Kitchen Exhaust Fan Ductwork: Expert Tips for Clean Air: 1 Minute to Plan Kitchen Exhaust Fan Ductwork Like a Pro

I’ve redesigned more kitchens than I can count, and clean air consistently hinges on getting the exhaust fan ductwork right. Fan specs are only half the story—duct sizing, routing, materials, and makeup air determine whether smoke and grease leave the space or linger. Done well, you’ll cook comfortably, protect finishes, and extend equipment life.

Performance isn’t guesswork. The WELL v2 standard notes the health value of effective ventilation and filtration, and sets targets that reinforce adequate air changes and pollutant removal for occupant well-being. In residential practice, a common benchmark is selecting a range hood delivering 100–150 CFM per linear foot of cooking surface for wall hoods and up to 200–300 CFM/ft for high-heat pro-style ranges—then preserving that capacity through properly sized, short, smooth duct runs. Steelcase research on work environments also underscores how air quality affects cognition and satisfaction; while focused on offices, the principle applies at home: cleaner air elevates focus and comfort in task-heavy zones like kitchens.

Grease management matters as much as flow. The NKBA’s kitchen planning guidelines recommend capturing area coverage and correct mounting heights (typically 24–30 inches above electric cooktops and 30–36 inches over gas, always per manufacturer). In practice, every elbow, rib, and restriction taxes the fan and increases noise. Smooth, rigid duct, carefully sized, keeps static pressure low so rated CFM can reach the exterior termination without stalling.

Right-Size the Duct for Real CFM

Start with the hood’s rated CFM, then follow manufacturer’s duct diameter requirements—usually 6 inches for 300–400 CFM, 8 inches for 600–900 CFM, and 10 inches for 1000+ CFM. Oversizing slightly helps, undersizing kills performance. Each 90° elbow can add the equivalent of 5–10 feet of straight duct in pressure loss; keep total effective length within the hood’s specification. The goal is a short, direct route to the outdoors with minimal turns.

Use Rigid, Smooth-Wall Duct—Avoid Flex

Grease aerosols stick to ridges. Flexible duct creates turbulence, increases static pressure, and becomes a cleaning nightmare. I specify 26–28 gauge galvanized steel or stainless rigid duct with sealed, screwed joints. Where code allows and runs are long, consider welded stainless for durability in high-output kitchens. Insulate runs that pass through unconditioned spaces to minimize condensation and maintain thermal stability.

Vent to the Outdoors—Never to the Attic

All cooking exhaust should terminate outside via a roof cap or wall cap with a backdraft damper. Discharging into an attic, crawlspace, or garage deposits grease, raises humidity, and risks mold. Roof penetrations require proper flashing and slope; wall penetrations should avoid prevailing wind and be at least 10 feet from operable windows when local codes demand. Confirm termination free area (the net opening) matches duct diameter to prevent bottlenecks.

Keep the Run Short and Straight

Plan the route during layout. Position the hood on a wall that allows the shortest path to an exterior face. If you’re revising cabinetry, use a room layout tool to simulate hood placement, duct orientation, and clearances. Limit to two elbows when possible, and use long-radius fittings to reduce turbulence.

Balance Makeup Air for High-CFM Hoods

Once a hood exceeds about 400–600 CFM, negative pressure can backdraft gas appliances and starve combustion. Many jurisdictions require coordinated makeup air—either passive (ducted, tempered) or mechanical supply linked to the hood. Aim to provide roughly 80–100% of exhaust volume as makeup air, delivered diffusely so it doesn’t fight capture. Tie-in with the HVAC contractor early; I’ve solved countless whistling doors and cold drafts by tempering makeup air and distributing it above the cooking zone.

Mind Noise: Static Pressure and Sones

Hood noise rises when ducts are undersized, long, or kinked. Select external or inline blowers for open-plan kitchens; relocating the motor outside or in the attic can drop perceived noise by several sones. Keep transitions smooth and avoid reducers unless specified. A quieter system encourages consistent use, which directly means cleaner indoor air.

Capture Area and Mounting Height

For effective capture, the hood should be as wide as the range and ideally deeper (18–24 inches) with good side containment. Island hoods benefit from larger canopies or perimeter capture technology. Maintain mounting heights per the appliance and NKBA guidance; too high and you lose capture, too low and you risk heat damage or poor ergonomics.

Seal, Support, and Fire Safety

Seal joints with high-temperature metal tape (UL 181) and sheet metal screws; avoid mastic that can soften with heat. Provide adequate hangers or straps every 6–8 feet on horizontal runs. Keep clearances to combustibles per code and manufacturer. In multifamily or commercial applications, grease ducts may require fire-rated enclosures and cleanout doors—coordinate with local mechanical code.

Filters, Cleaning, and Maintenance

Baffle filters outperform mesh for grease capture and airflow. Clean monthly for frequent cooking; heavy-use kitchens may need weekly attention. If you notice increased smoke or odor, inspect for blocked filters, stuck backdraft dampers, or accumulated grease in the duct. An annual professional inspection is sensible in high-output settings.

Material Choices and Sustainability

Galvanized steel is durable and cost-effective; stainless resists corrosion near the coast or with steam-heavy cooking. Specify low-VOC sealants around penetrations, and choose ENERGY STAR hoods where available. Efficient capture at lower flow rates reduces energy use and improves acoustic comfort. When planning cabinetry around the duct chase, consider recycled-content panels and finishes that withstand humidity.

Ergonomics and Behavior

Users turn hoods on when they’re quiet and effective. Provide a simple control scheme with a dedicated light and variable speed. Preheat the hood: switch it on a couple of minutes before cooking to establish pull. Bright, glare-free task lighting over the cooktop (300–500 lux, warm-neutral 3000–3500K) supports visibility without overpowering the hood lights.

Commissioning Checklist

- Verify duct diameter matches spec for the selected CFM.

- Count elbows and confirm total effective length is within limits.

- Check termination damper swing and free area; add bird screen only if code allows and airside loss is acceptable.

- Confirm makeup air strategy for hoods over 400–600 CFM.

- Measure airflow at the hood (flow hood or anemometer) and note sones at typical speeds.

- Inspect for air leaks, vibration, and proper support.

References to Guide Design Decisions

For broader health and performance guidance on ventilation strategies, WELL v2 provides science-backed guidance on air quality, filtration, and ventilation. Steelcase research explores how environmental quality affects performance and comfort in task-heavy spaces, useful when balancing noise and airflow in open-plan homes.

FAQ

Q1: What duct size should I use for a 900 CFM hood?

A: Typically 8 inches is the minimum, with 10 inches preferred for long runs or multiple elbows. Always follow the hood manufacturer’s duct spec; undersizing increases noise and slashes actual CFM.

Q2: Are flex ducts acceptable for short runs?

A: I avoid flex entirely for kitchen exhaust. Even short sections add turbulence and trap grease. Use smooth, rigid metal with sealed joints.

Q3: How many elbows are too many?

A: Keep to two or fewer 90° elbows. Each elbow adds significant equivalent length. If you must turn multiple times, increase duct diameter and use long-radius fittings.

Q4: Do I need makeup air for a 600 CFM hood?

A: In many regions, yes. Negative pressure can backdraft gas appliances and degrade IAQ. Provide tempered makeup air linked to the hood or via a dedicated passive intake sized to the exhaust.

Q5: What mounting height is best over a gas range?

A: Commonly 30–36 inches, depending on the hood and burner output. Check both range and hood manuals; staying within the capture sweet spot reduces spill.

Q6: Can I vent horizontally through a wall instead of the roof?

A: Yes, if the wall route is shorter and code-compliant. Ensure proper termination with a backdraft damper and adequate clearance from windows and make-up air intakes.

Q7: Why is my hood loud but ineffective?

A: The duct may be undersized, overly long, or obstructed. Dirty filters, restrictive terminations, or reducers cause high static pressure. Verify duct diameter, clean filters, and inspect the damper.

Q8: Are baffle filters better than mesh?

A: Baffles generally provide superior grease separation and maintain airflow longer. They’re easier to clean and less prone to clogging with heavy cooking.

Q9: What’s the ideal color temperature for hood lights?

A: Warm-neutral (3000–3500K) delivers comfortable task lighting without harsh glare. Pair with broader kitchen lighting in the 300–500 lux range for safe visibility.

Q10: How often should I clean the duct?

A: Residential ducts with baffle filters and proper capture typically need inspection annually. Heavy-use kitchens may require more frequent checks; filters should be cleaned monthly or as needed.

Q11: Can I share the exhaust duct with the bathroom fan?

A: No. Kitchen exhaust must be dedicated due to grease and odor load. Combining paths invites cross-contamination and code violations.

Q12: Will an inline or external blower reduce noise?

A: Yes. Moving the motor away from the canopy significantly lowers perceived noise. Pair with smooth, properly sized duct to keep sones low.