22.3 Ventilation, Exhaust, Makeup Air, Grease Ducts & Kitchen Exhaust

22.3 Ventilation, Exhaust, Makeup Air, Grease Ducts & Kitchen Exhaust

Trade C’s air-side install cluster is ventilation ductwork (fire codes, scale, sheet metal / fibrous glass / flex, fittings, outlets, hanging), exhaust and makeup, grease-handling ducts, and kitchen exhaust. Chapters 18.4 and 19.4 already built the hood and the welded duct. This section is the Trade C install: how that system sits in the building with outdoor air, makeup, and fire codes. The 2026 CBT list names NFPA 90A 2015, NFPA 90B 2024, NFPA 96 2021, SMACNA HVAC Duct Construction Metal and Flexible 4th 2020, SMACNA Fibrous Glass 8th 2021, and FBC Mechanical 2023 Chapters 4, 5, and 6.

Quick Answer: 90A is commercial AC/vent fire protection; 90B is dwelling warm-air; 96 is grease. Outdoor air follows people + area (ASHRAE 62.1 / FBC 403). Makeup ≈ exhaust, interlocked, and must not wreck hood capture. Grease duct is 16/18-gage welded, 500 fpm minimum, sloped, and light-tested (FBC 506.3.2.5, lamp ≥ 100 watts) before concealment.

Fire codes, scale, and which book is in your hand

NFPA 90A (2015), Standard for the Installation of Air-Conditioning and Ventilating Systems, is the commercial book: duct construction in fire-resistance-rated buildings, fire dampers, smoke dampers, ceiling radiation dampers, and smoke detection that prevents recirculating smoke (Section 22.4 quotes FBC 606, which points at 90A). NFPA 90B (2024) is warm-air heating and air-conditioning for one- and two-family dwellings and manufactured homes — lighter construction, still listed flex and still no grease duct. Using 90B tables on a three-story office is the wrong book. Using 90A as an excuse to skip NFPA 96 on a Type I hood is the other wrong book.

Scale on the drawings is still the architect’s 1/4 inch or 1/8 inch (Chapter 15). Exhaust and makeup ducts are measured centerline the same way supply ducts are. Velocity in a grease duct is not a Ductulator comfort-cooling pick: FBC 506.3.4 / NFPA 96 want not less than 500 fpm (and a listed maximum, commonly 2,500 fpm in the mechanical-code family) so grease stays airborne to the fan instead of plating in the elbow.

Sheet metal, fibrous glass, flex, fittings, outlets, hanging

Ordinary ventilation duct is the Chapter 19 product:

Fittings (elbows, wyes, takeoffs) match the duct’s pressure class (Section 19.3). Outlets are grilles, registers, and diffusers; FBC 603.18 wants a volume damper or other adjustment in the branch or at the device, with access. Hang straps from the building structure. Do not hang ventilation duct from a sprinkler pipe, a gas line, or another trade’s trapeze (Chapter 18.4: HVAC does not relocate sprinklers).

Fire and smoke dampers go where ducts penetrate fire-resistance-rated assemblies or smoke barriers, listed for the rating, installed in the plane of the assembly per the listing — that is FBC 607 / NFPA 90A, not a shop-made sliding plate. Combination fire/smoke dampers need power and a smoke signal; they are not a volume damper painted red.

Outdoor air, exhaust, and makeup (ASHRAE 62.1 qualitative)

ASHRAE Standard 62.1 is the commercial ventilation standard FBC Mechanical Chapter 4 is built from. The Ventilation Rate Procedure is qualitative on this exam: outdoor air for a zone is people outdoor air (cfm per person × occupants) plus area outdoor air (cfm per square foot × floor area). More people and more floor need more outdoor air. A locked 10 percent OA damper on a 20-ton rooftop is not a 62.1 calculation. Rest rooms, locker rooms, and janitor closets also have local exhaust rates in FBC Table 403.3.1.1; that exhaust is not optional because the rooftop “has some OA.”

