By Daniel Watkins, Noble Chimney Sweepers.
Every duct in a house carries air. One of them carries hot grease. The kitchen range hood duct is the only residential exhaust run that accumulates a flammable coating as a normal part of doing its job, and it is the one almost nobody thinks about until the fan gets loud or the kitchen stops clearing. In New Lenox homes with a real ducted hood over a gas range, that duct has been collecting a film since the day the house was finished.
This is not the same conversation as duct cleaning for the heating system. A supply or return duct collects settled dust. A range hood duct collects vaporized cooking oil that cools on the metal and hardens into a sticky, then tarry, then varnish-like layer. The behavior is much closer to creosote in a chimney than it is to dust in a register.
Why grease builds where it does
Grease deposits where the airstream slows and cools. Vaporized oil leaves the pan as part of the plume, travels with the exhaust, and condenses on the first cool surface it touches for long enough. That means the heaviest buildup is rarely in the middle of a straight run. It concentrates at the filters, in the hood cavity itself, at the first elbow above the hood, at any transition where duct size changes, and at the termination cap where outside air chills the metal.
A long horizontal run through a soffit is the other classic trouble spot. Horizontal duct gives the plume time to cool and gives gravity a chance to pool residue in the bottom of the pipe. That is also where a sag in flex duct, which does not belong on a range hood at all, turns into a grease trap.
What a correctly built range hood duct looks like
It should be smooth-wall rigid metal, sized to the hood manufacturer specification, running as short and as straight as the house allows, and terminating outdoors through a dedicated cap with a backdraft damper. No flex duct, no corrugated dryer-style pipe, and no screen over the outlet. A mesh screen on a grease exhaust is a guaranteed clog, the same way a screened dryer cap mats over with lint.
It must terminate outside. Not into an attic, not into a soffit, not into a crawl space, and not into a wall cavity. Grease-laden air dumped into an attic coats sheathing and insulation, and it is a mess that never gets cleaned up.
The other item people miss is make-up air. A powerful hood pulling several hundred cubic feet per minute out of a tight house has to get that air back from somewhere. In a house with a fireplace or a natural-draft appliance, the easiest path can be down the chimney, which pulls combustion byproducts into the room. That is the same negative-pressure mechanism that makes a fireplace smoke, and it is worth understanding before anybody upsizes a hood.
What a grease-loaded duct actually costs you
Three things, in order of how often they show up. First, performance: a coated duct has a smaller effective diameter and more friction, so the hood moves less air and the kitchen stops clearing steam and smoke. Second, odor: the layer itself goes rancid over years, and warm air moving across it carries that smell back into the house. Third, and least common but most serious, fire. A grease layer is fuel in a metal chimney, and a stovetop flare-up that reaches the hood has a path.
The first two are what bring people to the phone. The third is why the work matters.
How the duct gets cleaned
The filters come out and get degreased. The hood cavity, the fan housing, and the blower wheel get cleaned by hand, because a caked blower wheel is often the actual reason a hood got weak and noisy. The duct itself gets brushed and scraped mechanically, accessed from the hood end and from the termination, with the system under negative pressure so nothing released inside the duct ends up in the kitchen.
Straight runs come clean readily. Long soffit runs with multiple elbows take longer and occasionally need an access panel added. Hardened varnish in a duct that has never been touched in twenty years does not wipe off, and that is a real limitation worth saying out loud rather than discovering halfway through.
How often, and who to call in New Lenox
There is no single interval that fits every kitchen, because the driver is how you cook rather than how long you have lived there. Heavy frying, wok cooking, or searing several times a week loads a duct far faster than a household that mostly bakes. As a working rule, a heavy-use kitchen deserves a look every year or two, and a light-use kitchen every three to five. Pull a filter and look at the first foot of duct above the hood. If a fingertip comes back tacky rather than dusty, it is time.
We handle kitchen exhaust alongside our regular air duct cleaning work, and it is separate from a dryer vent job even though both are exhaust runs, because the deposit and the method are different. Our full service list is published, and so is our pricing, so you know the number before we start. We are certified, bonded and insured, owner-operated, with 14 years in this trade, and we serve New Lenox and the neighboring Will County towns. Call (708) 432-5747, 7 days a week, 7 AM to 7 PM.
Frequently asked questions
Does a recirculating hood need duct cleaning?
No, because there is no duct. A recirculating hood filters air through a charcoal cartridge and blows it back into the kitchen. It needs the grease filter washed and the charcoal cartridge replaced on the manufacturer schedule, but there is nothing behind it to clean.
Can I run a range hood into the attic?
No. Grease-laden exhaust dumped into an attic coats sheathing, framing, and insulation, adds a fuel load where you least want one, and carries moisture along with it. A range hood has to terminate outdoors through its own cap.
Is flexible duct acceptable for a range hood?
It should not be used. The corrugations catch grease, the material sags between supports and pools residue, and the added friction cuts airflow. Smooth-wall rigid metal is the correct material, with as few turns as the layout allows.
Why did my hood get loud and weak at the same time?
That pairing usually points at the blower wheel rather than the motor. Grease builds unevenly on the fan blades, which throws the wheel out of balance and reduces how much air each blade can move. Cleaning the wheel often restores both the noise level and the airflow.
Can a range hood make my fireplace smoke?
Yes, in a tight house. A strong hood depressurizes the room, and if the easiest replacement air path is down an unused or cold flue, the chimney reverses and brings smoke or odor into the house. Cracking a window near the fireplace while the hood runs will usually confirm it.
How do I know if my duct needs cleaning without opening anything?
Take the metal filter out and look and feel above it. Dust is normal. A tacky or sticky film on the duct wall means deposits have started, and a brown varnish layer means it has been a long time. Weak smoke clearing at the cooktop is the other everyday signal.
Is the screen on my exhaust cap supposed to be there?
Not on a grease exhaust. A screen mats over with residue and strangles the airflow, exactly the way a screened dryer vent cap mats over with lint. A proper cap uses a hinged backdraft damper to keep weather and pests out instead.
Do you clean the heating ducts at the same visit?
They can be combined, but they are separate jobs with different tools and different pricing, and one does not include the other. Plenty of households do both in one visit simply to get it over with, and we are happy to schedule it that way.
Published October 13, 2026