7.2 Type II Hoods, Kitchen Exhaust Fans & Makeup Air

Type II Hoods, Kitchen Exhaust Fans & Makeup Air

Section 7.1 covered Type I hoods and grease ducts. PSI's outline says "type 1 and type 2," and the Type II side plus the makeup-air requirements in Michigan Mechanical Code Section 508 are where a lot of otherwise-competent installers lose points — and where a lot of real restaurants end up with doors that will not open and hoods that will not capture.

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Type II Hoods

MMC Section 507.3: Type II hoods are installed over cooking or dishwashing appliances that produce heat, steam, or products of combustion and do not produce grease or smoke — steamers, steam-jacketed kettles, pasta cookers, and dishwashing machines.

Type II hoods split into two practical sub-types:

• Type II condensate hoods over dishwashers, steamers, and kettles — their job is to capture water vapor before it condenses on the ceiling.
• Type II heat/fume hoods over equipment producing heat and products of combustion without grease.

Construction

The critical discipline is the trigger, not the gauge. The question is always: does this appliance produce grease or smoke? If yes, everything jumps to Type I — heavier hood, welded grease duct, 18-inch clearances, suppression system. If no, Type II construction is permitted and the installed cost drops dramatically. Putting a fryer under a Type II hood is one of the most serious mistakes in commercial kitchen work.

Type II hoods also have their own overhang table. Wall-mounted canopy Type II hoods typically require 6 inches of end overhang and 12 inches of front overhang; island canopies require 12 inches on the applicable sides; eyebrow hoods require 12 inches of front overhang; backshelf, proximity, and pass-over hoods require 6 inches of end overhang and a 10-inch front setback.

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Exhaust Fans

Grease service (Type I)

• Upblast centrifugal roof fans are the standard. The fan housing serving a Type I hood must be constructed of steel, be hinged or otherwise arranged for inspection and cleaning, and drain grease to a listed grease collection device that is not integral to the duct.
• The fan must have a suitable means of restraint so it cannot fall over when hinged open, and the roof curb must be flashed properly.
• The discharge opening must be not less than 40 inches above the roof surface (MMC Section 506.3.13.1), and the discharge must be directed away from the roof surface and away from air intakes.
• In-line fans are permitted where listed for grease service; a cleanout is required on both the inlet and outlet side of an in-line fan.
• Dampers are prohibited in grease duct systems except where specifically required by the code.

Non-grease service (Type II)

Ordinary centrifugal or axial exhaust fans and standard Chapter 6 ductwork are acceptable, with terminations meeting the general exhaust outlet separation rules from openings into the building and from outdoor air intakes.

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Makeup Air (MMC Section 508)

Section 508 is short, and it contains the two rules that make or break a kitchen.

508.1 — General

Makeup air systems shall be automatically controlled to start and operate simultaneously with the exhaust system, and the amount of makeup air supplied to the building from all sources shall be approximately equal to the amount of exhaust air for all exhaust systems serving the building.

Two obligations in one sentence:

1. Interlock. The makeup air unit and the exhaust fan must start together. The most common field failure is a makeup air unit on its own thermostat or a manual switch, so the cooks run the hood in January and leave the MAU off because it is blowing cold air. The building goes deeply negative.
2. Balance. Total makeup air from all sources must approximately equal total exhaust from all systems serving the building — hoods, dishwasher exhaust, toilet exhaust, general kitchen exhaust. Designers routinely balance the hood against its dedicated MAU and forget the other exhaust.

508.1.1 — Makeup air temperature

Makeup air shall be tempered to within 10°F of the design setpoint temperature of the conditioned kitchen area, subject to the exceptions the code states.

In Michigan this is not a comfort nicety. A 4,800 cfm hood pulling untempered 5°F outdoor air into a kitchen represents roughly:

$1.08 \times 4{,}800 \times (75 - 5) = 362{,}880\ \text{Btu/h}$

of heating demand. That is why kitchen makeup air units in Michigan are large direct-fired or indirect-fired gas heaters, and why they must be included in the building's gas load and venting design from the beginning rather than discovered at rough-in.

What happens when the kitchen is unbalanced

Best practice, and the target of the code's "approximately equal" language, is to run the dining area slightly positive relative to the kitchen so cooking odors migrate toward the hood rather than into the dining room, while the building overall stays close to neutral.

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Commissioning Checklist

1. Confirm the automatic operation device (heat sensor or equivalent) starts both the hood exhaust and the makeup air when cooking appliances are in use.
2. Measure and record hood exhaust cfm and MAU supply cfm; compare with design and with all other exhaust in the building.
3. Verify capture and containment visually with a smoke test along the full hood perimeter, under real cooking conditions.
4. Measure building pressure relative to outdoors at the entry, and kitchen relative to dining.
5. Verify makeup air discharge temperature against the 10°F requirement at a cold outdoor condition, or confirm the burner staging that will achieve it.
6. Prove the fire suppression interlock: on discharge, fuel and power to appliances under the hood shut down and the makeup air shuts down per the listing.
7. Deliver a third-party balance report where the enforcing agency requires one — many Michigan jurisdictions do on restaurant projects.