Range Hood Installation Requirements: The Complete Guide for Homeowners and Contractors | Jaspector

A range hood is one of the most misunderstood appliances in a kitchen. Homeowners pick a model that looks good and fits the cabinet opening, then wonder why it barely moves the smoke. Or they install a powerful commercial-style hood without makeup air and start getting backdrafting from the water heater. Or they run a flexible duct through six elbows and a 30-foot run and gut the motor's performance entirely.

This guide covers range hood installation from first principles. If you are a homeowner doing your own research before hiring someone, or a contractor who wants a single reference for code-compliant installations, this is for you.

1. CFM Sizing: How Powerful Does the Hood Need to Be?

CFM — cubic feet per minute — is the airflow rating of a range hood. Getting this number right is the most important decision you will make. Undersize the hood and it cannot capture cooking effluent. Oversize it and you create makeup air problems and unnecessary noise.

The Basic Rules

Gas cooktops are sized by total BTU output. Add up all the burners at their maximum output, then divide by 100. That gives you the minimum CFM.

Example: Four burners at 15,000 BTU each = 60,000 BTU total. 60,000 / 100 = 600 CFM minimum.

Electric cooktops are sized by wattage. Use 1 CFM per 10 watts of total cooktop wattage.

Example: A 7,200-watt induction cooktop needs at least 720 CFM.

Island hoods — any hood mounted over an island or peninsula without a wall behind it — require 50% more CFM than a wall-mount hood for the same cooktop. Open air allows more heat and vapor to escape laterally before the hood can capture it. A cooktop that needs 400 CFM under a wall hood needs 600 CFM under an island hood.

Commercial-style or high-output gas ranges are a special case. Ranges with sealed burners rated above 15,000 BTU each, or with a griddle or wok burner at 25,000+ BTU, can produce dramatically more heat. Apply the same BTU/100 rule, but do not let the result anchor you — if a single burner produces 25,000 BTU, that burner alone demands a 250 CFM contribution before you add the others.

Minimum CFM by Cooktop Type

Cooktop Type	Typical BTU / Wattage	Minimum CFM (Wall Hood)	Minimum CFM (Island Hood)
Basic 4-burner electric, coil or smooth	7,000–9,000W	200–300 CFM	300–450 CFM
Induction, standard	7,200–9,600W	300–400 CFM	450–600 CFM
Gas, basic 4-burner (36,000–48,000 BTU)	36,000–48,000 BTU	360–480 CFM	540–720 CFM
Gas, mid-range (48,000–60,000 BTU)	48,000–60,000 BTU	480–600 CFM	720–900 CFM
High-output gas / pro-style (60,000–100,000 BTU)	60,000–100,000 BTU	600–1,000 CFM	900–1,500 CFM
Commercial-style 6-burner or with griddle	100,000–150,000 BTU	1,000–1,500 CFM	1,500+ CFM

These are minimums for code compliance and functional performance. Real-world cooking — especially high-heat stir fry or searing — benefits from more airflow than the formula suggests. If a client is buying a pro-style range, do not round down.

What Manufacturers Advertise vs. What You Get

Rated CFM is measured with no ductwork attached. Every foot of duct and every elbow reduces effective airflow. A hood rated at 600 CFM may deliver 400 CFM at the cooktop in a real installation with a typical duct run. Size up slightly to account for this. The duct sizing section below explains how to minimize losses.

2. Duct Sizing: Getting Air Out Efficiently

The duct is the second biggest variable in hood performance. A well-sized, short duct run allows the motor to operate near its rated output. An undersized or overly long duct chokes airflow and raises static pressure until the motor is working hard to move far less air than its rating suggests.

Why Duct Size Matters So Much

Range hood blowers are centrifugal fans. They are designed to operate within a specific static pressure range. When duct resistance is too high — from an undersized duct, excessive length, or too many fittings — the fan's operating point moves up the pressure curve and down the airflow curve. A 600 CFM hood running through a 4-inch duct will deliver something closer to 200 CFM at the exterior. The motor is not broken. It is just fighting physics.

