How to Choose the Right Kitchen Hood: Size, CFM & Venting
Learning how to choose a kitchen hood starts with accepting that the brochure specs rarely match what happens once ductwork, cooking style, and local code get involved. Most buyers pick a model based on width and finish, then discover the blower cannot push air through three elbows and a roof cap. The result is a loud fan that clears nothing.
This guide walks through the decision logic that pros use so you size, select, and install a system that actually works.
HVI testing shows that a hood rated at 600 CFM at 0.1 inch static pressure often delivers under 300 CFM in a typical residential duct run. That gap explains why so many kitchens stay smoky even with the fan on high. Let’s start by mapping the variables that decide everything else.
Quick Answer
Choose a hood by matching CFM to your cooktop and duct run. Pick ducted over recirculating whenever possible. Size the blower for real static pressure not free air ratings.
Check local code for make-up air triggers above 400 CFM. Install rigid duct with minimal elbows.
Why Choosing a Kitchen Hood Feels Harder Than It Should Be
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Marketing focuses on finish color and smart features. Physics cares about static pressure and capture geometry. A 36-inch pro-style hood with a 1200 CFM blower sounds impressive until you run 25 feet of duct with two 90-degree elbows and a roof cap.
The same hood on a 15-foot straight wall run will outperform it every time. Most showrooms never show you the duct layout. That omission is where projects go sideways.
The industry rates airflow at 0.1 inch water column. Real installations often sit at 0.3 to 0.5 inches. Every elbow adds 15 to 30 equivalent feet.
Flex duct doubles the resistance of rigid pipe. A wall cap can add 50 equivalent feet alone. These numbers are not optional.
They are the difference between a kitchen that clears and one that doesn't.
The 5-Minute Framework: Match Your Hood to Your Kitchen, Not the Marketing
Start with five questions. Write the answers down. They become your spec sheet.
- What is your cooktop fuel type and maximum BTU or kW?
- Is the hood going on an exterior wall, an island, or under cabinets?
- Can you run rigid duct to the outside? How many elbows and how far?
- Do you sear, wok-fry, or grill indoors multiple times a week?
- What is your local make-up air threshold? Many codes trigger at 400 CFM.
If you cannot duct outside, stop looking at ducted models. Recirculating is your only path. If you have a 60,000 BTU gas range and an island, you need a deep canopy, a remote blower, and a make-up air plan.
That decision tree saves hours of browsing.
How Kitchen Ventilation Actually Works: Capture, CFM, and the Duct Reality
A hood does two things. It creates a low-pressure zone that pulls the thermal plume into the canopy. Then it pushes that air through ductwork to the outside.
Capture depends on canopy depth, overhang, and mounting height. Transport depends on blower curve and duct resistance. Most failures happen in transport.
The thermal plume rises in a cone. A canopy that extends 3 to 6 inches past the cooktop edge catches more of it. Mounting height matters.
Gas cooktops need 24 to 30 inches clearance. Electric and induction can go 20 to 24 inches. Higher mounting means wider plume spread.
You need more CFM to grab it.
CFM is not a single number. It is a curve. At 0 inches static pressure the blower moves maximum air.
At 0.5 inches it moves far less. HVI publishes curves at 0.1, 0.25, and 0.375 inches. Always compare at your calculated static pressure.
Manufacturer free-air ratings are marketing fiction.
Your Kitchen's Decision Variables: Cooktop, Layout, Duct Route, and Cooking Style
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Your cooktop drives the baseline. A 30-inch electric coil needs far less than a 48-inch gas pro-range with a charbroiler. The rule of thumb is 100 CFM per linear foot for wall-mounted hoods.
Island hoods need 150 CFM per foot because the plume spreads 360 degrees. A 36-inch gas cooktop on an island starts at 450 CFM before duct penalties.
Duct route is the silent killer. A straight 10-foot run through a band joist with a wall cap is easy. A 30-foot run through the attic with three elbows and a roof cap is a different animal.
