7.3 Type II Hoods, Filters & Kitchen Exhaust Rates

507.3 is the Type II charging language: install Type II hoods above light-duty cooking appliances, dishwashers, and appliances that produce heat or moisture and do not produce grease or smoke as a result of the cooking process. Install Type II above appliances that produce products of combustion without grease or smoke. A Type I hood may be installed where Type II is required, but that Type I must meet all Type I rules. Exception inside 507.3: where such a Type I serves only dishwashers and appliances that require Type II, the Type I shall not be required to have fire suppression or grease filters. That exception does not let you park a fryer under the same canopy.

Light-duty, from Chapter 2, is the oven-and-steam list: gas and electric ovens (standard, bake, roasting, revolving, retherm, convection, combination convection/steamer, countertop conveyorized baking/finishing, deck, pastry), steam-jacketed kettles, pasta cookers, compartment steamers (pressure and atmospheric), and cheesemelters. The moment the appliance is a griddle, fryer, broiler, wok, or solid-fuel unit, you are back in 507.2 and Section 7.1.

Type II construction, condensate, ducts, termination

507.3.1 materials: steel not less than 0.0296 inch (0.7534 mm) (No. 22 gage), stainless not less than 0.0220 inch (0.5550 mm) (No. 24 gage), copper sheets not less than 24 ounces per square foot, or other approved material and gage. Compare that with Type I hood steel at 18 gage and grease-duct steel at 16 gage — three different stacks.

507.3.2: supports shall carry the hood, unsupported duct, effluent, and personnel. 507.3.3: joints, seams, and penetrations are built as in Chapter 6, sealed on the interior, and provide a smooth surface that is readily cleanable and watertight. Type II is a condensate hood. Standing water in a raw Pittsburgh lock is a 507.3.3 fail even though grease-weld rules do not apply.

506.4.1: ducts and plenums serving Type II hoods shall be rigid metallic materials. Construction, installation, bracing, and supports follow Chapter 6. Ducts under positive pressure and ducts conveying moisture-laden or waste-heat-laden air shall be constructed, joined, and sealed in an approved manner. Flexible duct and a jump to a wood-truss attic are not 506.4.1.

506.4.2 Type II terminations — every item, not a menu:

1. Not less than 3 feet in any direction from openings into the building.
2. Not less than 10 feet from property lines or buildings on the same lot.
3. Not less than 10 feet above grade.
4. If above a roof, not less than 30 inches above the roof surface (Type I rooftop discharge is 40 inches — do not swap them).
5. Not less than 30 inches from exterior vertical walls.
6. Protected against local weather.
7. Not directed onto walkways.
8. Exterior-wall opening protectives as required by the IBC.

501.3 still governs every mechanical exhaust system: discharge outdoors, not a public nuisance, not readily re-entrained, not into an attic or crawl space, not onto walkways. Field fail: a dishwasher Type II “vented” with flex into a soffit or attic. That is a 501.3 / 506.4.2 fail, not a “condensate detail.”

501.2: single or combined Type II exhaust systems for food-processing operations shall be independent of all other exhaust systems. You may combine Type II kitchen hoods with each other; you may not dump them into the toilet riser or the hazardous exhaust shaft.

Exhaust rates — 507.2.10 and 507.3.4

Net quantity of exhaust air is total hood exhaust minus any air supplied directly into the hood cavity. Compensating-hood short-circuit air does not count as capture. 507.2.10: where heavy-, medium-, and light-duty appliances share one Type I hood, use the exhaust rate for the heaviest-duty appliance for the entire hood. Extra-heavy-duty is its own independent hood (507.2.2) and is not mixed.

Confirmed 2024 IMC minimum net cfm per linear foot of hood:

Work three numbers so the table sticks. An 8-foot wall canopy over light-duty ovens: 8 × 200 = 1,600 cfm. The same 8 feet as a single island: 8 × 400 = 3,200 cfm. An 8-foot Type II over a flight dishwasher: 8 × 100 = 800 cfm. Move a medium-duty griddle under that wall canopy and 507.2.10 forces 8 × 300 = 2,400 cfm — and the hood becomes Type I.

