Type 2 hood requirements in the International Mechanical Code fit in one section. Section 507.3 puts a Type II hood, as the code writes it, above dishwashers and above appliances that produce heat or moisture but no grease or smoke, and above appliances that produce products of combustion without grease or smoke. It lets the heat-and-moisture group out of the hood where the HVAC design carries their load, and exhausts the space instead at 0.70 cfm per square foot, counting each such appliance as at least 100 square feet.
The decision that matters is made before that section is opened. Whether an oven is a Type 2 duty depends on what comes off it and on the duty class the code assigns it in Section 202. A conveyor pizza oven is medium-duty and needs a Type I hood however clean it looks; a deck pizza oven or a convection oven is light-duty and needs a Type I hood only when the cooking produces grease or smoke; and any single appliance that needs a Type I hood makes the whole hood a Type I.
This guide sets out what the 2021 edition requires of a Type 2 hood and its duct, which appliances the code puts on which side of the line, the three routes to no hood at all, and the documents that prove the classification. It does not classify a specific appliance or size a Type I system; the code official rules on the first, and the CFM table and the fan sit on commercial kitchen hood CFM requirements. The concept behind the two types is explained in Type 1 hood vs Type 2.
One disclosure: Polygee builds in-duct electrostatic precipitators and a hood with the cells built in. A true Type 2 duty has no grease and no smoke, so there is nothing in it for our equipment to do. We write this guide because a hood classified as Type 2 over cooking that turns out to smoke is where many of the complaints we are later asked about begin.
What Section 507.3 requires of a Type 2 hood
A Type II hood is a heat-and-moisture hood, and the 2021 code gives it two triggers, one release and one substitute. The first trigger is dishwashers and appliances that produce heat or moisture and do not produce grease or smoke as a result of the cooking process. The second is appliances that produce products of combustion and do not produce grease or smoke; a gas-fired steamer or kettle is the usual case. The release applies only to the first group: where the heat and moisture loads from those appliances are incorporated into the HVAC system design or into a separate removal system, the hood is not required. The substitute is general exhaust. Spaces containing cooking appliances that do not require Type II hoods are exhausted at 0.70 cfm per square foot, and for that calculation each appliance that is not required to be under a Type II hood is counted as occupying not less than 100 square feet, with that added area exhausted at the same rate.
Section 507.1 frames the choice. Hoods are Type I or Type II and are designed to capture and confine cooking vapors and residues. Where any cooking appliance under a single hood requires a Type I hood, a Type I hood is installed; where a Type II hood is required, either type may be installed. Once a Type I hood is installed, the whole system, hood, ducts, exhaust equipment and make-up air, falls under Sections 506 through 509, which is the difference in cost and construction that this classification decides.
| Document | Product scope | What it supports | What it does not prove |
|---|---|---|---|
| IMC 2021, Section 507.3 | Type II hoods and the spaces that do without them | When a heat-and-moisture hood is required, when the HVAC route replaces it, and the general exhaust that follows | Whether a given appliance produces grease or smoke; that is a fact about the cooking |
| IMC 2021, Section 507.2 | Type I hoods | That grease- or smoke-producing appliances and every medium-, heavy- and extra-heavy-duty appliance need a Type I hood | Anything about a Type II duct or hood |
| IMC 2021, Section 202 duty definitions | Light-, medium-, heavy- and extra-heavy-duty cooking appliances | Which class an appliance type belongs to, by name | What a light-duty appliance emits on your menu |
| UL 710B | Recirculating systems and the 5 mg/m³ grease test | The ventless route and the electric-appliance exception to a Type I hood | Compliance of a Type II hood, which UL 710B does not address |
This table answers which document governs which step. It does not answer the classification of your equipment, which the next section takes up.
