Range hood duct cleaning is the removal of condensed cooking grease from the metal duct that runs from the hood above your cooktop to a cap on the roof or an exterior wall. Most Denver homes have never had it done. The filters get rinsed now and then, but the duct behind them collects a slow, sticky layer of grease for years, directly above the one open flame most kitchens use every day.
We are a chimney crew, and we look at this duct the way we look at a flue. It carries hot, dirty air out of the house, it builds a combustible deposit as it works, it has a termination outside that weather and animals attack, and it fails quietly, with weak suction, lingering odors and drips, long before anyone looks inside it.
What Actually Builds Up Inside a Range Hood Duct
When oil hits a hot pan, part of it vaporizes and part of it breaks into tiny airborne droplets. The hood pulls that mixture up along with steam, smoke and combustion gases from a gas burner. What the filter misses travels into the duct as an aerosol.
The duct is cooler than the air entering it, especially in a Colorado winter when it passes through an unheated attic or an exterior wall. The grease vapor condenses on the cool metal the way water beads on a cold glass. Over years the film thickens, collects dust and lint, and hardens into a dark, waxy coating that can be a quarter inch thick at the elbows and the cap.
That coating is fuel. A grease fire that flares on the cooktop can be drawn into the hood by the fan, ignite a saturated filter, and follow the duct toward the roof. Commercial kitchens are required under NFPA 96 to have hoods and ducts cleaned on a schedule for exactly this reason. The layer also narrows the duct, holds odor, and draws insects and rodents to the cap.
Ducted, Recirculating and Downdraft: Three Very Different Systems
A ducted hood has a round or rectangular metal duct leaving its top or back, visible in the cabinet above. A recirculating hood has no duct, just a grille that blows filtered air back into the room. A downdraft unit rises out of the countertop behind the cooktop and pulls air down into a duct under the floor.
Table 1. How the three common range hood types compare on what needs cleaning, how well they remove cooking byproducts, and the problems we see most often in Front Range homes.
| Hood type | What needs cleaning | Effectiveness | Typical problems |
|---|---|---|---|
| Ducted (vented outdoors) | Filters, hood interior, fan, full duct run, backdraft damper, exterior cap | Best. Removes grease, moisture, odor and combustion gases from the house | Grease in long or flex runs, stuck damper, clogged or hail-damaged cap, attic terminations on remodels |
| Recirculating (ductless) | Grease filters, charcoal filters, fan housing | Limited. Traps some grease and odor, removes no moisture or combustion gases | Charcoal filters never replaced, grease saturating the fan, steam and gas byproducts staying in the kitchen |
| Downdraft | Filters, under-floor duct run, remote blower, exterior cap | Moderate. Fights the natural rise of hot air, weak on tall pots and back burners | Long horizontal runs collecting grease, blower buried in a crawlspace, ground-level cap clogging with debris |
Warning Signs Your Range Hood Duct Needs Cleaning
- Weak suction. Hold a sheet of paper under the hood on high. A healthy ducted hood holds it firmly against the filter. If it barely lifts, the duct or damper is restricting flow or the fan is coated.
- Grease drips from the hood. Drips on the cooktop or streaks down the hood's edge mean the filter is saturated and liquid grease is running back out.
- Odor that lingers. If cooking smells hang around for hours or return when the fan is off, the duct is holding odor in its grease film, or the damper is stuck open and the duct is breathing back into the room.
- Visible buildup at the exterior cap. Dark drips below the cap, a greasy sheen on the siding, or a louver glued half open all mean the whole duct is coated.
- A hood that whistles or roars. A new whistle on a familiar fan usually means deposit has shrunk the duct or the damper only partly opens.
These overlap with the signs of a restricted dryer vent, for the same reason: a fan pushing air through a duct that has slowly closed up. Our guide to dryer vent clogged warning signs covers that parallel system, and a home with one neglected exhaust duct usually has both.
The Homeowner Maintenance Routine
The order matters, because you want to remove grease at the source before it travels into the parts you cannot reach.
- Clean the grease filters weekly or monthly. Mesh and baffle filters come out with a latch or a slide. Soak them in hot water with a degreasing dish soap, scrub, rinse and dry fully.
- Wipe the hood interior. With the filters out, wipe the metal around the fan opening with a degreaser and a plastic scraper. Do not spray liquid into the motor housing. If the fan blades are caked, the duct behind them is too.
- Reach the first duct section. Wipe the first foot or two of duct with a degreaser-soaked rag on a stick, and check the hood collar for gaps or foil tape that has let go.
- Check the backdraft damper. The flap should swing freely and fall closed on its own. Stuck open, it lets Front Range wind blow down into the kitchen all winter; stuck shut, the fan moves no air. Clean the hinge and seat, and replace a bent or missing flap.