Exhaust without makeup pulls the building negative. Doors whistle, combustion appliances spillage-risk, and a Type I hood loses capture (the plume spills into the dining room). FBC 508.1: the amount of makeup air supplied to a commercial kitchen shall be approximately equal to the exhaust; makeup shall be supplied by gravity or mechanical means; windows and doors shall not be used as the makeup path; makeup shall be automatically controlled to start and stop with the exhaust; and makeup shall not reduce the effectiveness of the exhaust system. NFPA 96 adds a performance cap: replacement air shall keep building negative pressure from exceeding 0.02 in. w.c. while the exhaust runs. Type I exhaust stays independent of the HVAC return (you do not share a grease duct with a comfort return). Makeup may be tempered; dumping unconditioned 95°F, 80-grain Florida air on the cook line is a comfort failure, not a code pass.

Worked makeup. A listed Type I canopy is 2,400 cfm. Makeup is approximately 2,400 cfm, interlocked. If the dining rooftop is only 1,200 cfm of outdoor air, it is not the kitchen makeup. A transfer grille from the dining room can be part of the replacement air only where the code allows transfer — it still must not steal capture at the hood lip and it still must not use the corridor as a duct except for the narrow FBC 601.2 exceptions.

Grease ducts and kitchen exhaust — install, do not re-shop Chapter 19

Type I hoods (grease) and Type II hoods (heat/steam, no grease) were fabricated in Section 19.4. Field-welded Type I duct remains carbon steel not less than No. 16 gage or stainless not less than No. 18 gage, continuous liquid-tight external weld (FBC 506.3 / NFPA 96). Slope not less than 1/4 inch per foot toward the hood or a reservoir; if the horizontal run exceeds 75 feet, slope not less than 1 inch per foot (Section 18.4). Cleanouts on horizontal duct are not more than 20 feet apart and not more than 10 feet from changes of direction greater than 45° (Section 19.4). Minimum in-duct velocity 500 fpm. Fans: motor out of the airstream, UL 762 (or constructed as grease duct), upblast fans hinged with the listing’s duct height (commonly 18 inches of duct above the roof and 40 inches of clearance to the roofline in the NFPA 96 / FBC family — read the listing on the exam table).

FBC Mechanical 506.3.2.5 Grease duct test. Before the duct is used or concealed, a leakage test is required. Ducts shall be considered to be concealed where installed in shafts or other inaccessibility. A light test or an approved equivalent shall show that welded and brazed joints are liquid-tight. A light test passes a lamp of not less than 100 watts through the entire section to be tested. Many Florida fire departments witness that test — that is the “fire dept smoke/light test” the outline pairs with kitchen exhaust. A theatrical smoke candle in a 26-gage snap-lock trunk is not a 506.3.2.5 test.

Clearance to combustibles (commonly 18 inches unless a listed enclosure or wrap reduces it), listed grease filters, and a fire-extinguishing system interface (FBC 509 / NFPA 96) are part of the install. HVAC contractors do not design the wet-chemical system as fire-protection contractors, but they interlock the exhaust fan, makeup, and fuel/electric shutoff the way the listing and NFPA 96 require. Type I exhaust does not get a fire damper that would trap grease; shaft and enclosure rules in 506.3.11 replace that thought.

Florida scenario

A certified Class B shop in Orange County fits a 2,400 cfm Type I canopy, welded 16-gage grease duct up a rated shaft with listed wrap, an upblast UL 762 fan, and 2,400 cfm of interlocked tempered makeup for a 150-seat restaurant. Outdoor air for the dining room is a separate 62.1 / FBC 403 number on the rooftop (people + area) — that rooftop is not the hood makeup. The grease duct is light-tested with a ≥100-watt lamp before the shaft is closed; the AHJ’s fire inspector stands there. Ordinary 2 in. w.g. SMACNA supply to the dining room is hung from structure, sealed, and balanced (Section 22.4). Substituting fibrous-glass board for the grease riser, using the dining-room OA damper as makeup, or skipping the light test because “it is welded so it cannot leak” are the three ways this ticket fails. The hood is well under 25 tons of cooling, so Class B may hold the mechanical permit; the grease rules are still 96 / Chapter 5, not 90B.

Traps: (1) Using NFPA 90B on a commercial kitchen. (2) Makeup from an open window. (3) 26-gage snap-lock “grease duct.” (4) Hanging duct from the sprinkler main. (5) Treating dining-room OA as hood makeup. (6) Concealing a grease duct before the 100-watt light test.