The appliance decides: the duty classes in Section 202
The code sorts cooking appliances into four duty classes by name, and only the lightest class can ever be a Type 2 duty. Section 507.2 requires a Type I hood over every medium-, heavy- and extra-heavy-duty cooking appliance and wherever the cooking produces grease or smoke; that leaves the light-duty class as the only one whose hood type turns on the menu.
| Duty class (IMC 2021, Section 202) | Appliances the code names | Hood required | What it means for the build |
|---|---|---|---|
| Light-duty | Gas and electric ovens (standard, bake, roasting, revolving, retherm, convection, combination convection/steamer, countertop conveyorized baking/finishing, deck and pastry); steam-jacketed kettles; pasta cookers; compartment steamers, pressure and atmospheric; cheesemelters | Type I where the cooking produces grease or smoke; otherwise Type II for heat and moisture, or the HVAC route with general exhaust | The menu decides: a deck pizza oven baking cheese and pepperoni is a Type I duty in practice; a bake-only oven is not |
| Medium-duty | Electric discrete-element ranges, with or without oven; gas and electric hot-top ranges, griddles and double-sided griddles; fryers, including open deep-fat, donut, kettle and pressure fryers; gas and electric conveyor pizza ovens; tilting skillets; rotisseries | Type I, always | A conveyor pizza oven is never a Type 2 duty, whatever it bakes |
| Heavy-duty | Electric and gas under-fired broilers and chain broilers; gas open-burner ranges, with or without oven; wok ranges; smokers and smoker ovens; over-fired broilers and salamanders | Type I, always | The heavy-duty exhaust rate applies, as set out in the CFM table |
| Extra-heavy-duty | Any appliance using open-flame combustion of solid fuel at any time | Type I, always, with its own separate system | Wood-fired pizza and charcoal grills are covered in solid fuel cooking exhaust requirements |
| Not a cooking duty class | Dishwashers and potwashers | Type II, for heat and moisture, unless the HVAC route is taken | A dishwasher Type II hood exhausts at least 100 cfm per linear foot of hood under Section 507.5.5 |
This table answers which class an appliance type sits in by name. It does not answer what your light-duty appliance emits; that is a question for the menu and the code official, and it is where the search question below comes from.
Does a pizza oven need a hood, and which type?
The duty class answers before the menu does. A conveyor pizza oven, gas or electric, is medium-duty by name and needs a Type I hood whatever it bakes. A deck pizza oven is light-duty, so it needs a Type I hood when the cooking produces grease or smoke, which a cheese-and-cured-meat menu does, and a Type II hood, or the HVAC route, when it does not. A wood-fired oven is solid fuel, extra-heavy-duty, with a separate system of its own. Electric countertop ovens sold as ventless stand on a UL 710B listing or a grease-effluent test, described in the next section, not on the word on the box.
Three routes to no hood at all
The code offers three ways for a cooking appliance to stand without a hood, and each carries a document.
- The heat-and-moisture route in Section 507.3. A design decision recorded in the HVAC calculation, with the space exhausted at 0.70 cfm per square foot and each such appliance counted as 100 square feet or more. It never applies to appliances that produce products of combustion.
- The exception to Section 507.2. A Type I hood is not required for an electric cooking appliance where an approved testing agency provides documentation that the appliance effluent contains 5 mg/m³ or less of grease when tested at an exhaust flow rate of 500 cfm in accordance with UL 710B; the document is the test report for that appliance, and its heat and moisture are then answered by a Type II hood or the HVAC route under Section 507.3.
- The factory-built recirculating system listed to UL 710B. The ventless oven or countertop unit with its own filters and fire protection, which Section 507.1 excuses from eight of Section 507’s construction and capacity subsections and whose space is ventilated as a kitchen with each appliance counted as 100 square feet; what that listing does and does not prove is in UL 710B vs UL 710.
Section 507.1 adds two narrower cases: appliances with integral down-draft exhaust listed for the application under NFPA 96, and smoker ovens with integral exhaust systems that are listed and installed to their instructions.