- Inspect the exterior cap twice a year. Clear leaves, lint and cobwebs from the screen, make sure the louvers open and close, and look for nests.
Where the Job Needs a Professional
Everything past the first reachable section is a different job. Cleaning it properly means running rotary brushes sized to the duct through the entire length, often from both ends, with degreaser to soften hardened deposits and a vacuum capturing what the brushes free. If your duct has elbows, crosses an attic, or terminates on the roof, it belongs with a professional, and the equipment is close to what we use for dryer vent cleaning.
How the Duct Route Traps Grease
Grease condenses where air slows down and where metal is cold, so the geometry of the run decides how fast the coating grows.
Long Horizontal Runs and Elbows
Hot air wants to rise. Every foot of horizontal duct is a foot where grease-laden air is coasting instead of climbing, and every foot in a cold attic or soffit is chilled metal for it to condense on. Each elbow forces the air to turn, and grease droplets, heavier than air, slam into the outside of the bend. Elbows are where we find the thickest deposits.
Flex Duct
Flexible duct is the worst material for a range hood. The ribbed interior catches grease on every ridge, it cannot be brushed without tearing, and the plastic and foil versions are not rated for grease exhaust at all. If your hood is connected with flex, the fix is replacing it with smooth rigid metal, not cleaning it.
Undersized Duct and Cap
When a powerful new hood is connected to the old four inch duct that served the original builder-grade fan, air speeds up, pressure climbs, and the cap's louvers rattle without opening fully. The turbulence at the restriction is where grease drops out of the airstream.
Ranked from best to worst so you can judge your own setup:
- Short, straight, rigid metal duct through the roof with a matched cap. Coats slowly and cleans easily.
- Rigid metal with one or two elbows to a wall cap. Normal maintenance handles it.
- Long horizontal rigid run through an attic or under a floor. Needs a regular schedule and benefits from insulation.
- Any run with an undersized section or cap. Needs correction, not just cleaning.
- Any run using flex duct. Replace it.
The Exterior Termination: Caps, Damper Flaps and Uninvited Guests
The cap takes the weather. On the Front Range that means hail denting louvers, monsoon rain driving sideways into a wall cap, freeze-thaw cycles seizing a damper hinge, and wind that can hold a light flap open all night.
The cap is also where the smell of grease meets the outdoors. Birds, squirrels and mice find a warm, food-scented opening irresistible, and a duct with a missing screen or bent louver is an easy nest site. A nest blocks the exhaust, dries into tinder, and puts animal waste upstream of your kitchen. If you hear scratching above the hood, do not run the fan. Our guide to birds and animals in the chimney explains why some species are protected and how removal is handled, and our animal removal service page explains how we handle the job humanely.
Code Basics in Plain Terms, and the Attic Venting Problem
Exact code language varies by jurisdiction and edition, but the residential mechanical rules for range hood exhaust agree on a few points, each for a reason this article has already covered.
- The duct must be metal with a smooth interior. Grease is combustible and has to be cleanable. Plastic and ribbed flex fail both tests.
- The duct must terminate outdoors. Not into an attic, crawlspace, wall cavity or garage.
- There must be a backdraft damper so outdoor air cannot blow back down the duct when the fan is off.
- Very high capacity hoods need makeup air, because a hood that moves a large volume can depressurize a tight house.
The attic problem comes up on remodels more than you would expect. A kitchen gets an island hood or a relocated range, the installer has no easy path to the roof or an exterior wall, and the duct is run up into the attic and left open. Every bit of grease, steam and combustion gas is delivered into the insulation, which mats down with grease while the sheathing above it stains and can grow mold. The attic gains a fuel-soaked patch right under the roof deck.
The only way to know where your duct goes is to follow it. If you cannot find a cap outside that matches your hood, or the attic has a duct end pointing at the insulation, it needs to be extended to a proper termination. We look for this when a chimney inspection takes us into the attic.
Range Hood Venting, Indoor Air and Gas Ranges at Altitude
A gas burner is an open flame inside your living space. It produces water vapor, nitrogen dioxide and, when combustion is incomplete, carbon monoxide. The range hood is the appliance that is supposed to take all of it outside.
Denver's altitude matters here. With less oxygen in every breath of air a mile up, a burner not adjusted for elevation burns a little richer and incomplete combustion is more likely. A hood with a grease-choked duct pulls less air, so more of those byproducts stay in a tightly built home with the windows shut all winter. We cover the broader picture in our guide to fireplace and indoor air quality, and detection in carbon monoxide and chimney safety in Colorado. The short version: if you cook with gas, run the hood every time you light a burner, keep the duct clear so the hood actually moves air, and have a CO alarm near the kitchen as well as the bedrooms.