This is also why flexible ("flex") duct is a bad choice for range hood applications. The corrugated interior surface creates dramatically more friction than smooth rigid duct. If flex duct must be used — which it should not be — it must be fully extended with no sags or bends, and the CFM must be derated accordingly. Most manufacturers void the warranty if flex duct is used.

CFM to Duct Diameter Reference

Rated Hood CFM	Minimum Round Duct Diameter	Notes
Up to 200 CFM	4 inches	Absolute minimum; only for light-use ductless conversions
200–300 CFM	5 inches	Common for basic under-cabinet hoods
300–400 CFM	6 inches	Most common residential installation
400–600 CFM	7 inches	Mid-range gas cooktops
600–900 CFM	8 inches	High-output gas, induction
900–1,200 CFM	10 inches	Pro-style ranges
1,200+ CFM	12 inches	Commercial applications, large kitchen islands

When in doubt, size up. A larger duct than required costs almost nothing extra and reduces noise, reduces static pressure losses, and extends motor life.

Round vs. Rectangular Duct

Many installations use rectangular (also called "flat") duct to fit inside cabinet runs or wall cavities. Rectangular duct is acceptable, but round duct is preferred whenever possible. Round duct has better airflow characteristics — less turbulence, more efficient cross-section — and leaks less at joints.

If you must use rectangular duct, use the equivalent diameter to match it to the CFM table above. Equivalent diameter for a rectangle is calculated as:

D_e = 1.30 × (a × b)^0.625 / (a + b)^0.25

Where a and b are the duct dimensions. Practically speaking, use this table for common sizes:

Rectangular Duct Size	Round Equivalent
3.25" × 10"	~6 inches
3.25" × 14"	~7 inches
6" × 10"	~8 inches
8" × 10"	~9 inches
10" × 10"	~10 inches

One more note: reduce the number of duct transitions. Every transition from round to rectangular and back introduces turbulence and restriction. If your run requires it, fine — but do not add transitions unnecessarily.

3. Duct Routing: Getting the Air Outside

Shortest Path to Exterior

The goal is to get from the hood outlet to the exterior wall or roof cap in the fewest feet and fewest bends possible. A straight, short run through an exterior wall is ideal. The longer and more convoluted the path, the more airflow you lose.

Equivalent Duct Length

Every fitting in the duct run adds resistance equivalent to a certain number of feet of straight duct. This is called equivalent length. When planning a route, add up the total equivalent length and compare it to the manufacturer's maximum rated duct length.

Fitting Type	Equivalent Length Added
90-degree elbow, 5" and smaller	10 feet
90-degree elbow, 6"–8"	8 feet
90-degree elbow, 10" and larger	6 feet
45-degree elbow	5 feet
Offset (two 45s back to back)	8 feet
Roof cap / wall cap	10–20 feet (check manufacturer)
Damper (inline)	5 feet
Each foot of straight duct	1 foot

Example: A hood outlet in the back of a cabinet, running up through the cabinet and out the wall above, with two 90-degree elbows and a wall cap, with 8 feet of straight duct. Using 6-inch duct:

8 feet straight duct: 8 feet
Two 90-degree elbows at 8 feet each: 16 feet
Wall cap: 15 feet
Total equivalent length: 39 feet

Most hoods rated at 400–600 CFM specify a maximum equivalent duct length of 50–100 feet. Check your specific model. At 39 feet equivalent, this installation is fine. At 100 feet equivalent — which is easy to accumulate with a long attic run and multiple elbows — the hood will be dramatically underperforming.

Exterior Termination

Wall caps are used when the duct exits through an exterior wall. They should include a backdraft damper (either built in or installed inline) that prevents cold outside air from flowing back into the kitchen when the hood is off. In cold climates, this also prevents condensation inside the duct.

