Kitchen Exhaust Ductwork: Sizing, Materials, and Code Requirements

Kitchen exhaust ductwork is one of the most code-regulated areas of HVAC. The stakes are higher than comfort ductwork - grease-laden air is a fire hazard, and undersized exhaust duct creates indoor air quality problems that affect health. Whether you are installing a residential range hood or a commercial kitchen hood system, the duct sizing, material, and installation must meet specific requirements.

Residential Kitchen Exhaust: The Basics

Residential kitchen exhaust is governed by the IRC (International Residential Code) Section M1503. The requirements are straightforward but frequently violated:

• Exhaust duct must be made of galvanized steel, stainless steel, or copper. No aluminum, flex duct, or PVC.
• Duct must be airtight with no screws or fasteners protruding into the airstream (screws collect grease).
• Duct joints must be sealed with approved methods - typically mastic or listed HVAC tape. No duct tape.
• The duct must terminate outside the building. Never terminate in an attic, crawl space, soffit, or garage.
• A backdraft damper is required at the wall or roof cap to prevent outside air from entering when the hood is off.

How Much CFM Does a Range Hood Need?

The CFM (cubic feet per minute) rating of the range hood determines the duct size. Here are the sizing guidelines:

Against-the-wall hoods: The standard rule is 100 CFM per linear foot of range width. A 30" range needs a minimum of 250 CFM, a 36" range needs 300 CFM, and a 48" professional range needs 400-600 CFM.

Island hoods: Because there are no walls to help contain rising fumes, island hoods need approximately 150 CFM per linear foot. A 36" island hood should have at least 450 CFM.

High-BTU burners: For gas ranges with burners exceeding 15,000 BTU, the rule changes to 1 CFM per 100 BTU of total burner output. A professional range with 80,000 BTU total output needs an 800 CFM hood, regardless of width.

Never downsize the duct below the hood outlet diameter. If the hood has an 8" round outlet, the duct must be 8" round or equivalent rectangular area for its entire length. Reducing the duct size increases static pressure, reduces airflow, and makes the hood louder.

Duct Routing and Length Limits

Every foot of duct and every fitting adds resistance. Most residential range hoods are rated at a specific CFM with a maximum equivalent duct length. Typical limits:

• Maximum straight run: 30-50 feet depending on the hood CFM and duct size.
• Each 90-degree elbow adds approximately 10 equivalent feet.
• Each 45-degree elbow adds approximately 5 equivalent feet.
• Wall or roof cap adds approximately 5 equivalent feet.

A typical installation with a wall cap, two 90-degree elbows, and 8 feet of straight duct has an equivalent length of approximately 8 + 10 + 10 + 5 = 33 feet. Check the hood manufacturer's installation guide for the maximum allowable equivalent length at the rated CFM.

Shorter, straighter duct runs perform dramatically better. Every eliminated elbow recovers 10 equivalent feet of capacity. Route the duct directly out the nearest exterior wall whenever possible rather than running through the attic to a roof cap.

Commercial Kitchen Exhaust: A Different World

Commercial kitchen exhaust is governed by the IMC (International Mechanical Code) Chapter 5 and NFPA 96 (Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations). The requirements are far more stringent than residential.

Grease Duct Requirements (Type I Hoods)

Type I hoods are used over cooking equipment that produces grease-laden vapors: fryers, grills, broilers, ranges, and woks. The exhaust ductwork from a Type I hood is classified as grease duct, and the requirements are severe:

• Material: Minimum 16-gauge carbon steel (welded black iron) or 18-gauge stainless steel (Type 304 or 316). Galvanized steel is NOT permitted for grease duct - the zinc coating breaks down at the temperatures involved.
• Joints: All joints must be continuously welded (liquid-tight). No mechanical joints, no drive cleats, no slip connections. The weld must be on the exterior of the duct.
• Slope: Horizontal runs must slope back toward the hood at a minimum of 1/4" per foot to drain grease back to the hood filters and grease collection system.
• Cleanout access: Cleanout openings are required at every direction change and at intervals not exceeding 20 feet on horizontal runs and every floor on vertical runs.
• Clearance to combustibles: Minimum 18 inches of clearance from grease duct to any combustible material (wood framing, insulation, etc.). This clearance can be reduced to 6 inches with a listed grease duct enclosure (fire-rated wrap system).
• Fire dampers prohibited: Fire dampers are NOT allowed in grease duct. Instead, the duct itself is the fire-rated assembly.

Fire Wrap and Enclosure Systems

When grease duct passes through combustible construction, it must be enclosed in a fire-rated shaft or wrapped with a listed fire-rated grease duct enclosure system. Common listed systems include ceramic fiber blanket wraps that provide 1-hour or 2-hour fire ratings. These wraps allow the 18" clearance to combustibles to be reduced to as little as 3 inches in some listed assemblies.

The fire wrap must cover the entire duct length where it passes through or is adjacent to combustible construction, including through roof penetrations. This adds significant cost to commercial kitchen exhaust installations - fire wrap material runs $15-25 per linear foot plus installation labor.

