7.1 Type I Commercial Kitchen Hoods & Grease Duct Systems
Key Takeaways
- Type I hoods are required where commercial cooking appliances produce grease or smoke; Michigan Mechanical Code Section 507.2.3 requires steel not less than 0.0466 inch (No. 18 gage) or stainless steel not less than 0.0335 inch (No. 20 MSG).
- Grease ducts must be steel not less than 0.0575 inch (No. 16 gage) or stainless steel not less than 0.0450 inch (No. 18 gage), with all joints, seams, and penetrations made by a continuous liquidtight external weld.
- Grease duct systems serving a Type I hood must be designed for an air velocity of not less than 500 feet per minute (MMC Section 506.3.4).
- Where an enclosure is not required, grease ducts must keep not less than 18 inches clearance to combustible construction and not less than 3 inches to noncombustible construction (MMC Section 506.3.6).
- Grease ducts slope not less than 1/4 unit vertical in 12 units horizontal toward the hood, increasing to 1 unit in 12 where the horizontal run exceeds 75 feet, and exhaust outlets terminating above a roof discharge not less than 40 inches above the roof surface.
Type I Commercial Kitchen Hoods & Grease Duct Systems
PSI's HVAC Equipment content outline names "commercial kitchen exhaust systems for type 1 and type 2" directly. Michigan restaurant work is a large part of the mechanical contracting market, grease exhaust is one of the highest fire-risk systems a mechanical contractor installs, and Chapter 5 of the Michigan Mechanical Code is where all of it lives.
Where a Type I Hood Is Required
MMC Section 507.2: a Type I hood is required where cooking appliances produce grease or smoke as a result of the cooking process. That covers griddles, fryers, ranges, broilers, smokers, wok ranges, and — per the code commentary — conveyor and deck-style pizza ovens.
Compare with Section 507.3, the Type II trigger: Type II hoods go over cooking or dishwashing appliances that produce heat, steam, or products of combustion but no grease or smoke — steamers, kettles, pasta cookers, and dishwashing machines. Section 7.2 of this guide covers Type II.
Duty classifications determine exhaust rate:
| Duty | Typical appliances |
|---|---|
| Light | Ovens, steam-jacketed kettles, pasta cookers, compartment steamers |
| Medium | Griddles, double-sided griddles, fryers, conveyor ovens, ranges |
| Heavy | Gas underfired broilers, gas chain (conveyor) broilers, electric and gas woks |
| Extra heavy | Solid-fuel appliances — charcoal, wood, mesquite, briquette broilers |
Two structural rules follow from the table:
- Extra-heavy-duty (solid fuel) appliances require a separate, independent exhaust system. A solid-fuel char broiler may not share a hood or duct with gas or electric appliances.
- The exhaust rate for the highest duty-rated appliance under the hood applies to the entire hood. One fryer under a hood otherwise covering ovens pulls the whole hood up to the medium-duty rate.
Hood Construction and Sizing
| Item | Requirement (MMC Chapter 5) |
|---|---|
| Type I hood material | Steel not less than 0.0466 inch (No. 18 gage) or stainless steel not less than 0.0335 inch (No. 20 MSG) |
| Joints and seams | External hood joints, seams, and penetrations must be made with a continuous external liquidtight weld down to the lower outermost perimeter |
| Clearance to combustibles | Not less than 18 inches from a Type I hood to combustible construction, with listed reduction methods permitted |
| Overhang | Canopy-type hoods must overhang or extend a horizontal distance of not less than 6 inches beyond the edge of the cooking surface on all open sides |
| Supports | Noncombustible, capable of carrying the hood, the connected ductwork, the effluent load, and the possible weight of personnel working in or on the hood |
| Grease filters | Listed, removable, installed at an angle not less than 45 degrees from horizontal, draining to a collection trough/cup; minimum lowest-edge height above the cooking surface is set by code table |
Exhaust rate — MMC Table 507.5
Net exhaust flow rates for unlisted Type I hoods, in cfm per linear foot of hood:
| Hood type | Light duty | Medium duty | Heavy duty | Extra-heavy duty |
|---|---|---|---|---|
| Wall-mounted canopy | 200 | 300 | 400 | 550 |
| Single island canopy | 400 | 500 | 600 | 700 |
| Double island canopy (per side) | 250 | 300 | 400 | 550 |
| Eyebrow | 250 | 250 | not permitted | not permitted |
| Backshelf / pass-over | 300 | 300 | 400 | not permitted |
Worked example. A 12-foot wall-mounted canopy hood over a bank of fryers and griddles (medium duty) with one gas underfired broiler (heavy duty) at one end. The highest duty under the hood is heavy duty, so the entire hood is rated at 400 cfm per linear foot:
Listed hoods are a different matter. A UL 710 listed hood is exhausted at the flow rate on its listing label, which is frequently well below the unlisted table value because the manufacturer has engineered the capture and containment. Always check for a listing label before applying Table 507.5.
