to the fan, with cleanout access along the way and the clearances and enclosure covered in the next sections. Confirm the gauge, the slope, and the materials against the adopted edition of NFPA 96 and the listing of any factory-built duct, because the AHJ checks these on the rough-in before the duct is closed up.

What clearance does a grease duct need to combustibles?

The common requirement is 18 in of clearance from a grease duct to combustible construction, and it exists because the duct gets hot, hot enough during a duct fire to ignite wood framing, paper-faced insulation, or anything else sitting against it. The air gap is the simplest fire protection there is: keep the burnable material far enough from the hot steel that it cannot light.

You rarely get 18 in of free air in a real building, so the standard allows the clearance to be reduced when you add a listed protection method. That means a listed grease-duct wrap rated for reduced clearance, a listed clearance-reduction system, or a fire-rated enclosure around the duct. The reduction is only as good as the listing, so the wrap or system has to be installed exactly per its instructions and its tested clearance, not a number someone remembers from another job.

This is the detail inspectors zero in on, because it is where a duct that passed the weld check still fails the building. A grease duct run tight against floor joists with no wrap and no rated shaft is a fire waiting for an ignition source it already has inside it. Confirm the required clearance and the listed reduction against NFPA 96 and the product listing, and confirm the building construction around the duct actually matches what the protection was tested for.

The grease-duct wrap and the fire-rated shaft

When a grease duct passes through the building or runs near combustibles, it gets enclosed, and there are two common ways to do it. One is a listed grease-duct fire wrap, a flexible insulation blanket wrapped around the duct and rated to a fire-resistance period, commonly a 2-hour rating. The other is a fire-rated shaft, a built enclosure of rated construction that the duct runs inside as it travels through floors and occupied spaces.

The wrap does two jobs at once. It is tested to maintain the duct's fire-resistance rating from the inside, holding a duct fire in for the rated period, and it is also listed to reduce the clearance to combustibles, so the wrapped duct can run closer to building materials than a bare one. That dual rating is why the wrap is so common on tight retrofits: it solves the enclosure and the clearance in one product. It only works installed to the listing, with the right number of layers, the right overlaps, and the listed banding.

The fire-rated shaft is the other path, more common in new construction and multi-story buildings, where the duct rises through a rated shaft enclosure from the kitchen to the roof. Either way, the goal is the same: the grease duct travels through the building inside something rated to contain a duct fire for long enough for people to get out and the fire service to respond. Which method applies is a code and design call, so verify the required rating and the method against the adopted code and the listing.

Cleanout access panels

A grease duct is useless if you cannot clean it, so NFPA 96 requires cleanout access panels along the run. The duct collects grease over its whole length, and the cleaner has to be able to reach every interior surface to take it back to bare metal. No access means an uncleanable stretch, which means a section that quietly loads with fuel between every service.

The access goes where grease collects and where the cleaner needs to reach. Panels are required at intervals along horizontal runs, commonly not more than 12 ft apart, and at each change of direction where grease builds up and where a brush cannot pass a bend. Vertical risers get access at the base and, in tall runs, on the way up so each floor's section can be reached. The panels are built of the same material and gauge as the duct and made grease-tight, because an access panel is still part of the sealed grease channel when it is closed.

Missing or buried access is a real failure with two faces. One, the cleaner cannot reach the duct, so a section never gets cleaned and the grease builds toward a fire. Two, a panel that leaks or was added without being made grease-tight becomes the weep point the welded duct was supposed to eliminate. Confirm the spacing and the panel construction against the adopted edition of NFPA 96, and confirm on site that the panels are actually accessible, not hidden above a hard ceiling someone built over them.

The rooftop grease exhaust fan

At the top of the duct is the exhaust fan, and on a grease system it is almost always a rooftop upblast fan or a utility set built for grease duty. Upblast means it throws the exhaust up and away from the roof surface, so the grease-laden air is not dumped onto the membrane right at the fan. The fan pulls the whole system, holding the duct under negative pressure so any leak draws air in rather than pushing grease out, which is part of why a tight, welded duct and a working fan go together.