Each 90-degree elbow costs 15 to 30 equivalent feet. The roof cap adds 50 to 100. Flex duct adds 50 percent more resistance per foot.
Draw your duct path. Count the fittings. Calculate equivalent length before you shop.
Cooking style multiplies the baseline. Light simmering and boiling use a 1.0 multiplier. Frequent searing and frying use 1.3.
Wok cooking or indoor grilling use 1.5 to 2.0. A 30-inch gas cooktop with heavy use needs 600 CFM at the hood. After a 60-foot equivalent duct run you need a blower rated for 900 CFM at 0.375 inches.
Hood Type Decision Tree: Wall, Island, Insert, Downdraft, or Recirculating?
Wall-mount canopy hoods are the standard. They anchor to an exterior wall. Duct runs are usually short.
They come in widths from 24 to 60 inches. Depths range from 18 to 24 inches standard. Pro models go 27 to 30 inches deep for better capture.
If you have an exterior wall above the cooktop, start here.
Island hoods hang from the ceiling. They need 360-degree capture. Canopies are deeper, typically 27 to 30 inches.
Blowers are larger. Duct runs go up through the roof or down through the floor joists. Both add length and elbows.
Island installs cost more in labor and duct material. Plan for a remote blower to keep noise down.
Insert hoods (liners) hide inside custom cabinetry. You buy the power pack separately. This lets you match the hood face to your cabinet run.
Power packs come in internal, inline, and remote blower configurations. Verify the liner is UL listed with the specific power pack. Unlisted combos fail permitting.
Downdrafts pull air down and back. They rise behind the cooktop when in use. Capture efficiency is 50 to 60 percent of overhead at best.
They struggle with tall pots and front burners. Use only when overhead is impossible. They work best with low-profile cooking and rear burners.
Recirculating hoods filter air through charcoal and return it to the room. They remove odor but not heat, moisture, or combustion byproducts. Charcoal saturates in three to six months with heavy use.
They are the only option when ducting is structurally impossible. Do not choose them for high-heat gas cooking.
Sizing Workflow: Calculate Real CFM, Map Equivalent Duct Length, Pick the Right Blower
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Start with cooktop width in feet. Multiply by 100 for wall mount or 150 for island. That gives baseline CFM.
A 36-inch wall cooktop needs 300 CFM baseline. A 36-inch island needs 450 CFM baseline. Next apply cooking style multiplier.
Light use stays at 1.0. Moderate searing uses 1.3. Heavy wok or griddle uses 1.5 to 2.0.
A 36-inch island with heavy use needs 450 times 1.5 equals 675 CFM at the hood.
Now map the duct run. Measure straight length in feet. Add equivalent feet for each fitting.
A 90-degree elbow adds 15 to 30 feet. A 45-degree elbow adds 8 to 15 feet. A wall cap adds 30 to 50 feet.
A roof cap adds 50 to 100 feet. Flex duct multiplies its actual length by 1.5 to 2.0. Sum everything for total equivalent length.
A typical island run with two elbows and a roof cap hits 80 to 120 equivalent feet fast.
Look up the blower curve at your static pressure. Equivalent length converts to static pressure. Rough rule: 100 equivalent feet equals about 0.375 inches water column.
Find a blower that delivers your required CFM at that pressure. If your hood needs 675 CFM at 0.375 inches, the blower must show 675 CFM at 0.375 inches on the HVI curve. Free air rating is useless here.
Many 900 CFM free-air blowers drop to 400 CFM at 0.375 inches.
Choose blower type after you know the pressure. Internal blowers work fine up to 0.25 inches. Inline blowers handle 0.375 to 0.5 inches better.
Exterior blowers handle the highest pressure and move noise outside. Match the blower to the duct reality not the marketing sheet.
The Make-Up Air Fork: When 400 CFM Triggers Code, Cost, and Comfort Trade-Offs
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IRC section G2407 requires make-up air when exhaust exceeds 400 CFM. Many local codes adopt this. Some trigger at 300 CFM.