Hood, duct and termination rules for a Type 2 system
A Type 2 system is lighter than a Type 1 in every dimension, and the code still writes down its metal, its joints and its outlet.
| Section (IMC 2021) | What it requires | What it means for the build |
|---|---|---|
| 507.3.1 Type II hood materials | Steel not less than 0.0296 inch (No. 22 gage), stainless steel not less than 0.0220 inch (No. 24 gage), copper sheet of not less than 24 ounces per square foot, or other approved material and gage | Lighter gage than a Type I hood; no grease filters, no fire suppression, no grease-tight welding requirement |
| 507.3.3 Joints, seams and penetrations | Constructed as set forth in Chapter 6, sealed on the interior of the hood, smooth, readily cleanable and watertight | Condensate is the enemy of a Type II hood; the inside has to shed it |
| 506.4.1 Ducts | Ducts and plenums of rigid metallic materials, built, installed, braced and supported to Chapter 6; ducts under positive pressure and ducts conveying moisture-laden or waste-heat-laden air constructed, joined and sealed in an approved manner | Ordinary sheet-metal duct, sealed for moisture; none of the grease duct rules in grease duct requirements apply |
| 506.4.2 Type II terminations | To the hood manufacturer’s instructions and: at least 3 feet from building openings; at least 10 feet from property lines and from other buildings on the same lot; at least 10 feet above grade; at least 30 inches above the roof surface; at least 30 inches from exterior vertical walls; protected against the weather; not directed onto a walkway; compliant with the International Building Code on exterior wall opening protectives | Shorter distances than the Type I outlet in commercial kitchen exhaust termination requirements, and no grease-handling at the fan |
| 507.5.5 Dishwashing appliances | The minimum net airflow for Type II hoods used for dishwashing appliances is 100 cfm per linear foot of hood length | The one Type II airflow figure the code states; other Type II duties are sized to the heat and moisture load |
| 507.1, Exception 1 | A factory-built hood listed and labeled to UL 710 and installed to Section 304.1 need not comply with 507.3.1 and 507.3.3, among other construction sections | A listed hood carries its construction on its label |
This table answers what a Type 2 hood and its duct are built and terminated to. It does not answer the exhaust airflow for a non-dishwashing Type II duty, which the code leaves to the heat and moisture load and the manufacturer, and which a Type I duty replaces with the rates in commercial kitchen hood CFM requirements.
When the answer is a Type 1 hood, and what follows from it
Most Type 2 questions end in a Type 1 answer for one of four reasons, and each one changes the whole exhaust system, not only the hood. The appliance is in the medium or heavy class by name, as a conveyor pizza oven, a griddle or a rotisserie is. A light-duty appliance cooks something that produces grease or smoke, as a deck oven with cheese and cured meat or a convection oven roasting chicken does. One Type I appliance shares the hood with the Type 2 duties, which makes the hood a Type I under Section 507.1. Or the fuel is solid, which makes the appliance extra-heavy-duty with a separate system of its own.
Once the hood is a Type I, the exhaust rate comes from the duty table, the filters have to be listed grease filters, the duct becomes a grease duct with its own clearances and access, the fire suppression system follows under the fire code, and the outlet moves to the longer Type I distances. The whole submittal is collected in commercial kitchen hood code requirements. Only at that point does anything Polygee makes enter the picture. An in-duct electrostatic precipitator sits in a Type I grease duct after the filters to take the fine grease mist and smoke that pass them. It has no role on a true Type 2 duty, and it is the wrong first response to a Type 2 hood that is smoking, because that symptom means the classification, the capture or the airflow is wrong, in the order set out in how to reduce smoke from commercial kitchen exhaust. Where a Type I duct does carry visible smoke to the outlet, the unit and its documents are on the kitchen electrostatic precipitator page.