Roof caps (also called roof jacks or roof vents) are used when the duct must exit through the roof. This is common with island hoods or homes where the range is on an interior wall. Roof caps must be rated for kitchen exhaust — they run hotter and carry grease. Use a cap with a motorized or spring-loaded damper, not a passive louvered vent, which may not seal well enough.

Never terminate a range hood duct into an attic, crawl space, wall cavity, or soffit. This is a fire and moisture hazard. The grease-laden air will condense on framing, create mold conditions, and is a significant fire risk. Many jurisdictions treat this as a code violation, and for good reason.

Backdraft dampers should always be present — either built into the hood, built into the wall/roof cap, or installed as a standalone inline damper. Do not rely on wall cap louvers alone. Louvers can stick open, be pushed open by wind, or fail to seal in cold weather. A spring-loaded or motorized inline damper is the reliable choice.

4. Recirculating (Ductless) Hoods: What They Are and When to Use Them

How They Work

A ductless hood draws air through a grease filter, then through an activated charcoal (carbon) filter, and recirculates the cleaned air back into the kitchen. There is no exterior duct connection. The charcoal filter adsorbs odors and some particulate. The grease filter catches grease before it reaches the charcoal.

When Ductless Is Acceptable

Ductless hoods are acceptable in specific situations:

Over electric or induction cooktops only (see below on gas)
In apartments or condos where exterior penetrations are not permitted
In kitchen islands or locations where exterior ducting would require a prohibitively long run or extensive structural work
Light cooking use — occasional meals, not daily high-heat cooking

When Ducted Is Required or Strongly Preferred

Ductless hoods are not recommended — and in some jurisdictions not code-compliant — over gas cooktops. The reason is combustion byproducts. A gas burner produces carbon monoxide, nitrogen dioxide, and water vapor. Charcoal filters do not remove these gases. A ductless hood above a gas range recirculates combustion gases back into the kitchen air. This is a health concern, not just a performance concern.

California's Title 24 and several other state energy codes also require ducted ventilation for new construction kitchens above a certain threshold.

Even where ductless hoods are technically permitted, their performance is inferior for all but light use. A ductless hood can never remove moisture from the kitchen — it only filters it. Over time, heavy cooking loads saturate the charcoal filter, and the filter becomes ineffective at odor removal. Charcoal filters must be replaced on a schedule (typically every 6–12 months for regular use), and this maintenance is frequently neglected.

CFM Ratings Are Misleading for Ductless Hoods

A ductless hood rated at 400 CFM does not deliver 400 CFM of cleaned, filtered air to the kitchen. That CFM rating describes how fast the fan can move air through the filter — but a loaded charcoal filter has high resistance. As the filter loads up with use, actual airflow drops significantly below the rated value.

More importantly, CFM is a measure of air movement, not filtration effectiveness. A ductless hood moving 400 CFM of recirculated air is not equivalent to a ducted hood moving 400 CFM of kitchen air to the exterior. The ducted hood is removing grease, moisture, and combustion gases from the building envelope. The ductless hood is only partially filtering them and returning them to the same space.

Do not compare ductless and ducted hood CFM ratings as if they represent the same thing. They do not.

Filter Types

Baffle filters are stamped aluminum or stainless steel channels that force air through directional changes, causing grease to separate and drain into a collection trough. They are durable, cleanable, and far more effective at grease separation than mesh filters. Baffle filters are preferred for any medium to heavy cooking use.

Mesh filters are layers of fine aluminum mesh. They catch grease but clog faster than baffle filters and require more frequent cleaning. They are common on cheaper hoods.

Charcoal (activated carbon) filters are used in ductless hoods for odor adsorption. They are not washable — they must be replaced. A charcoal filter that is not replaced becomes a grease-saturated block that may restrict airflow and no longer adsorbs odors effectively.

5. Makeup Air: The Overlooked Requirement

What Is Makeup Air?