A second interaction catches people off guard. A powerful hood on high can pull enough air out of a tight house to reverse the draft in a fireplace, wood stove or naturally vented water heater. If your fireplace smokes only when the kitchen fan is running, that is what is happening. Our guide to furnace and water heater chimney venting covers backdrafting in tight houses in depth. A clean duct lets the hood work at a lower speed with less pull on the rest of the house.
How Often to Clean a Range Hood Duct
There is no single interval, because the load depends on how you cook. A household that boils pasta puts almost no grease into the duct in a year. One that pan-fries, sears on cast iron or runs a wok on a high-output burner several times a week can load a duct in a single winter.
Table 2. Typical range hood maintenance intervals by cooking style. These are working guidelines from what we see in the field, not a standard, and a long or complicated duct run shortens every interval.
| Cooking style | Grease filters | Hood interior and first duct section | Full duct and cap |
|---|---|---|---|
| Light: mostly boiling, baking, occasional sauté | Monthly | Twice a year | Every 3 to 5 years, or when signs appear |
| Moderate: daily stovetop cooking, some frying, gas range | Every 2 weeks | Quarterly | Every 2 to 3 years |
| Heavy: frequent frying, searing, wok cooking, high-output burners | Weekly | Monthly | Every 1 to 2 years |
| Very heavy: home food business, large household, daily deep frying | Weekly or more | Monthly | Annually, with a camera check of the run |
If you have just bought a home and do not know the history, treat the duct as due. A camera look at the run and the cap tells you in minutes whether you inherited a clean system or a decade of someone else's frying. Small restaurants, church kitchens, break rooms with a cooktop and multi-family buildings with shared exhaust runs fall under the commercial rules and a stricter schedule; our commercial services page covers those systems.
What a Professional Range Hood Duct Cleaning Involves
Here is the sequence a proper cleaning follows, so you know what to ask about.
- Trace the run. Confirm where the duct goes and terminates, note material, diameter, elbows and any flex or attic problems, and put a camera down it first.
- Protect the kitchen. Cover the cooktop and seal the hood opening to a vacuum so freed grease does not fall onto the range.
- Remove filters and fan. Degrease the blades and housing, and clean and check the hood collar and damper.
- Brush and degrease the run. Rotary brushes work the full length from both ends where possible, with a vacuum capturing the debris.
- Service the cap. Clean the termination, free the louvers or flap, inspect the screen, and note hail damage.
- Verify airflow. Reassemble, run the hood, and compare airflow before and after.
- Report what was found, including any routing problems and a realistic interval for the next cleaning.
The cost of a duct cleaning depends on the length and access of the run, so we quote it after tracing the duct. Our chimney and fireplace pricing page shows how we price our standard services.
Frequently Asked Questions
Can a dirty range hood duct really start a fire?
Yes. Condensed grease is fuel, and it sits in a metal tube directly above the cooktop with a fan pulling flame and heat toward it. A stovetop grease fire that reaches a saturated filter can travel into the duct.
How do I know if my hood is ducted or recirculating?
Look in the cabinet above or behind the hood for a metal duct. If there is none and the hood has a grille that blows air back into the room, it is recirculating. If you can see a duct but cannot find a cap on the roof or walls, follow it in the attic, because it may terminate there illegally.
Can I clean the range hood duct myself?
You can clean the filters, the hood interior, the fan blades, the first reachable section of duct and the damper flap, and inspect the exterior cap. The full run, especially anything with elbows, an attic section or a roof cap, needs rotary brush equipment, degreaser, containment and often roof access.
Why does my kitchen still smell hours after cooking?
Either the grease film inside the duct is holding odor and releasing it back into the room, or the backdraft damper is stuck open and the duct breathes back into the kitchen whenever wind pushes on the cap. Cleaning the run and freeing the damper fixes both.
Is flexible duct acceptable for a range hood?
No. Ribbed flex traps grease on every ridge, cannot be brushed clean, and the plastic and foil types are not rated for grease exhaust. Residential mechanical rules call for smooth metal duct. Replacing flex with rigid metal is the fix.
Does a range hood affect my fireplace or water heater?
It can. A powerful hood on high can pull enough air out of a tight house to reverse the draft in a fireplace, wood stove or naturally vented water heater. If your fireplace smokes only when the kitchen fan runs, that is what is happening.
Have Your Range Hood Duct Looked At
If your hood feels weak, drips, whistles or leaves the kitchen smelling like dinner the next morning, the duct is telling you something. We clean and inspect residential exhaust ducts across Aurora, Denver and the Front Range with the same rotary equipment and cameras we use on dryer vents and flues.
Call Adam Chimney Sweep at (720) 207-9232 or use our contact page to schedule, and if you would like the dryer vent checked on the same visit, say so when you book.