Which document proves what
The classification of a hood is proved by documents about the appliance and the design, not by the hood itself.
| Evidence | What it supports | What it does not prove on its own | Product-condition scope | Buyer action |
|---|---|---|---|---|
| The appliance schedule with duty class and fuel, read against Section 202 | Which appliances are medium-, heavy- or extra-heavy-duty and therefore Type I by name | What a light-duty appliance emits on your menu | The listed appliances | Mark the class beside every appliance before the hood is drawn |
| The menu statement for light-duty appliances | The basis on which the code official accepts a Type II hood, or none, over an oven | Compliance if the menu changes | That menu | Get the official’s classification in writing with the menu attached |
| The HVAC load calculation for the heat-and-moisture route | That the release in Section 507.3 applies and the general exhaust is provided | Anything about grease or smoke | The appliances included in it | Keep it with the mechanical drawings |
| UL 710B recirculating system listing | The ventless route in Section 507.1 | Compliance of a hooded installation | The listed package | Read the listing conditions and the space ventilation that comes with it |
| Grease effluent test documentation at 500 cfm under UL 710B | The exception to a Type I hood for that electric appliance | A Type II hood decision; a different appliance | The tested appliance | Ask the appliance maker for the agency report, not the brochure |
| The balance report and the termination drawing | That the Type II exhaust and its outlet were built to Sections 506.4 and 507.5.5 | The classification itself | The installation | Check the outlet against the eight items in 506.4.2 |
This table answers which record settles which question. It does not answer what the inspector will see on the day, which is smoke or no smoke at the hood; the classification has to match that.
Where this guide stops
- Edition. Section numbers and wording are from the 2021 International Mechanical Code; some states amend or replace Section 507 (Minnesota applies NFPA 96 and ASHRAE 154 in its place). Check the edition and amendments your jurisdiction has adopted. Canada arrives at NFPA 96 by a different route again, set out in NFPA 96 in Canada.
- The classification. The duty classes are the code’s; whether a light-duty appliance produces grease or smoke on your menu is the code official’s call, and this guide does not make it.
- Type II airflow. Apart from the 100 cfm per linear foot for dishwashers, the code leaves the Type II exhaust rate to the heat and moisture load; this guide gives no other figure.
Buyer record: what to have ready, what to ask for, what triggers a re-check
- Have ready: the appliance schedule with fuel and the Section 202 class marked; the menu for every light-duty oven; whether any Type I appliance shares the hood; whether the HVAC design carries the heat and moisture load; the outlet position against the eight items in 506.4.2.
- Ask for: the code official’s classification in writing; the UL 710B listing or the 5 mg/m³ test report where a no-hood route is claimed; the hood label where a listed hood is used; the balance report after installation.
- Re-check when: the menu adds anything that renders fat or chars; an appliance is swapped for one in a higher duty class; a Type I appliance is added under the hood; the fuel changes to solid; or the jurisdiction adopts a new edition.
Frequently Asked Questions
- What appliances require a Type 2 hood?
- Under Section 507.3 of the 2021 International Mechanical Code, a Type II hood goes above dishwashers and above appliances that produce heat or moisture but no grease or smoke as a result of the cooking process, and above appliances that produce products of combustion without grease or smoke. The heat-and-moisture group can be released from the hood where the HVAC design or a separate removal system takes their loads; the products-of-combustion group cannot. Typical Type II duties are steamers, steam kettles, pasta cookers and dishwashing machines, when they produce no grease or smoke. The moment an appliance produces grease or smoke, or belongs to the medium, heavy or extra-heavy duty classes, Section 507.2 requires a Type I hood instead.
- Does a commercial convection oven need a hood?
- It depends on what it cooks, because the 2021 International Mechanical Code lists convection ovens among light-duty cooking appliances. A light-duty appliance needs a Type I hood only where the cooking produces grease or smoke, which roasting meats, bacon or anything that spits fat does. An oven that only bakes bread or reheats needs a Type II hood for its heat and moisture, and even that can be dropped where the HVAC design carries the load, in which case the space is exhausted at 0.70 cfm per square foot with the oven counted as at least 100 square feet. The code official rules on the menu, not the nameplate, and some states amend Section 507 or apply ASHRAE 154 instead, so confirm the adopted text and the classification in writing before the duct is run.