Grease Duct Systems (MMC Section 506.3)
This is where the exam questions concentrate. Memorize the numbers.
| Item | Requirement | Section |
|---|---|---|
| Material | Steel not less than 0.0575 inch (No. 16 gage) or stainless steel not less than 0.0450 inch (No. 18 gage) | 506.3.1.1 |
| Joints, seams, penetrations | Continuous liquidtight weld or braze made on the external surface of the duct | 506.3.2 |
| Supports | Noncombustible, attached to the structure; bolts, screws, rivets, and other mechanical fasteners must not penetrate the duct walls | 506.3.3 |
| Air velocity | Not less than 500 feet per minute in the duct system (exception for transitions not exceeding 3 feet) | 506.3.4 |
| Separation | A separate grease duct system for each Type I hood, with a narrow exception for interconnected hoods on the same story, in the same or adjoining rooms, with no rated-assembly penetrations and no solid-fuel appliances | 506.3.5 |
| Clearances | 18 inches to combustible construction; 3 inches to noncombustible construction and to gypsum board on noncombustible framing, where an enclosure is not required | 506.3.6 |
| Slope | Not less than 1/4 unit vertical in 12 units horizontal (2%) toward the hood or a grease reservoir; not less than 1 unit in 12 (8.3%) where the horizontal duct run exceeds 75 feet | 506.3.7 |
| Cleanouts | Openings not more than 20 feet apart and not more than 10 feet from a change of direction greater than 45 degrees | 506.3.8 |
| Enclosure | Where the duct passes through other parts of the building, a fire-resistance-rated shaft or a listed field-applied enclosure evaluated to ASTM E2336 | 506.3.11 |
| Termination above roof | Discharge opening not less than 40 inches above the roof surface | 506.3.13.1 |
Three points that catch installers:
- No fasteners through the duct wall. Hangers must clamp or be welded to external attachments. A single self-tapping screw through a grease duct creates a grease-wicking leak path directly into the concealed space.
- The weld is external and liquidtight. An internally sealed or mastic-sealed joint is not a grease duct joint.
- Velocity is a minimum, not a maximum, in the 2021 code. Older code editions required 1,500 fpm; the current requirement is not less than 500 fpm. If a question quotes 1,500 fpm, it is quoting either an older edition or a local amendment such as Chicago's.
Fire Protection Interlocks
A Type I hood is a fire-suppression system host, and the mechanical contractor's work has to coordinate with it:
- A UL 300 listed wet-chemical suppression system protects the cooking surfaces, the plenum, and the duct collar.
- On discharge, the system must shut off the fuel or power to all appliances under the hood through a mechanical gas valve or shunt trip.
- The exhaust fan normally continues to run on system discharge (so the wet chemical is drawn across the plenum and duct) while makeup air is shut down — confirm the specific listing and the fire code official's direction, because this interlock is frequently field-verified at final inspection.
- An automatic operation device — a heat sensor or equivalent — must start the hood and makeup air systems when the cooking appliances are in use.
- Access panels on the grease duct must be located so the suppression piping and the duct interior can both be reached.
Scope reminder. Installing the wet-chemical suppression system itself is fire suppression work under classification (i). Installing the hood, duct, fan, and makeup air is HVAC Equipment and Ductwork work. On a Michigan restaurant job, make sure the right license is on the right permit.
A 12-foot wall-mounted canopy Type I hood covers a row of fryers and griddles (medium duty) plus one gas underfired broiler (heavy duty). The hood is not listed. Using Michigan Mechanical Code Table 507.5, what is the minimum net exhaust flow rate?
A grease duct serving a Type I hood runs 90 feet horizontally above a suspended ceiling before turning up to a rooftop fan. Which combination of requirements applies under Michigan Mechanical Code Section 506.3?
An inspector rejects a newly fabricated grease duct because the contractor hung it with self-tapping sheet metal screws through the duct wall into strap hangers, and sealed the transverse joints with high-temperature silicone on the inside. Which two code provisions were violated?
A restaurant design places a solid-fuel mesquite broiler at one end of a 16-foot wall canopy hood that also covers gas ranges and a fryer battery. What does the Michigan Mechanical Code require?