Every cubic foot of air a range hood exhausts must be replaced by air entering the home from somewhere. In older, leaky construction, this happened naturally through gaps in the building envelope. In modern tight construction — especially homes built to energy codes from the 2010s onward — the building envelope is sealed well enough that a powerful range hood creates measurable negative pressure inside the house.

That negative pressure has real consequences.

When Makeup Air Is Required

Most jurisdictions follow the International Residential Code (IRC), which requires a makeup air system when a range hood exceeds 400 CFM. Some jurisdictions set the threshold at 300 CFM. California's Title 24 has its own requirements.

If your hood exceeds the local threshold and you do not have makeup air, you are typically not code-compliant for new construction or major remodels. Even if permits are not pulled, the practical problems will appear.

What Happens Without Makeup Air

In a tight house, running a 600 CFM hood without makeup air can depressurize the kitchen area enough to:

Cause combustion appliances (water heater, furnace, gas range) to backdraft — pulling combustion gases, including carbon monoxide, from the flue back into the living space
Cause doors to stick or not close properly
Pull radon or soil gases from crawl spaces or slabs
Stress weatherstripping and gaskets throughout the house

Backdrafting from a gas water heater or furnace is the most dangerous consequence. A hood that is powerful enough to require makeup air but does not have it is not a performance problem — it is a safety problem.

Passive Makeup Air

The simplest form of makeup air is a dedicated exterior air inlet — a duct with a damper that opens when the hood runs. The damper is connected to the hood's power circuit so it opens automatically. Outside air enters through the duct, typically located near the kitchen.

The downsides of passive makeup air: it introduces unconditioned air directly into the living space. In winter, that means cold air. In summer, hot, humid air. This affects comfort and can affect HVAC system sizing.

Passive systems are adequate for moderate climates and hoods in the 400–600 CFM range.

Active (Conditioned) Makeup Air

An active makeup air unit draws outside air and conditions it — heating it in winter, cooling it in summer, and sometimes dehumidifying it — before introducing it into the kitchen. This is the correct approach for:

High-CFM installations (600 CFM and above)
Climates with cold winters or hot humid summers
Homes where comfort and HVAC load are priorities

Active makeup air units are a significant additional cost. Budget $1,000–$3,000 for the unit, plus installation. For a home with a 1,000 CFM hood in a cold climate, an active makeup air unit is not optional — it is the right way to do the job.

Coordination With HVAC

Any makeup air system must be reviewed alongside the home's HVAC. Introducing a large volume of unconditioned air — even temporarily while cooking — can affect heating and cooling loads, duct pressure balancing, and filtration. A mechanical engineer or experienced HVAC contractor should be involved in makeup air design for high-CFM installations.

6. Mounting Height: Why It Matters

Standard Mounting Heights

Hood Type	Minimum Height	Recommended Height	Maximum Height
Over electric cooktop	24 inches	24–30 inches	30 inches
Over gas cooktop	30 inches	30–36 inches	36 inches

Height is measured from the cooktop surface to the bottom of the hood.

Capture Efficiency

A range hood works by creating a zone of negative pressure below it that captures rising cooking effluent — grease particles, steam, smoke, and odors. The lower the hood is to the cooktop surface, the smaller the capture zone and the more effective the capture. As you raise the hood, the capture zone expands and becomes less defined. Effluent plumes can escape laterally before reaching the capture zone.

Most manufacturers specify a recommended mounting height for their specific model based on its capture area and motor power. Exceed that height and you are degrading performance. A hood that captures 90% of cooking effluent at 30 inches may capture 60% at 36 inches.

Fire Safety Clearances

Height requirements for gas cooktops are not arbitrary. The combustion zone of a gas burner produces radiant heat, and the base of a hood that is too close can reach temperatures that pose a fire risk — especially hoods with plastic components, grease filters, or wiring inside the housing.