- What exhaust does a kitchen need for cooking appliances that have no Type 2 hood?
- Section 507.3 answers with general exhaust. Where a heat-and-moisture appliance is left without a Type II hood because the HVAC design or a separate removal system carries its load, the space containing it is exhausted at 0.70 cfm per square foot, and for that calculation each appliance not required to be under a Type II hood is counted as occupying not less than 100 square feet, with that added area exhausted at the same rate. The release applies only to the heat-and-moisture group; an appliance that produces products of combustion without grease or smoke, such as a gas steamer or kettle, still needs its Type II hood. The HVAC load calculation is the document that proves the route was designed rather than assumed.
- What are the duct and termination rules for a Type 2 hood?
- A Type II hood is built of steel at least 0.0296 inch, No. 22 gage, or stainless steel at least 0.0220 inch, No. 24 gage, or copper of 24 ounces per square foot, with joints and seams sealed inside and left smooth, cleanable and watertight. Its ducts and plenums are rigid metallic materials built to Chapter 6, sealed in an approved manner where they carry moisture-laden or waste-heat-laden air or run under positive pressure. The outlet follows the hood manufacturer’s instructions and Section 506.4.2: at least 3 feet from building openings, 10 feet from property lines and other buildings on the same lot, 10 feet above grade, 30 inches above the roof surface and 30 inches from exterior vertical walls, protected from the weather and not directed onto a walkway. A Type II hood for dishwashing exhausts at least 100 cfm per linear foot of hood.
Did the classification come back as Type 1?
If it stayed Type 2, there is nothing to buy from us; file the HVAC calculation and the balance report. If the menu or the duty class has moved an oven or a cookline under a Type I hood and visible smoke at the outlet is the concern, send us the appliance schedule with the classes marked, the design exhaust airflow, the duct route and where on it a unit can sit, and the outlet position. Our project team will come back with the model whose rated airflow band fits the flow, its resistance at that flow for the fan selection, the documents that model carries, and a plain statement of where an in-duct unit will and will not help on that cookline.
Talk to our project team →Law manages kitchen exhaust filtration projects at Polygee, from spec review through duct-side ESP sizing and installation support. Polygee, founded in 2004, builds commercial kitchen exhaust filtration equipment. This article summarizes model-code provisions for planning purposes only — it is not a compliance determination, so confirm the classification of a specific appliance with your engineer, your local code official and the edition of the code adopted where you build.
References & further reading
- International Code Council — 2021 International Mechanical Code, Section 507.1 General, 507.2 Type I hoods and 507.3 Type II hoods. codes.iccsafe.org
- International Code Council — 2021 International Mechanical Code, Section 202 General Definitions (light-duty, medium-duty, heavy-duty and extra-heavy-duty cooking appliances). codes.iccsafe.org
- International Code Council — 2021 International Mechanical Code, Sections 506.4 Ducts serving Type II hoods, 507.3.1 Type II hood materials and 507.5 Capacity of hoods. codes.iccsafe.org
- UpCodes — 2021 IMC as adopted, Sections 506.4.2 Type II terminations and 507.5.5 Dishwashing appliances. up.codes
- Minnesota Office of the Revisor of Statutes — Minnesota Rules 1346.0507, Section 507 Commercial Kitchen Hoods (current text replaces IMC 507.2 to 507.6.1 with NFPA 96 and ASHRAE 154). revisor.mn.gov
- City of Twin Falls Building Department — Commercial Kitchen Hoods (Types 1 and 2) design guide, quoting IMC 507.3. tfid.org
- Polygee — kitchen exhaust filtration product range and third-party test reports. polygeeesp.com
Related reading
- Type 1 hood vs Type 2 — the concept behind the two hood types and what goes after a Type 1.
- Commercial kitchen hood code requirements — the full Type 1 submittal, from exhaust rate to termination.
- UL 710B vs UL 710 — the ventless listing that is the third route to no hood.
- Solid fuel cooking exhaust requirements — the extra-heavy-duty class and its separate system.