Commercial Exhaust CFM Sizing

Commercial hood CFM is calculated based on hood type and dimensions, not cooking equipment BTU. The two methods:

Method 1 - CFM per linear foot of hood:

Method 2 - CFM per square foot of hood opening: Typically 150-250 CFM per square foot of hood face area, depending on cooking process and hood design.

A typical 12-foot wall canopy hood over a heavy-duty cooking line (grill, fryers, range) would require approximately 3,600-4,800 CFM. That volume demands a duct in the range of 20" x 20" rectangular or 22" round.

Makeup Air Requirements

Both residential and commercial codes now require makeup air when exhaust exceeds certain thresholds.

Residential (IRC M1503.6): Makeup air is required when the kitchen exhaust rate exceeds 400 CFM. The makeup air system must be interlocked with the exhaust hood and provide approximately the same volume of air as the exhaust. This prevents the house from going into negative pressure, which can cause backdrafting of gas appliances - a serious safety hazard.

Commercial (IMC 508): The building HVAC system must provide enough outdoor air to compensate for the exhaust. Typically, 80-85% of the exhaust CFM is provided as conditioned makeup air through the HVAC system or a dedicated makeup air unit (MAU). The remaining 15-20% is transfer air from the dining room, which creates a slight negative pressure in the kitchen to keep cooking odors from migrating to the dining area.

Makeup air ductwork is standard HVAC duct - galvanized rectangular or round with standard connections. It does not have the welded-joint and fire-wrap requirements of grease duct.

Common Kitchen Exhaust Mistakes

• Using screws in the duct interior. Sheet metal screws protruding into the airstream collect grease and create fire ignition points. Use external rivets or welds only.
• Reducing duct size below the hood outlet. This is both a code violation and a performance killer. The duct must never be smaller than the hood outlet.
• Terminating duct in the attic or soffit. Grease-laden exhaust in an enclosed space is a fire waiting to happen. Always terminate outside with a proper cap.
• Using flex duct for range hood exhaust. Flex duct is not code-approved for kitchen exhaust. It collects grease, sags, and cannot be cleaned.
• Ignoring makeup air. A 600 CFM hood without makeup air depressurizes a modern tight home enough to backdraft a gas water heater or furnace.

Velocity Check: Worked Examples

Once you have a CFM and a duct size, check the velocity. Velocity (FPM) = CFM / duct area in square feet, and the area of a round duct is 0.7854 x (diameter in inches / 12)². Here are the minimum duct sizes from the residential table above, run through that formula:

The minimum sizes all land between roughly 1,270 and 1,720 FPM. That moves grease-laden air out quickly, but the rows near 1,700 FPM leave the least room for a long run. Friction loss rises with the square of velocity, so the 600 CFM hood on 8" round burns through its equivalent-length allowance much faster than the 450 CFM hood on the same duct. If the run is long and the hood manufacturer allows a larger duct, stepping the 600 CFM hood up to 10" round drops velocity to 1,100 FPM and cuts friction per foot by more than half.

Commercial check. The 12-foot heavy-duty canopy above needs 3,600 to 4,800 CFM. A 20" x 20" duct has 400 square inches of area, or 2.78 square feet, so it runs at 1,296 to 1,728 FPM. A 22" round duct has 2.64 square feet and runs at 1,364 to 1,818 FPM. Both are well above the 500 FPM minimum for grease duct, so either size keeps grease moving toward the fan.

Order the Right Exhaust Duct

Kitchen exhaust duct must be the correct size, material, and connection type - there is no room for compromise. Use the PMX Ductwork Designer to configure straight sections, elbows, transitions, and square-to-round adapters in galvanized or stainless steel with the exact dimensions your installation requires.

Frequently Asked Questions

What size duct does a range hood need?

A range hood needs a duct at least as large as its outlet, sized to its CFM: 6-inch round for 250 to 300 CFM, 7-inch for about 360 CFM, 8-inch for 450 to 600 CFM and 10-inch for 900 CFM. Never reduce the duct below the hood outlet anywhere in the run, because that raises static pressure, cuts airflow and makes the hood louder.

Can you use flex duct for a range hood?

No. Flex duct is not approved for kitchen exhaust because it collects grease, sags and cannot be cleaned. Residential range hood duct must be galvanized steel, stainless steel or copper, with sealed joints and no screws protruding into the airstream. It must terminate outside the building through a wall or roof cap with a backdraft damper, never in an attic or soffit.

How long can a range hood duct run be?

Most residential range hoods allow 30 to 50 feet of equivalent duct length, depending on hood CFM and duct size. Count each 90-degree elbow as about 10 feet, each 45-degree elbow as 5 feet and the wall or roof cap as 5 feet. A run with 8 feet of straight duct, two 90-degree elbows and a cap is about 33 equivalent feet.

When is makeup air required for a kitchen exhaust hood?

Makeup air is required for a residential range hood when the exhaust rate exceeds 400 CFM. The makeup air system must be interlocked with the hood and supply roughly the same volume of air it exhausts, so the house does not go negative and backdraft gas appliances. Commercial kitchens typically supply 80 to 85 percent of exhaust CFM as conditioned makeup air.