The National Fire Protection Association (NFPA) and most manufacturers specify a minimum of 30 inches from a gas burner to the bottom of the hood. Do not go below this. If the hood appears to require a lower mounting position to fit the kitchen layout, the solution is to select a different hood — not to mount it below the minimum clearance.

Tall Ceilings

Standard chimney sections for wall-mount and island hoods are designed for 8-foot or 9-foot ceilings. If the ceiling is 10 feet or higher, chimney extensions are required. Most manufacturers sell extensions for their specific models. Generic extensions may fit but create a visual mismatch. If ordering a hood for a tall-ceiling kitchen, order the extension at the same time.

7. Electrical Requirements

Voltage and Circuit Type

Most residential range hoods run on 120V. Commercial and high-CFM residential hoods may require 240V — check the model specifications.

Outlet vs. hardwired: Many hoods are designed to plug into a standard 120V outlet inside the cabinet above the hood or within the chimney chase. Others are designed to hardwire directly to a junction box. Both are acceptable if the installation matches the hood's design. Do not hardwire a plug-in hood by cutting the cord — install the outlet the hood is designed to use.

Dedicated circuit: IRC and NEC do not require a dedicated circuit for a range hood specifically. Range hoods share circuits with other kitchen outlets in many residential installations. However, for hoods above 600 CFM — which often have multiple motors — a dedicated circuit is good practice. Some high-CFM hoods require it per the manufacturer.

GFCI Requirements

NEC 210.8 requires GFCI protection for all receptacles within 6 feet of a kitchen sink. Depending on the kitchen layout, the outlet for the range hood may fall within this zone. Check the distance from the cooktop outlet location to the nearest sink. When in doubt, use a GFCI-protected outlet — it is never wrong to add protection.

For hardwired hoods, GFCI protection is typically not required at the junction box, but verify with your local jurisdiction's interpretation of NEC.

Lighting Circuits

Some range hoods include LED lighting systems that require low-voltage drivers. These are self-contained. No additional wiring is needed beyond the hood's main power supply.

8. Hood Types: Structural and Installation Requirements

Under-Cabinet Hoods

Under-cabinet hoods mount to the underside of a cabinet directly above the cooktop. They are the most common type in residential kitchens and the simplest to install.

Structural requirements: The existing cabinet provides the mounting surface. Standard under-cabinet hoods are relatively light (15–40 lbs) and the cabinet is adequate support for all but the heaviest commercial-grade units.

Duct routing: Duct typically exits the top of the hood into the cabinet, then routes through the cabinet interior and up through the wall or ceiling. This requires cutting through the cabinet back or top.

Key consideration: Under-cabinet hoods reduce usable cabinet space. The hood itself takes vertical space, and the duct running through the cabinet above removes shelf space.

Wall-Mount Hoods (Chimney Hoods)

Wall-mount hoods attach to the wall above the cooktop, with a decorative stainless chimney section that extends to the ceiling. There is no cabinet above — the hood and chimney are freestanding from wall brackets.

Structural requirements: Wall-mount hoods are heavier (50–150 lbs for the full assembly) and must be secured to wall studs. A single 2x4 stud may not be adequate for the largest hoods — blocking (a horizontal piece of lumber between studs) installed during rough-in is the right approach. For heavy hoods on a finished wall, a steel bracket spanning two studs with lag bolt attachment is the minimum.

Duct routing: Duct exits the top of the hood housing and runs up inside the chimney section to exit at or above the ceiling line. On an exterior wall, a short horizontal run to a wall cap is ideal. On an interior wall, the duct must route through the attic or ceiling cavity to an exterior termination.

Island Hoods (Ceiling-Mount)

Island hoods hang from the ceiling above a cooktop in a kitchen island or peninsula. They have no wall behind them, which is why they require 50% more CFM than wall-mount equivalents.

Structural requirements: This is where island hoods differ most significantly from other types. The hood must be suspended from ceiling joists. For an 8-foot ceiling with joists 16 or 24 inches on center, and a hood directly below the joists, this is manageable. For 10-foot or higher ceilings, or joists running the wrong direction, custom steel support may be required.

