2021 IMC Sections 506 to 509
A commercial kitchen exhaust system is a fire safety system dressed up as ventilation. Grease from the cooking line coats every surface it touches, and a grease fire inside a duct can travel through a building faster than almost any other mechanical failure. Sections 506 through 509 of the 2021 IMC therefore treat the hood, duct, fan, makeup air and fire suppression as one assembly, and the figure below follows it from the cooking surface up to the roof. Local adoption governs, and the fire official may review kitchen exhaust as well.
A Type I hood is required over cooking appliances that produce grease or smoke, and the code names medium-duty, heavy-duty and extra-heavy-duty appliances (507.2). Chapter 2 sorts the equipment. Light duty covers ovens, kettles and steamers. Medium duty covers griddles, fryers and electric ranges. Heavy duty covers under-fired broilers, gas open-burner ranges, woks and smokers. Extra-heavy duty means open-flame combustion of solid fuel. A Type II hood goes over dishwashers and other heat- or moisture-producing appliances that make no grease or smoke, and over appliances that produce combustion products but no grease (507.3). Where cooking appliances do not require a Type II hood, the space is exhausted at 0.70 cfm per square foot, counting each such appliance as at least 100 square feet.
If any appliance under one hood needs Type I, the whole hood is Type I, and then the entire system (hood, duct, fan and makeup air) follows Sections 506 through 509 (507.1). An extra-heavy-duty hood cannot cover other duty classes and needs its own exhaust system (507.2.2).
A Type I hood is steel of at least No. 18 gage or stainless steel of at least No. 20 MSG, supported by noncombustible members, with external joints and seams welded or brazed liquid tight (507.2.3 to 507.2.5). It keeps 18 inches from combustible construction. A hood listed for less follows its listing, and gypsum board on noncombustible structure needs no clearance if a smooth noncombustible surface covers the wall 18 inches around the hood (507.2.6).
Grease filters are listed, readily removable and set at least 45 degrees from horizontal over a drip tray (507.2.8). Their lowest edge sits at least 6 inches above a cooking surface without exposed flame, 24 inches over exposed flame and burners, and 42 inches over charcoal and charbroil appliances (Table 507.2.8). A canopy hood overhangs the appliance by at least 6 inches on every open side, with its front lip no more than 4 feet above the cooking surface (507.4.1). A non-canopy hood is no more than 3 feet above the surface and set back no more than 1 foot (507.4.2). Each exhaust outlet in a hood serves no more than 12 feet of hood (507.1.5).
The exhaust rate depends on the hood type and on the heaviest-duty appliance under it; mixing lighter appliances under the same hood does not lower the rate. The code gives cubic feet per minute per linear foot of hood; the table shows two common types. An island canopy needs more air because no wall helps contain the plume, and an eyebrow hood is not allowed over heavy or extra-heavy appliances. A Type II hood over a dishwasher is 100 cfm per linear foot (507.5.5).
| Duty of the heaviest appliance | Wall-mounted canopy (cfm per ft) | Single island canopy (cfm per ft) |
|---|---|---|
| Light | 200 | 400 |
| Medium | 300 | 500 |
| Heavy | 400 | 600 |
| Extra-heavy | 550 | 700 |
Worked example. A 10-foot wall-mounted canopy covers a medium-duty fryer and a heavy-duty gas open-burner range. The heavy-duty appliance governs, so the net exhaust is 400 x 10 = 4,000 cfm. Makeup air must be about 4,000 cfm too, and one outlet is enough because an outlet may serve 12 feet of hood. As a single island canopy the same hood would need 600 x 10 = 6,000 cfm. Slope has arithmetic too: a 40-foot horizontal run at 1/4 inch per foot rises 10 inches from the hood end, while an 80-foot run needs 1 inch per foot and rises 80 inches, which is why long runs are a design problem to catch early.
Grease ducts are steel of at least No. 16 gage or stainless steel of at least No. 18 gage (506.3.1.1). Joints, seams and penetrations receive a continuous liquid-tight weld or braze on the external surface, (506.3.2). Screws, bolts and rivets must not penetrate the duct wall, and supports are noncombustible (506.3.3). Before any part is used or concealed, the permit holder runs a light test, passing a lamp of at least 100 watts through the entire duct to show the welds are tight (506.3.2.5). Air velocity stays at 500 feet per minute or more so grease travels rather than settles (506.3.4). Each Type I hood gets its own grease duct unless the hoods share one story and the same or adjoining rooms, cross no rated assembly and serve no solid-fuel appliance (506.3.5).