California Title 24 requires it at 300 CFM with demand control. If your hood exceeds the threshold you must provide a way for replacement air to enter. Cracking a window does not count.
The system must be interlocked so it runs when the hood runs.
Passive make-up air uses a barometric damper on an intake duct. It opens when the house goes negative. It costs less to install but brings in untempered air.
In winter that means cold drafts. In summer it brings humidity. Passive works best in mild climates or with smaller hoods just over the threshold.
Powered make-up air uses a supply fan that matches the hood CFM. It can include a heater or ERV core to temper the air. This adds $2,000 to $6,000 installed but protects comfort and heating equipment.
It also prevents backdrafting of gas water heaters and furnaces. That safety benefit alone often justifies the cost.
Size the make-up air to match the hood CFM not the threshold. A 600 CFM hood needs 600 CFM of supply. Undersizing defeats the purpose.
The interlock wiring must be done by an electrician. The damper or fan must be UL listed. Permits are required for the duct penetration and electrical work.
Skipping this step fails inspection and risks carbon monoxide.
Blower Location: Internal, Inline, or Exterior — Noise, Power, and Install Implications
Internal blowers sit inside the hood canopy. They are easiest to install. No extra wiring or mounting.
They work well for duct runs under 0.25 inches static pressure. Noise stays in the kitchen. Typical sones at working speed range from 4 to 8.
If you cook quietly this may be fine. For open-concept spaces the drone can be intrusive.
Inline blowers mount in the duct run, usually in the attic or crawlspace. They remove the motor from the kitchen. Noise drops 2 to 4 sones at the hood.
They handle higher static pressure better. Installation needs accessible power and a mounting platform. Vibration isolation hangers are a must.
Duct connections must be airtight.
Exterior blowers mount on the roof or wall. They are the quietest in the kitchen. They handle the highest static pressure.
They cost more and need weatherproof housing. Roof mounts need a curb and flashing. Wall mounts need a sturdy bracket.
Both need a disconnect switch at the unit. Maintenance means roof or ladder access.
Match blower location to your duct pressure and noise tolerance. Internal for short straight runs. Inline for medium runs with attic access.
Exterior for long complex runs or noise-sensitive layouts. The price jumps at each step. Internal adds zero.
Inline adds $300 to $800. Exterior adds $800 to $2,000 plus roof work.
Filter Strategy: Baffle for Grease, Charcoal for Ductless, and Why Mesh Fails
Baffle filters are the standard for ducted hoods. Stainless steel vanes force air to change direction. Grease droplets hit the vanes and drain into a trough.
They capture 85 to 95 percent of grease particles. They are dishwasher safe. They last the life of the hood.
Pressure drop is low so they do not choke airflow. Clean them monthly for heavy use.
Mesh filters are layers of aluminum or stainless mesh. They trap grease in the fibers. They clog fast.
Pressure drop rises quickly. They capture less grease than baffles. They are a fire hazard when loaded.
Most pro hoods no longer offer them. If your hood came with mesh, upgrade to baffle. The difference is immediate.
Charcoal filters are for recirculating hoods only. They adsorb odors and some gases. They do not capture grease.
A grease pre-filter (usually mesh) sits in front. Charcoal saturates in three to six months. Once saturated it passes odor and restricts airflow.
Replace on schedule. No visual indicator tells you when. Mark the calendar.
Hybrid filters combine a baffle front with a charcoal cartridge behind. They work for convertible hoods that can run ducted or ductless. In ducted mode remove the charcoal.
In ductless mode keep it. The charcoal still needs replacement. The baffle still needs cleaning.
This is the only valid use of charcoal in a ducted-capable hood.
Features That Perform vs. Specs That Distract: HVI Ratings, Sone Curves, Lighting, Controls
HVI certification is the only rating that matters. It verifies airflow at three static pressures. It verifies sound at working speeds.