The mounting weight for a large island hood can exceed 150 lbs once the chimney is included. Standard ceiling drywall is not structural. The hood must be attached to framing — either existing joists or added blocking between joists. If the ceiling is above an attic, access is usually available to add blocking. If it is a second-floor subfloor, blocking must be added from below before drywall closes the cavity.

Chimney section height: Island hoods typically come with chimney sections sized for 8- or 9-foot ceilings. For higher ceilings, extensions are required. Island hood chimneys are also wider than wall-mount chimneys to accommodate the duct running down through the center.

Duct routing: The duct exits the top of the island hood housing and runs vertically up through the chimney section, through the ceiling, and into the attic or floor cavity above. From there, it must route to an exterior termination — typically a roof cap. Minimize bends in this overhead run. A single 90-degree elbow at the top (transitioning from vertical to horizontal in the attic) plus a straight run to the roof cap is the target.

Comparison Table

Hood Type	Typical Weight	Structural Mount	Duct Exit	CFM Requirement vs. Wall
Under-cabinet	15–40 lbs	Cabinet underside	Top or rear, into cabinet	Same as wall hood
Wall-mount chimney	50–150 lbs	Wall studs / blocking	Top, inside chimney	Same as wall hood
Island / ceiling	80–200 lbs	Ceiling joists / blocking	Top, up through chimney	+50% vs. wall hood

9. Insert and Liner Hoods

What They Are

An insert (also called a liner) is a bare-bones range hood blower assembly designed to be installed inside a custom cabinet enclosure or a decorative housing. The homeowner or cabinet maker builds the exterior surround; the insert provides the motor, filters, controls, and duct connection.

Inserts are common in high-end kitchens where the designer wants a custom cabinet facade over the hood, or in period homes where a stainless chimney would look out of place.

Blower Location Options

Internal blower: The blower motor is housed inside the insert unit itself. This is the most common configuration. The motor noise is present in the kitchen.

Inline blower: The motor is installed somewhere in the duct run — typically in the attic, in a ceiling chase, or near the exterior wall — not in the hood housing. Quiet inserts route air from the cooktop to a remote inline blower, and then to the exterior. The kitchen hears only the air movement, not the motor.

Remote (exterior) blower: The blower is mounted at the exterior termination, outside the building envelope. It pulls air through the duct from the cooktop. Remote blowers are the quietest option — the motor is outside entirely — but they must be rated for outdoor use and are harder to service. They also create a constant negative pressure in the duct when not running, which requires a well-sealing backdraft damper at the hood end to prevent drafts.

Noise Reduction

If kitchen noise is a priority — open-plan homes where the kitchen is part of the living space, high-end clients who want near-silent operation — specify an inline or remote blower. The sone rating of a hood at maximum speed is a major comfort factor. Most internal blowers run 4–8 sones at full speed. A well-installed inline blower setup can achieve 1–2 sones at the cooktop, with the motor noise occurring in the attic where no one is standing.

Custom Cabinet Considerations

The cabinet housing for an insert must be non-combustible internally or lined with non-combustible material if it is directly adjacent to the heat from the insert. Most inserts specify minimum cabinet dimensions and clearances. The cabinet must also provide a straight duct connection from the insert outlet — routing the duct through the cabinet with multiple bends defeats the purpose of a powerful insert.

10. Fire Code Clearances

Minimum Distance: Cooktop to Hood

The minimum distance from the cooking surface to the bottom of the hood is a fire safety requirement, not a performance recommendation. Grease accumulates in filters and on surfaces inside the hood. If the hood is too close to an active burner, this grease can ignite.