The fan is built to be cleaned. A rooftop grease fan sits on a hinged base, the curb mount, so the cleaner can tip the fan up and reach the fan housing, the wheel, and the top of the duct. A fan bolted down with no hinge and no access is a fan whose interior never gets cleaned, and the wheel loads with grease until it is out of balance and out of code. The motor is kept up out of the greasy airstream so it is not running in the grease it is pulling.

Grease that reaches the fan has to be contained, not left to run onto the roof. NFPA 96 requires the fan to drain to a grease receptacle, commonly a container not exceeding 1 gal, mounted so it is visible and can be emptied. Grease on the roof is both a slip-and-fire hazard and a sign the containment is missing or overflowing. When you walk a roof, the grease cup and the absorbent pads under the fan tell you fast whether anyone is maintaining the system below.

Grease containment, from the cup to the roof

Grease containment runs the length of the system, because grease comes out of the air at every stage and each stage needs somewhere to put it. At the hood it is the filter trough and the grease cup. Along the duct it is the slope to a low-point drain. At the fan it is the drain to the rooftop receptacle. The design assumes grease will collect and gives it a controlled place to go, instead of letting it find the building.

The rooftop is where containment most often breaks down in plain view. An upblast fan with no grease cup, an overflowing receptacle, or no absorbent containment under it lets grease spread across the membrane. That is a fire hazard on the roof, a slip hazard for anyone working up there, and a sure sign the kitchen's maintenance has slipped. Many roofs over busy kitchens now use a containment system, a curb-mounted pad and tray, specifically to keep the grease off the membrane and out of the roof drains.

Containment is a maintenance item, not a one-time install. The cups fill, the pads saturate, and the troughs need emptying on the same kind of routine as the cleaning. A containment system that nobody empties is the same as no containment a few months in. Note the rooftop and hood containment condition whenever you are on the system, because it is the cheapest tell for how the whole kitchen is being run.

How often must a kitchen grease duct be cleaned?

Cleaning frequency is set by how hard and how greasy the kitchen cooks, and NFPA 96 ties it to cooking volume and fuel type rather than a single calendar number. The common intervals run from monthly for the heaviest grease load to annual for the lightest, with the system cleaned by qualified, certified personnel and taken back to bare metal each time. The frequency is a floor, and the AHJ can require more often based on what an inspection finds.

This is the single most important ongoing requirement in the whole standard, and it is the one that prevents the fire. Every other detail, the weld, the clearance, the wrap, is there to contain a fire or keep it off the building. Cleaning is the one that keeps the fuel from accumulating in the first place. A perfectly built, perfectly clearanced grease duct that nobody cleans is just a well-protected fuel load waiting for an ignition source it gets every service hour.

Bare metal is the standard, and it matters. A cleaning that wipes the hood and leaves the duct and fan loaded is not a cleaning, it is paperwork. The whole grease path, hood, filters, plenum, the full duct run through every access panel, and the fan, has to be cleaned to bare metal, and the cleaner leaves a service sticker showing the date and the company. Skip the cleaning, or under-clean it, and you own the grease fire. There is no softer way to say it. Confirm the required frequency and the qualifications of the cleaner against NFPA 96 and the AHJ.

The volume-based cleaning and inspection schedule

NFPA 96 sets the cleaning interval against the type and volume of cooking, and the heavier the grease load, the shorter the interval. The intervals below are the common benchmarks. Confirm them against the adopted edition and the AHJ, since the standard is the source and the jurisdiction can tighten it.

Solid-fuel cooking, wood and charcoal, carries the most frequent interval, commonly monthly, because it produces the most grease and creosote and adds spark and ember risk. High-volume operations, the charbroilers, woks, and 24-hour kitchens, fall around quarterly. Moderate-volume sit-down restaurants run around semi-annual. Low-volume operations, seasonal kitchens, churches, day camps, fall around annual. The menu on the sign does not set the interval. A diner that runs a charbroiler ten hours a day loads grease like a high-volume operation no matter what it calls itself.

Separate from the cleaning is the inspection. NFPA 96 calls for the exhaust system to be inspected by a qualified person on a schedule that tracks the same volume tiers, and the inspection is what decides whether the system actually needs cleaning before the next scheduled date. When an inspection finds grease building faster than expected, the interval gets shortened. The schedule is a starting point that the inspection keeps honest, not a number you set once and forget.

