Grease fires start fast kitchen fires cause 18 % of home‐fire deaths each year. Yet many homes still run a flex duct for range hood exhaust that traps grease, melts, and silently breaks code.
I’m here to show you why that shortcut backfires and how a safer duct keeps airflow high, heat resistance strong, and insurance valid—stick around for a clear fix.
Keynote: Flex Duct for Range Hood
Using a flex duct for a range hood is unsafe and violates building codes. Its ridged design traps grease, increasing fire risk, and hampers airflow, reducing efficiency. Rigid metal ducts are the recommended choice for safety and optimal performance.
What Is Flex Duct? (Simple Definitions for Clarity)
A flex duct is bendable, corrugated tubing used in HVAC to snake around framing.
Materials
- Metal: thin aluminum or semi-rigid steel—more heat-tolerant but still ridged.
- Non-metal: PVC, vinyl, polyester—cheap, combustible, and banned for cooking exhaust.
Flex duct’s ridges grab airflow but also collect grease and cut durability. True rigid ducting stays smooth, stainless, and fire-safe.
Why Flex Duct Is a Risky Choice for Range Hoods (Pain Points Front & Center)
Safety hazards: Grease settles in every ridge, turning the hose into a fuse; plastic layers can melt at stovetop temperatures.
Performance issues: Corrugations increase static pressure, reducing CFM, so your exhaust fan roars yet moves less air.
Maintenance headaches: You can’t scrub inside a floppy hose; sagging sections hide corrosion and leaks.
Result? Higher fire risk, lower airflow, and warranty voids.
Flex Duct vs. Rigid Duct: Which Performs Better? (Quick Comparison)
| Feature | Rigid Metal Duct (Steel/Copper) | Flexible Duct (Foil/Plastic/Semi-Rigid Alum.) |
|---|---|---|
| IRC Compliance | Compliant | Prohibited |
| Interior Surface | Smooth | Corrugated |
| Airflow Efficiency | High | Low |
| Grease Potential | Low | High |
| Fire Risk | Low | High |
| Durability | High | Low |
| Cleanability | Feasible | Nearly impossible |
| Install Ease | Slower | Faster |
| Cost Up-Front | Higher | Lower |
Rigid duct behaves like a clear artery; flex duct clots up fast.
What the Rules Say: Building Codes You Can’t Ignore
The 2024 International Residential Code (IRC M1503) demands a smooth, non-combustible metal duct that vents outdoors. That stipulation alone bans flex, semi-rigid, and plastic hose.
Fail to comply and you face failed inspections, denied claims, and potential liability if a hood vent fire spreads.
When Flex Duct Might Be a Last Resort
Only consider metal flex if every box below is true:
- Run length is under two feet.
- Walls stay perfectly straight—no kinks, no attic loops.
- You cook on an electric cooktop with lower flame temps.
Even then, rigid metal outlasts and outperforms.
Choosing the Right Duct: Size, Material & More
Duct Size by CFM
| Range-Hood CFM | Minimum Round Diameter |
|---|---|
| Up to 400 CFM | 6 inches |
| 401–600 CFM | 6–7 inches |
| 601–900 CFM | 8 inches |
| 901–1200 + CFM | 10 inches + |
Match the hood vent, never pinch diameter, and avoid extra elbows to protect airflow.
Best Materials
- Stainless steel: durable, corrosion-proof, grease-resistant.
- Galvanized steel: budget pick with solid heat resistance.
Shorter, straighter ductwork slashes static pressure and noise.
How to Install Your Range Hood Duct Safely (Step-by-Step Guide)
For Rigid Duct
- Plan the most direct exit—roof or wall—skip the attic whenever possible.
- Limit bends to two 90-degree elbows; use 45-degree pairs for smoother turns.
- Join sections with screws, then seal seams with UL-rated mastic or foil tape—not cloth “duct” tape.
If You Must Use Flex (Not Recommended)
- Choose metal-only hose rated for kitchen exhaust.
- Pull it taut; remove all sags, and support every four feet with steel straps.
- Inspect monthly for grease, dents, or loose clamps.
A proper backdraft damper stops cold air and pests once cooking ends.
Keeping Your Duct Clean & Safe (Maintenance Musts)
- Monthly: Soak aluminum filters in hot, soapy water to catch grease before it hits the duct.
- Yearly: Open exterior cap; shine a flashlight to check for grease rings or rust.
- Every 2–3 years: Hire a pro with rotary brushes to scour rigid pipe—skip this step and buildup becomes flammable.
Regular care extends duct life and preserves peak CFM.
Better Alternatives to Flex Duct
- Semi-rigid metal: Code-compliant in a few jurisdictions when kept smooth and short.
- Ductless hoods: Carbon filters recirculate air when venting outside is impossible, though they struggle with gas-stove smoke.
- Make-up air systems: Required above 400 CFM to balance pressure and keep combustion appliances safe.
Choose rigid whenever an outdoor route exists; choose ductless only when walls truly prevent ductwork.
Conclusion
Cut corners with a flex duct for range hood and you invite grease fires, noise, and code violations. Steel pipe costs more today but saves repairs, refunds, and heartache tomorrow. Vent smart, vent safe—because risking your kitchen’s future for a few bends of cheap hose simply isn’t worth the heat.
Flexible Duct for Range Hood (FAQs)
What is a flexible duct in HVAC?
Flex duct is a lightweight, bendable tube of foil-wrapped plastic or thin metal helix used to move air. Its corrugated interior raises static pressure and needs tight support to prevent kinks inside residential HVAC runs.
Which trade installs rigid duct to vent kitchen hoods?
HVAC contractors or sheet-metal technicians typically fabricate and install rigid steel ducts that vent household kitchen hoods. Commercial settings may add fire-suppression specialists, but the HVAC trade still handles core ductwork fabrication tasks.
Can an 8′ rigid duct be split into two 6“ flex ducts?
You can split an eight-inch rigid trunk into two six-inch branches, matching area, if airflow balance is maintained. However, kitchen vents demand smooth metal; replacing branches with six-inch flex violates code and increases friction losses.
Can I vent a stove hood and bathroom fan through the same duct?
Building codes forbid connecting a range-hood duct to any bathroom fan, dryer, or other exhaust system. Grease and moisture must exit through independent ducts, preventing backflow contamination and ensuring inspection approval.
Can flexible ducting be used for kitchen exhaust?
Flexible ducting is prohibited for kitchen exhaust; IRC requires smooth galvanized, stainless, or copper pipe to the outdoors. Flex ridges trap grease, raise fire risk, and create a frequent code-violation noted by inspectors.
Katie Lee has over 20 years of experience in the kitchen. She helps homeowners find the right appliances for their needs to sets up a perfect kitchen system. She also shares helpful tips and tricks for optimizing appliance performance.