It verifies capture efficiency per ASTM E3087. A hood without HVI listing is an unknown. Manufacturers who skip HVI often inflate free-air CFM by 30 to 50 percent.
Check the directory before you buy.
Sone ratings tell you loudness. One sone is a quiet refrigerator. Three sones is a quiet office.
Six sones is a loud conversation. Ten sones is a busy restaurant. Look for the sone rating at the speed you will actually use.
Boost mode is always loud. Working speed should be 3 to 5 sones for open kitchens. Lower is better but often means lower CFM.
LED lighting should be 3000K to 4000K color temperature. Lumens matter more than watts. Aim for 500 to 800 lumens per linear foot of cooktop.
Dimmable is essential. Night cooking at full blast ruins the mood. Sealed LED modules last 50,000 hours.
If the driver fails you replace the module not the bulb. Check warranty on the light engine.
Controls should be tactile and readable with greasy fingers. Mechanical rocker switches survive forever. Capacitive touch looks clean but fails with wet hands.
RF remotes are great for island hoods. ADA-compliant remotes mount at counter height. Smart features (Wi-Fi, voice) add convenience but introduce obsolescence risk.
The hood should work without the app.
Heat sentry auto-starts the fan if the canopy gets hot. It protects the motor and electronics. It saves you when you forget to turn it on.
Delay-off runs the fan 5 to 20 minutes after cooking. It clears residual plume. Both features are standard on mid-range and up.
Do not pay extra for them.
Filter cleaning indicators are timers not sensors. They count run hours. Reset after cleaning.
They are helpful reminders not diagnostics. Variable speed with at least three steps lets you match airflow to the task. Boost is for the first minute of searing.
Working speed does the rest. Two-speed hoods force you to choose between weak and loud.
Common Mistakes: Undersized Duct, Oversized Hood, Flex Tape, Missed Permits, Silent Failure
Undersized duct is the number one killer. A 6-inch round duct cannot carry 900 CFM without massive pressure loss. Step up to 8-inch or 10-inch for runs over 600 CFM.
Rectangular duct needs equivalent cross-section. Do not reduce duct size at the hood collar. That bottleneck chokes the whole system.
Oversized hoods trigger make-up air costs for no gain. A 1200 CFM hood on a 30-inch electric cooktop wastes energy and money. It depressurizes the house.
It backdrafts the water heater. It costs $4,000 more in make-up air gear. Match the hood to the cooktop not the ego.
Flex duct, duct tape, and screws inside the airstream are code violations. Flex traps grease and burns in a fire. Tape fails from heat and grease.
Screws catch grease and block cleaning. Use rigid galvanized or stainless. Seal joints with UL 181 mastic or foil tape.
Support every 4 feet.
Missing permits and fire dampers fail inspections. Any duct penetrating a fire-rated wall or ceiling needs a UL 555 damper. Multi-family buildings always require this.
Skipping the permit risks insurance denial after a grease fire. The $200 permit saves $200,000 in liability.
Silent failure means the hood runs but captures nothing. Causes: mounting too high, canopy too shallow, blower choked by duct, make-up air starved. Test with a tissue at the canopy edge while cooking.
If it does not pull, the system is broken. Fix the root cause not the symptom.
Cost Breakdown: Hood, Duct, Make-Up Air, Labor — Where the Money Actually Goes
Basic under-cabinet hood: $200 to $500. Mid-range wall or island: $500 to $1,500. Pro-style 36 to 48 inch: $1,500 to $4,000.
Custom insert plus remote blower: $3,000 to $8,000. These are unit prices only.
Duct materials: rigid 6-inch round $3 to $5 per foot. 8-inch $5 to $8. 10-inch $8 to $12. Roof cap $80 to $200. Wall cap $50 to $150.
Elbows $15 to $40 each. Mastic and tape $50. Total duct $200 to $800 typical.
Make-up air: passive damper kit $300 to $600. Powered supply fan $800 to $2,000. Tempered unit with heater or ERV $2,500 to $6,000.
Electrical and controls $300 to $800. Permit and inspection $150 to $400.
Labor: electrician $500 to $1,200 for dedicated circuit and interlock. HVAC or carpenter $800 to $2,500 for duct install and roof work. Custom cabinetry for insert $1,000 to $3,000.
Total installed range $1,500 to $15,000. Know your ceiling before you spec.
Code & Safety Checklist: IRC, UL, Fire Dampers, Clearances, and the Backdrafting Risk
IRC M1503 governs residential exhaust. Duct must be 26-gauge minimum galvanized or stainless. No flex for grease ducts.
Slope 1/4 inch per foot to exterior. Terminate outside not in attic or soffit. Wall cap at least 12 inches from openings.
Roof cap at least 3 feet from any intake.
UL 507 lists the fan. UL 710 lists commercial hoods. Residential hoods need UL 507.
Insert liners must be listed with the specific power pack. Unlisted combos are not approved. The label must be visible after install.
Inspectors check for it.
IRC G2407 requires make-up air over 400 CFM. Gas appliances need combustion air. Backdrafting pulls carbon monoxide into the house.
Test with a smoke pencil at the water heater draft hood while the hood runs. If smoke reverses you have a problem. Fix make-up air before you cook.
Clearances: 24 inches minimum above gas. 20 inches above electric or induction. Manufacturer specs override code if stricter. Combustible cabinet sides need 3 inches or a heat shield.
The hood bottom must not project beyond cabinet face. Check the manual.
Fire dampers: UL 555 rated. Required at each penetration of a fire-rated assembly. Shaft walls, floor-ceiling, garage separation.
Fusible link at 165°F. Access panel for inspection. Do not paint the link.
This is not optional in multi-family.
Maintenance Schedule: Monthly, Quarterly, Annual — Keep It Moving Air, Not Grease
Monthly: pull baffle filters. Run through dishwasher on heavy cycle. Degrease canopy interior with hot soapy water.
Wipe light lenses. Check that damper flap moves freely. Listen for bearing noise or vibration.
Quarterly: inspect duct exterior termination. Clear bird nests, lint, debris. Verify damper opens fully when fan runs.
Check roof cap screen for blockage. Tighten any loose duct clamps. Confirm make-up air damper or fan activates with hood.
Annual: pull the blower wheel if accessible. Clean dust and grease buildup. Lubricate motor bearings if not sealed.
Test airflow with anemometer at canopy face. Compare to commissioning numbers. Drop over 15 percent means duct or fan issue.
Replace charcoal filters if ductless. Schedule professional duct cleaning every 3 to 5 years for heavy use.
Final Decision Guide: Your Kitchen, Your Cooking, Your Hood — A Quick-Reference Flowchart
Start with duct feasibility. Can you run rigid duct to outside? No → recirculating only.
Yes → continue.
Measure cooktop width and fuel. Gas BTU total divided by 100 = minimum CFM. Electric width in feet times 100 (wall) or 150 (island) = baseline CFM.
Apply cooking multiplier: light 1.0, moderate 1.3, heavy 1.5 to 2.0.
Map duct route. Count elbows. Add equivalent feet.
Convert to static pressure. Pick blower that delivers required CFM at that pressure. Check HVI curve not free air.
Check make-up air threshold. If required CFM exceeds local limit (usually 400) add make-up air system. Size to match hood CFM.
Budget $2,000 to $6,000 for powered tempered.
Choose hood type by layout. Wall → wall mount. Island → island canopy.
Custom cabinet → insert. No overhead → downdraft (compromise). No duct → recirculating.
Pick blower location. Under 0.25 inches → internal. 0.25 to 0.5 with attic access → inline. Over 0.5 or noise critical → exterior.
Spec filters. Ducted → baffle. Ductless → charcoal plus grease pre-filter.
Convertible → hybrid.
Verify HVI listing. Confirm UL. Pull permit.
Install rigid duct. Seal with mastic. Interlock make-up air.
Commission with tissue test. Clean monthly. You are done.