Where no enclosure is required, a grease duct keeps 18 inches from combustible construction and 3 inches from noncombustible construction, including gypsum board on noncombustible structure (506.3.6). A duct that penetrates a ceiling, wall, floor or concealed space is enclosed from the point of penetration to the outlet, in a shaft, a listed field-applied wrap or a factory-built enclosure. The enclosure is rated at least 1 hour and not less than the assembly it penetrates, and it holds only that one duct (506.3.11). No enclosure is needed for a duct that penetrates only a non-rated roof or ceiling assembly. Inside a shaft the clearances become 18 inches to combustibles and 6 inches to noncombustibles (506.3.11.1). Fire dampers and smoke dampers are not installed in grease ducts. Every cleanout inside an enclosure needs an access panel marked ACCESS PANEL. DO NOT OBSTRUCT. (506.3.12). The duct slopes at least 1/4 inch per foot (2 percent) toward the hood or a grease reservoir, and 1 inch per foot where a horizontal run exceeds 75 feet (506.3.7).
Grease has to be reachable. Horizontal cleanouts are spaced no more than 20 feet apart and no more than 10 feet from a change in direction over 45 degrees. Each opening is at least 12 by 12 inches, with a liquid-tight steel door, a gasket rated for 1,500 degrees F, and no fasteners through the duct. A reservoir sits at the bottom of a horizontal duct or riser, is at least 12 inches long and 1 inch deep across the full width, slopes to a drain and has its own cleanout (506.3.7.1). An in-line fan has cleanouts on both sides within 3 feet (506.3.8.2), and a duct big enough to enter needs a 22 by 20 inch opening (506.3.8.1).
Fan housings are built like the duct unless listed to UL 762, and the motor stays out of the airstream (506.5.1). Upblast fans are hinged, on flexible weatherproof cable with a restraint, and the duct rises at least 18 inches above the roof (506.5.4). The discharge opening is at least 40 inches above the roof surface (506.3.13.1). The outlet stays at least 10 feet horizontally from the same or adjacent buildings and property lines and at least 10 feet above adjoining grade, and it is either 10 feet horizontally from or 3 feet above air intakes; where the discharge is directed away from those places, the horizontal distances drop to 5 feet (506.3.13.3). A wall termination is allowed only where it creates no nuisance or fire hazard, not where the IBC requires protected openings, and not within 3 feet of a wall opening (506.3.13.2). Type II hood outlets differ: 30 inches above the roof and from vertical walls, 3 feet from openings, 10 feet from lot lines and above grade (506.4.2).
Makeup air is supplied whenever the exhaust runs. The total supplied is about equal to the total exhaust, without weakening capture, and mechanical makeup starts automatically with the exhaust (508.1). Its temperature stays within 10 degrees F of the space unless the HVAC system can carry the load (508.1.1), and the plans show an air balance schedule (508.1.2). The hood fan starts automatically with the appliances or is interlocked to them (507.1.1). Before final approval the permit holder tests exhaust and makeup airflow and capture and containment with appliances hot, judged with smoke or steam (507.6). Appliances under a Type I hood also get an approved automatic fire suppression system under the IBC and IFC (509.1).
In the field
A church fellowship hall remodel adds a fryer and a gas range under a hood that is meant to be reused from a residential kitchen, with a flexible duct running into the attic. Check before anything is concealed: the hood is Type I and meets 507.5, the duct is welded No. 16 gage steel with no flex, the duct is enclosed where it crosses the ceiling, and the light test is done. Then walk outside: the outlet must be 40 inches above the roof and 10 feet from the neighboring building and every air intake.
1. Under 2021 IMC 507.2, a Type I hood is required over which appliance in a school cafeteria kitchen?
2. An inspector finds a restaurant grease duct made of carbon steel that measures 0.0466 inch (No. 18 gage) with welded seams. What is the minimum thickness for carbon steel grease duct under 506.3.1.1?
3. A 40-foot horizontal run of grease duct must slope at least how much under 506.3.7?
4. A horizontal grease duct run is 90 feet long. Under 506.3.7 its minimum slope is:
5. An unenclosed grease duct serving a Type I hood in a Group A-2 restaurant must keep what minimum clearance to combustible construction?
6. A 12-foot wall-mounted canopy hood covers a heavy-duty broiler and medium-duty fryers. Using 400 cfm per linear foot for heavy duty, the minimum net exhaust is:
7. Horizontal grease duct cleanouts are spaced no farther apart than:
8. A grease duct serving a Type I hood in a Group A-2 restaurant penetrates a 1-hour rated ceiling. Which protection follows 506.3.11?
9. An upblast fan on a grease duct discharges above a flat roof. The discharge opening must be at least how far above the roof surface (506.3.13.1)?
10. A canopy-type Type I hood must overhang the edge of the appliance at least 6 inches on open sides. The hood's front lower lip may be no more than what distance above the cooking surface?
11. A restaurant's Type I hood is complete. Before final approval, the permit holder must perform which test under 507.6?
12. Makeup air for a commercial kitchen exhaust system must be within how many degrees Fahrenheit of the space temperature, unless the HVAC system can carry the added load (508.1.1)?
13. Unless the discharge is directed away, a Type I grease duct outlet must be at least how far horizontally from an adjacent property line (506.3.13.3)?
14. Grease filters in a Type I hood must be installed at an angle from the horizontal of not less than: