Grease Buildup in Range Hood Ductwork: Fire Risk and Cleaning

Grease builds up in range hood ductwork when cooking vapor slips past the filters, cools inside the duct and sticks to the walls. A thin film is normal; a thick, sticky or crusty coating is a fire risk and an airflow problem. You can clean the parts you can reach yourself, but long, hidden or heavily coated runs are a job for a professional.

If there is fire in or around the hood right now, skip to What to do in a hood or duct fire.

Quick diagnosis

How grease gets into the duct

When you sear, fry or stir-fry, oil breaks into fine droplets and vapor. The grease filters catch much of it, but not all. The rest travels up the duct, cools on the metal walls and sticks.

Buildup is worse when:

• Filters are overdue for cleaning. A clogged filter lets more grease slip around its edges and makes the fan work harder.
• The fan runs on low, or is turned on after the pan is already smoking.
• The duct is long, has many elbows, or runs through a cold space where vapor condenses faster.
• The duct is flexible foil, which traps grease in its ridges, or has screws protruding inside that catch grease.
• The hood has no filters in place, or aftermarket filters that do not fit tightly.

Over time a soft film hardens into a dark varnish, and in bad cases, a thick layer that visibly narrows the duct.

Why it is a fire risk

Grease is fuel. A pan that flares up can send flame and very hot gases into the hood. Clean filters and a clean duct give that flame little to catch. A duct with a thick grease coating can ignite and burn inside the pipe, where you cannot reach it, and a duct fire can spread into the wall, ceiling or attic.

The grease filters are the main line of defense. Metal baffle and mesh filters slow flame and catch grease before it reaches the duct, which is one more reason never to run the hood without them.

What to do in a hood or duct fire

Keep this simple and conservative. Your safety comes before the kitchen.

1. Turn off the burner if you can do it safely.
2. Leave the hood fan off, or turn it off if you can reach the switch without reaching through flames. A running fan feeds air to the fire and can draw it deeper into the duct.
3. If the fire is still only in the pan, slide a metal lid or baking sheet over it and leave it covered until it cools completely. For a small grease fire, a Class K or Class B extinguisher or a generous amount of baking soda can put it out. Never use water, flour or sugar.
4. If flames have reached the hood, the filters or the duct, or you hear roaring or crackling in the duct, get everyone out and call 911. Close the kitchen door behind you if you can. Do not try to fight a fire inside ductwork.
5. Even if the fire seems out, call the fire department to check. Duct fires can smolder out of sight in walls, ceilings or the attic.

After any fire that reached the hood, do not use it again until the hood, duct and wiring have been inspected by a qualified professional.

Inspecting your ductwork

Turn off power at the breaker, not only at the hood’s switch, before reaching into the hood. Confirm it is off with a non-contact voltage tester.

1. Remove the grease filters.
2. With a flashlight, look up into the hood collar. Check the damper flap and the first 1–2 feet of duct you can see. A mirror helps.
3. Wipe a finger or cloth along the inside of the collar. A light film that wipes off easily is normal. A sticky coating you can scrape, or a hard, dark crust, is buildup.
4. Go outside and look at the wall or roof cap from a safe spot. Grease staining on siding or roofing below the cap, or a sticky flap, means grease is reaching the end of the run.
5. If the duct passes through an accessible attic, look at the joints. Grease seeping from seams is a sign of heavy buildup inside.

For a broader airflow check, including nests and crushed sections, see how to check if range hood ductwork is blocked.

DIY cleaning: the reachable sections

This covers what you can see and reach safely: the hood’s interior plenum, the collar, the damper, the first section of duct and the exterior cap.

Hood interior and collar

With power off at the breaker:

1. Protect the cooktop with a towel.
2. Scrape heavy deposits with a plastic scraper. Avoid metal scrapers that can gouge the duct or hood.
3. Wipe with hot water and degreasing dish soap, or a kitchen degreaser labeled safe for galvanized steel. Apply with a rag rather than spraying toward the motor, lights or controls.
4. Clean the damper flap until it swings freely.
5. Reach as far up the duct as you comfortably can with a rag. Do not force anything past an elbow.
6. Dry everything before restoring power.

Exterior cap

1. Only work from a ladder if you can do so safely; do not go onto a steep or wet roof.
2. Remove the cap’s cover if it is designed to come off.
3. Scrape and wipe grease from the flap and housing with degreaser and rags.
4. Confirm the flap opens and closes freely. See exterior vent flap stuck if it does not.

What not to do

• Do not use a lighter, torch or heat gun to soften grease.
• Do not run a stiff brush through a flexible foil duct; it tears.
• Do not pour degreaser into the duct from above and let it run down onto the motor.
• Do not remove or bypass any part of the hood’s safety features to reach deeper.

When to hire a professional duct cleaner

Call a professional kitchen exhaust or duct cleaning service if:

• You can see a thick or crusty coating past the collar.
• The duct is longer than you can reach, runs through closed walls or ceilings, or has several elbows.
• There has been any fire, even a brief flare into the hood.
• Grease is seeping from joints in the attic.
• The duct is flexible foil or the wrong material.

A professional will typically use rotary brushes, scraping tools and degreasers through the whole run, and can tell you whether the duct is sound or should be replaced. If it needs replacing, the IRC 2021 (M1503.4) requires galvanized steel, stainless steel or copper, and the duct must end outdoors (M1503.3). Check with your local building department about permits.

Replacement is often the better option for short, accessible runs of flexible duct or heavily varnished old duct, because new rigid duct is inexpensive relative to cleaning labor.

Prevention

• Clean filters on schedule: every month with heavy frying or wok cooking, every 1–3 months otherwise. See how to clean range hood filters.
• Run the fan every time you cook, starting before the pan heats up, and keep it on a few minutes after.
• Use a higher speed for high-heat cooking. More airflow keeps grease moving instead of settling. If high speed still is not enough, check your sizing with the CFM calculator.
• Use smooth rigid metal duct, keep it short (ideally under 30 equivalent feet, counting each 90° elbow as roughly 10–15 feet), and seal joints with foil tape with no screws protruding inside.
• Inspect the collar and cap once a year. A quick look with a flashlight catches buildup early.

If a stale smell persists even with a clean duct, see range hood smells bad.

The short version

A light film in your duct is normal; a sticky or crusty coating is fuel. Clean the parts you can reach, hand long or hidden runs to a professional, and keep the filters clean so less grease gets there in the first place. If fire ever reaches the hood or duct, leave the fan off, get out and call 911.