Cooktop Type	Minimum Hood Clearance (Bottom of Hood)
Electric (coil, smooth top, induction)	24 inches
Gas (standard residential)	30 inches
Gas (commercial-style, high-output)	36 inches
Solid fuel (wood, pellet)	36+ inches (verify with AHJ)

These are minimum clearances per NFPA 96 and typical manufacturer requirements. If a manufacturer specifies a greater clearance, follow the manufacturer — the stricter requirement governs.

Combustible vs. Non-Combustible Surround

The cabinet surfaces and walls adjacent to the range hood must meet clearance requirements based on material. The standards here vary by jurisdiction, but the general rule:

Non-combustible surround (tile, stone, cement board, metal): No additional clearance requirements beyond the standard hood mounting height.

Combustible surround (wood cabinets, painted drywall within 18 inches of the cooking surface): Must maintain clearances per NFPA 54 and local fire codes. Wood cabinets above the cooktop are the most common concern. The bottom of a cabinet that is above the cooktop but at the side — not directly above the burners — must still maintain minimum clearances from the burner edge. Check NFPA 54 Table 10.1.3.1 for your specific configuration.

Grease accumulation zones: The area directly above and around the cooktop, within the range hood cavity, will accumulate grease. Combustible surfaces that are part of the hood housing or are immediately adjacent must be protected or excluded from the installation area. This is why building a custom cabinet enclosure for an insert requires internal lining or material selection per the insert manufacturer's installation instructions.

11. Permits: When You Need One

Permit requirements vary by jurisdiction. The following are the situations where a permit is most commonly required and should be assumed necessary unless you have confirmed otherwise with your local building department.

Exterior Penetrations

Any new penetration through an exterior wall or roof requires a building permit in most jurisdictions. A range hood duct penetrating the exterior wall is a new penetration if it did not previously exist. This is true even if the hood itself is a simple replacement.

Exception: Many jurisdictions have a permit threshold where minor work below a certain cost does not require a permit. Check locally.

Makeup Air Systems

Any mechanical makeup air system — active or passive — is mechanical work that typically requires a permit and inspection. This is because makeup air systems interact with the home's combustion appliances (gas water heaters, furnaces) and improper installation can create safety hazards.

Electrical Work

New circuits, new outlet rough-in, or moving existing wiring requires an electrical permit in most jurisdictions. Plugging an existing hood into an existing outlet that is already code-compliant typically does not.

When to Pull Permits

If you are doing a full kitchen remodel, permits are almost certainly required for the range hood work as part of the broader project. If you are replacing an existing hood with the same duct configuration and no new penetrations, many jurisdictions allow this as a like-for-like replacement without a permit. Do not assume — call the building department and ask. The call takes five minutes and eliminates the risk of having to open walls for a post-hoc inspection.

12. Common Mistakes

Ductless Hood Over a Gas Range

This is the most common serious mistake in range hood installation. A ductless hood above a gas cooktop recirculates combustion byproducts — including carbon monoxide and nitrogen dioxide — back into the kitchen. No charcoal filter removes these gases effectively. This is a code violation in many jurisdictions for new construction and is a health hazard in all cases. If a customer insists on a ductless hood above a gas range because ducting is "too difficult," the right answer is to explain the issue clearly and either route a duct or recommend switching to an electric or induction cooktop.

Duct Too Long With Too Many Elbows

The installation looks fine. The hood is the right CFM. But someone ran a 7-inch duct through six 90-degree elbows and 35 feet of straight run to get it to the nearest exterior wall. At 35 feet plus 60 feet of elbow equivalent (six elbows at 8 feet each), the total equivalent length is 95 feet. The hood is likely delivering a fraction of its rated airflow. This mistake is invisible — the hood runs, it makes noise, it moves some air. But grease is not being captured effectively, and the kitchen smells like dinner for hours. The fix is to re-route the duct with fewer elbows and a shorter path, or to accept a lower-CFM hood that is properly sized for the restrictive duct run.

No Backdraft Damper

A hood installed without a backdraft damper at the wall cap or inline will allow outside air — cold in winter, insects, birds — to enter through the duct when the hood is off. In cold climates, this causes condensation inside the duct and can allow cold air to pour into the kitchen. In all climates, it allows odors, humidity, and animals to enter. This is easy and cheap to fix at installation time — install a damper. It is annoying and expensive to fix after the fact, because it requires either opening the wall or climbing into the attic.

Hood Too High Above the Cooktop

The hood is mounted at 42 inches above the cooktop because the homeowner wanted more visual clearance or because the installer was working around a tile backsplash. At 42 inches, the capture zone is dramatically larger than the hood face and capture efficiency collapses. The hood runs and the smoke and steam still fills the kitchen. Do not exceed the manufacturer's maximum mounting height. If the kitchen layout makes standard mounting heights difficult, the solution is a different hood — a larger model with a wider capture area, or a model specifically designed for high-mount applications.

Undersized Duct for the Hood

A contractor installs a 600 CFM hood and runs a 4-inch duct because that is what was already in the wall. The duct size mismatch creates enough static pressure that the hood cannot achieve anywhere near rated airflow. The motor runs hot and wears faster. This is not a rare mistake — it happens every time a high-output hood is installed as an upgrade without upgrading the duct. Matching duct size to hood CFM is non-negotiable.

No Makeup Air on a High-CFM Install

A 900 CFM island hood is installed in a well-sealed new construction home with no makeup air provision. The first time someone runs the hood on high during winter, the combustion water heater in the adjacent utility room backdrafts. This is the kind of mistake that does not always make itself obvious immediately, but creates chronic low-level combustion gas exposure. Any hood above 400 CFM (or the local threshold, whichever is lower) must include a makeup air plan.

13. DIY vs. Hire Out

What a Homeowner Can Reasonably Do

A homeowner with basic tool skills and comfort working in cabinets can handle:

Replacing a like-for-like under-cabinet hood using the same duct opening and same electrical outlet
Installing a range hood insert in a pre-built custom cabinet if the duct run is already in place
Cleaning and replacing filters on any hood type

What Should Be Hired Out

Any new exterior penetration for duct termination
Island hood installations requiring ceiling structural work
Any installation requiring a new electrical circuit
Makeup air system installation
Roof cap installations on pitched roofs (safety hazard without proper equipment)
Duct runs through attic or ceiling cavities in tight, complex layouts

Evaluating a Contractor's Knowledge

When hiring a contractor or HVAC technician for a range hood installation, ask these questions:

What duct diameter will you use for this CFM rating?
How will you calculate equivalent duct length for this run?
Is a backdraft damper included in the installation?
Does this hood require makeup air, and if so, how will you provide it?
Will this installation require a permit?

A contractor who cannot answer these questions fluently — or who dismisses them — should not be doing the installation. Range hood installation is not complex, but it requires understanding the airflow and building science principles above. An undersized duct or a ductless hood above a gas range is not a minor inconvenience. It is either a performance failure or a safety problem.

Summary: Installation Checklist

Item	Check
CFM sized by BTU/100 (gas) or W/10 (electric)
Island hood CFM sized at 1.5x wall hood minimum
Duct diameter matches CFM table
Total equivalent duct length within manufacturer spec
Rigid metal duct (not flex) used throughout
Exterior termination with backdraft damper
No termination into attic, soffit, or wall cavity
Mounting height within manufacturer range; minimum per fire code
Makeup air provided if hood exceeds 400 CFM (or local threshold)
Electrical outlet or hardwire per hood design
GFCI protection if outlet within 6 feet of sink
Permit pulled for new exterior penetration, new circuit, or makeup air
Chimney extension ordered for ceilings above 9 feet
Ceiling blocking installed for island hood before drywall

A range hood installation done right improves indoor air quality, protects the kitchen from grease accumulation, and — for high-output cooktops — is a meaningful life-safety measure. Done wrong, it is a system that runs, makes noise, and does almost nothing useful. The difference is in the sizing, the duct design, and the attention to the details above.