Solid-fuel cooking: the special rules

Solid-fuel cooking, wood, charcoal, and the like, is the harder hazard, and NFPA 96 treats it separately because it does two things gas and electric do not. It produces more grease and creosote, and it throws sparks and embers up into the system that can ignite the grease already there. The rules tighten to match.

The big one is separation. Exhaust systems serving solid-fuel cooking equipment are generally required to be separate from the rest of the kitchen exhaust, with their own duct and their own fan. The reasoning is direct: you do not want sparks from the charbroiler riding into the grease-loaded duct that serves the fryers and the rest of the line. A solid-fuel appliance under a shared hood with a shared duct is a code problem, not a layout preference.

Then the spark arrestor. Where the operation can throw sparks and embers, a spark arrestor is required ahead of the grease removal device to keep the embers out of the filters, the hood, and the duct. The grease filters over a solid-fuel appliance are also held farther off the cooking surface, commonly a minimum of 4 ft, to keep them out of the direct flame and ember stream. And because solid fuel loads grease fastest, it carries the most frequent cleaning interval. Confirm the separation, the spark arrestor, the filter height, and the cleaning frequency against NFPA 96 and the AHJ before you sign off on a wood or charcoal line.

The exhaust needs makeup air

An exhaust system that pulls thousands of CFM off the cookline only works if something puts that air back. Take the air out without replacing it and the kitchen goes negative to the outdoors, the exhaust fan moves less than its rating, the hood loses capture and spills smoke into the room, and in the worst case the negative backdrafts a gas appliance and pulls flue gas into the space. The exhaust and the makeup are two halves of one balance.

The mechanical code requires makeup air for a commercial Type I exhaust system regardless of its airflow (the 400 CFM trigger people quote is the residential range-hood rule, not the commercial one), and requires the makeup to start and run with the exhaust so the hood can never run without it. The kitchen is held slightly negative to the dining room so grease and odor stay on the kitchen side, and roughly neutral to the outdoors so the doors and the appliances behave.

The full treatment, sizing the makeup, tempering it, interlocking the fans, and balancing the kitchen with a gauge, is its own job covered in the makeup-air guide. For the exhaust side, carry one fact: a grease exhaust system that captures and drafts correctly depends on makeup air that was designed, installed, and balanced to match it. A fouling grease duct quietly raises the static the fan fights, which walks the balance negative over months, so the cleaning interval and the air balance are connected.

The fire suppression over the cookline

The grease system has a fire-extinguishing system aimed at it, and NFPA 96 requires one over the cooking equipment. It is a pre-engineered wet-chemical system with nozzles over the appliances and into the hood plenum and the duct, fusible links in the heat stream that trip it, a manual pull station, and an interlock that shuts off the fuel and power to the appliances when it fires. When a grease fire starts, the system lays a wet-chemical agent that smothers and cools the grease so it cannot reflash.

The exhaust and the suppression are designed together. The nozzles are placed to protect the same hood, plenum, and duct the grease loads, and the duct nozzle in particular is there because the duct is exactly where a fire wants to run. A clean duct and a working suppression system are the two halves of the duct's fire protection: cleaning keeps the fuel down, suppression fights the fire if it lights anyway.

The inspection, link replacement, nozzle aim, actuation test, and the gas-trip check are their own semi-annual scope covered in the hood suppression guide. For the exhaust side, the point is that the two systems share the same hazard and fail together. A tech who cleans the duct but never confirms the suppression, or services the suppression while the duct runs black, is treating half the problem. Note the suppression tag and the duct condition together, because each is the context for the other.

The code, the mechanical code, and the AHJ

More than one document governs a kitchen exhaust system, and they split the work. NFPA 96 is the standard for the grease side, the fire protection of the whole cooking operation. The mechanical code, commonly the International Mechanical Code, carries the commercial kitchen exhaust and makeup-air requirements in parallel, and the fire code adopts NFPA 96 by reference. The health department has its own interest in the hood and the food, separate from the fire side.