507.2.1 requires Type I hoods to bear a label with the minimum exhaust flow in cfm per linear foot that captures for the duty served. 507.1.1 lets the running rate follow the listing, 507.2.10, or 507.3.4, and allows engineered or listed variable-speed reduction only while capture of idle/standby appliances is maintained. 508.1 still replaces whatever you actually exhaust.

Kitchen occupiable-space outdoor air is a different number. Table 403.3.1.1 “Kitchens (cooking)” is 20 occupants/1000 ft², Rp 7.5, Ra 0.12, plus 0.70 cfm/ft² exhaust and Note b (no recirculation from the space). Hood cfm is not a substitute for forgetting the Table 403 row, and the Table 403 row is not a substitute for 507.2.10.

Filters, domestic appliances, when a hood is not required

Type I filters were covered in 7.1; they show up on M2 next to Type II stems because the distractor is always “mesh is fine if you wash it.” 507.2.8 is UL 1046, access for cleaning, Table 507.2.8 heights (0.5 / 2 / 3.5 feet), and 45-degree mounting with a drip tray. Mesh that is not UL 1046 listed is not a Type I grease filter. Type II hoods do not get grease filters so they can hide a fryer; they get a cleanable condensate path.

507.1.2 again: a domestic range hood used for commercial cooking is the wrong appliance. Domestic cooking used as domestic cooking follows Section 505 (including the 501.3 Exception 3 path for listed ductless domestic hoods where Chapter 4 ventilation is otherwise provided). A bed-and-breakfast residential range is not a 40-pound fryer line.

507.1 exceptions — when 2024 IMC does not require a hood (confirm each listing on the appliance, then still ventilate the room):

1. UL 710 factory-built exhaust hoods (Exception 1) still are hoods; they skip listed construction/geometry/rate sections, not the idea of a hood.
2. No hood (Exception 2) for: UL 710B factory-built recirculating systems installed per 304.1 (room = kitchen for Table 403.3.1.1; each appliance ≥ 100 ft²); cooking appliances with integral down-draft exhaust listed to NFPA 96; smoker ovens with integral exhaust listed and tested for the application.
3. UL 2162 ovens listed for wood fuel and vented per the manufacturer.
4. Electric cooking appliances listed to UL 197 for reduced grease emissions.
5. Commercial electric dishwashers with a self-contained condensing system listed to UL 921.
6. Heat and moisture from dishwashers and non-grease appliances incorporated into the HVAC design (or a separate removal system). Those spaces get exhaust at 0.70 cfm per square foot; each such appliance is not less than 100 square feet for that exhaust-area math, and that extra area is also exhausted at 0.70 cfm/ft².

A UL 921 condensing dishwasher needs no Type II hood. An ordinary hot-water flight machine that is not UL 921 still needs 507.3 and 100 cfm per linear foot, terminated per 506.4.2. A “we opened the rooftop unit a little” note is not Exception 6 unless the HVAC calculation actually carries the load at 0.70 cfm/ft².

507.1.7 still requires a performance test of exhaust rate, makeup, and capture before final approval. For Type II, capture is steam you can see. If the plume rolls out when the tray cart passes, 508.1 makeup or 507.3.4 cfm is wrong — do not pass it because “it is only a dishwasher.”

Field scenario. A school servery has a UL 921 condensing dishwasher (no hood required), a non-listed flight dishwasher under a Type II canopy whose duct is flex into the attic, and a wall canopy labeled 200 cfm/ft over a convection oven and a new double-sided griddle. The UL 921 unit is fine. The flight-machine duct fails 501.3 and 506.4.2. The mixed canopy fails 507.1 / 507.2 (the griddle is medium-duty Type I) and, even as Type I, would need 300 cfm/ft, UL 1046 filters, grease duct, and 507.2.11 suppression. Tab 507.3, 507.3.4, 507.1 Exceptions 2–6, and 506.4.2, then run the rate and attic items in the free ICC M2 practice questions.