Above all of them is the AHJ, the authority having jurisdiction, usually the fire marshal for the fire and grease side. The AHJ adopts a specific edition of each code, amends it locally, and inspects the kitchen against what was adopted. That is why the same detail can be enforced differently two towns apart, and why a number that is right in one jurisdiction is worth confirming in the next.

Practically, this means you cite the standard for the requirement and confirm the figure against the adopted edition before you commit to it. The weld, the clearance, the cleaning interval, the access spacing, all trace to NFPA 96 and the mechanical code, but the enforceable version is whatever the local AHJ adopted and amended. When the standard and a local amendment disagree, the local rule wins on that job.

What does an NFPA 96 inspection check?

An inspection walks the whole grease path and looks at the system as fire-protection equipment, not just ventilation. It starts at the hood and follows the grease to the roof, then checks the records that prove the system has been maintained between visits. The point is to catch the fuel load and the broken protection before either one finds an ignition source.

At the hood, the inspector checks the filters, that they are listed baffles and not mesh, that they are in place and not heavily loaded, and that the grease cup and trough are not overflowing. Along the duct, the check is the grease loading inside it, that the access panels are present, spaced, and reachable, and that the duct is welded and not weeping at a joint or a panel. The clearance to combustibles and the wrap or shaft get looked at wherever the duct runs near building materials. At the fan, it is the hinge access, the wheel and housing loading, and the grease containment on the roof.

Then the records. The cleaning sticker and the service reports show whether the system is being cleaned to bare metal on the right interval, and the suppression tag shows the fire system is current. An inspector who finds a black duct with no recent cleaning sticker has found the headline failure before measuring anything. The documentation is part of the inspection, because a system that is clean today but has no record is a system nobody can vouch for next month.

Where these systems actually fail

The failures repeat from kitchen to kitchen, and they rank by how often they cause the fire. At the top, every time, is grease. A duct cleaned too rarely, or cleaned but never taken to bare metal, builds a fuel load that turns a small cookline flare into a fire that runs the length of the system. Under-frequency cleaning is the number one cause of the grease fire, and it is the one that is entirely preventable with a calendar and a certified cleaner.

After grease comes the construction. An un-welded or leaking grease duct that weeps fuel into a building cavity. A duct run tight to combustibles with no wrap and no rated shaft, so the heat from a duct fire reaches framing. Missing or buried cleanout access, so a section of duct never gets cleaned at all. Each of these is a fire path that was built in and then hidden behind a ceiling.

Then the supporting systems. A negative kitchen from no makeup air, starving the hood so it spills and, at worst, backdrafts an appliance. A Type II heat hood hung over a grease appliance, with no grease duct and no suppression behind it. A suppression system that is missing, expired, or aimed at appliances that have since been moved. None of these are exotic. They are the same handful of failures on most kitchens that burn, and the inspection exists to find them while they are still findings instead of headlines.

Facility cafeterias and data-center kitchens

The grease rules do not relax because the kitchen is inside an office, a campus, a hospital, or a data center. A facility cafeteria that fries, grills, or charbroils is a commercial cooking operation under NFPA 96, with the same Type I hood, welded grease duct, clearances, fan, suppression, and cleaning schedule as a standalone restaurant. The fact that it serves employees instead of paying customers changes nothing about the fire.

What changes is the building around it. A cafeteria kitchen is a small, intense grease-and-ventilation load sitting inside a building tuned for something else, and in a data center that something else is a tightly controlled environment with its own pressure and fire-protection priorities. The grease duct still has to run to the roof inside a rated enclosure, the exhaust still pulls thousands of CFM that the building has to make up, and the kitchen's pressure cannot be allowed to upset the spaces it sits next to.

These kitchens also get overlooked precisely because the building is not a restaurant. The facility team runs the chillers and the generators and forgets the cafeteria has a grease duct on the same fire-safety footing as everything else. The cleaning interval still tracks the cooking volume, the inspection still happens, and the records still have to exist. Treat the cafeteria grease system as its own NFPA 96 system, on its own schedule, inside the larger facility, not as a kitchenette nobody thought to put on a cleaning contract.

What to document

The record is what proves the grease system is being maintained as the fire-protection equipment it is, and it is the first thing an inspector or an insurer asks for after a fire. Capture the system as built and as serviced: the hood type, the duct construction and protection, the fan and containment, and every cleaning and inspection with its date and who did it.

Keep the cleaning stickers and service reports together, because the chain of them is the proof that the system was taken to bare metal on the right interval. When the cooking volume changes, when an appliance is swapped, or when an inspection shortens the interval, write down what drove the change, so the next person reads why the schedule is what it is instead of guessing.

Common mistakes

• Skipping or stretching the grease cleaning interval, the number one cause of the grease fire, or cleaning the hood and leaving the duct and fan loaded.
• Running an un-welded, screwed, or sealed grease duct that weeps liquid grease into the building cavity.
• Running the grease duct tight to combustibles with no listed wrap and no fire-rated shaft.
• Leaving out cleanout access panels, or building a hard ceiling over them so a duct section can never be cleaned.
• Providing no makeup air or too little, so the kitchen goes negative, the hood spills, and an appliance can backdraft.
• Hanging a Type II heat-and-steam hood over a grease appliance, with no grease duct and no suppression.
• Leaving the suppression system missing, expired, or aimed at appliances that were moved after it was installed.
• Using mesh grease filters, or leaving an upblast fan with no grease containment so grease spreads on the roof.

Field checklist

Standards and references

The grease side of a commercial kitchen lives under NFPA 96, the standard for ventilation control and fire protection of commercial cooking operations. It governs the hood and grease removal devices, the welded grease duct and its clearances and enclosure, the exhaust fan, the requirement for a fire-extinguishing system over the cooking equipment, and the periodic inspection and cleaning. The exact section numbers shift between editions, so confirm them against the edition the jurisdiction adopted before citing them on a submittal or a report.

The mechanical code, commonly the International Mechanical Code, carries the commercial kitchen exhaust and makeup-air requirements that run alongside NFPA 96, including the makeup-air threshold and the interlock. NFPA 17A is the standard for the wet-chemical extinguishing system over the cookline, and NFPA 10 covers the portable Class K extinguisher in the kitchen. UL listings control the hardware: the listed baffle filters, the listed grease-duct wrap and clearance-reduction systems, and factory-built grease duct assemblies, which only deliver their rating when installed to the listing.

Above all of these is the AHJ, usually the fire marshal, who adopts the edition, amends it locally, and enforces it, and who can require cleaning more often than the schedule based on what an inspection finds. Cite the standard that controls the point, hedge the clearances and the cleaning frequencies to NFPA 96 and the AHJ, and treat the cleaning as the requirement everything else depends on, because it is the one that keeps the grease fire from having fuel.

Units and terms

A kitchen exhaust system borrows vocabulary from ventilation, sheet metal, and fire protection, so the same part shows up under different names across a drawing set, a manufacturer sheet, and a service ticket.

Airflow through the hood and duct is in CFM, cubic feet per minute. Duct steel is called out by gauge, with a lower gauge number meaning thicker metal. Clearance to combustibles is in inches. The grease duct is the welded steel path from hood to fan; the plenum is the space behind the filters; the grease removal device is the listed baffle or filter. Bare metal is the cleaning standard, meaning the surface is taken back to clean steel, not just wiped. The AHJ is the authority having jurisdiction, the official who adopts and enforces the code.

Type I hood

A grease hood over cooking appliances, with listed filters, a welded grease duct, clearances, and suppression

Type II hood

A heat-and-steam hood with no grease, over dishwashers or steam kettles, with no grease duct

Grease duct

The welded, liquid-tight steel duct carrying grease-laden vapor from the hood to the exhaust fan

Baffle filter

The listed grease removal device that turns the air sharply so grease droplets strike the blades and drain

Clearance to combustibles

The required air gap between a hot grease duct and burnable construction, commonly 18 in unless reduced by a listed method

Grease-duct wrap

A listed insulation blanket around the duct, rated for fire resistance and reduced clearance

Upblast fan

The rooftop grease exhaust fan that discharges up and away from the roof, hinged for cleaning

Bare metal

The NFPA 96 cleaning standard: the grease surfaces taken back to clean steel, not wiped

AHJ

Authority having jurisdiction, usually the fire marshal, who adopts and enforces the code

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