Range Hood Not Venting? 9 Causes of Weak Suction and Fixes

When a range hood isn’t venting properly, the cause is usually airflow being choked somewhere between the filter and the outdoors: dirty filters, a stuck damper, or a duct that’s blocked, crushed or too long. Work through the checks below in order. The first three fix most cases in under 30 minutes and cost nothing.

Quick diagnosis

Start with a 2-minute suction test

Before you take anything apart, measure the problem so you can tell whether each fix helped.

1. Turn the hood to its highest speed and let it run for 30 seconds.
2. Hold a single sheet of tissue or a thin strip of paper towel flat against the grease filter, then let go.
3. A healthy ducted hood holds it firmly against the filter. If it barely clings or drops, airflow is restricted.
4. Step outside while the fan runs and find the wall or roof cap. You should see the flap open and feel a steady stream of air.

Repeat this test after each fix below.

1. Clogged grease filters

This is the cause in most homes. Grease builds up slowly, so the loss of suction is easy to miss until smoke starts hanging in the air.

How to check

1. Pull the filters out (most slide or pop out with a latch).
2. Hold one up to a light. If you can’t see light through most of the mesh or between the baffles, it’s restricting airflow.
3. Run the tissue test again with the filters removed, for a few seconds only. A big jump in suction confirms the filters were the problem.

How to fix

1. Wash metal mesh or stainless baffle filters in hot water with degreasing dish soap, or run baffles through the dishwasher. Our guide to cleaning range hood filters has the full soak method and schedule.
2. Clean them every month with heavy or frying use, every 1–3 months otherwise.
3. Replace mesh filters that are torn, bent or permanently matted.

Don’t run the hood for long without filters. Grease goes straight into the blower and duct, which creates a fire risk.

2. Hood set too low, or turned on too late

This one isn’t a fault, but it looks like one. Many people cook on low speed, or switch the hood on once the pan is already smoking.

How to check

1. Note which speed you normally use while searing or frying.
2. Watch whether smoke rises straight into the hood or rolls out past the front edge.

How to fix

1. Turn the hood on about 5 minutes before you start cooking so it establishes an airflow path.
2. Use high speed for searing, stir-frying and anything that smokes.
3. Cook on the back burners when you can. The hood captures more from the rear of the cooktop.
4. Leave the fan running for 10–15 minutes after you finish.

3. Backdraft damper stuck at the hood

Most ducted hoods have a small flap at the top of the hood where it meets the duct. Grease can glue it shut, and installers sometimes leave the shipping tape on it.

How to check

1. Turn off power at the breaker, not only at the hood’s switch.
2. Remove the filters and shine a flashlight up into the collar.
3. Push the flap gently with a finger or a wooden spoon. It should swing open freely and fall closed.

How to fix

1. Remove any tape or packing material.
2. Wipe grease off the hinge and flap with a degreaser and let it dry.
3. If the flap is bent or its pin is broken, replace the damper. A generic round damper usually costs $10–30 (typical); match the collar diameter.

4. Exterior vent flap stuck or blocked

The flap on the outside wall or roof cap can stick from grease, paint, ice or a bird’s nest. If the flap doesn’t open, air has nowhere to go.

How to check

1. With the hood on high, look at the cap from outside. The flap should lift noticeably.
2. Look for a nest, leaves, a screen packed with lint and grease, or a flap painted shut.

How to fix

1. Clean and free the flap. Our stuck vent flap guide walks through each type of cap.
2. Remove fine-mesh screens that clog with grease; kitchen caps should use a flap, not a fine screen.
3. If the cap is on the roof, consider hiring a pro rather than working on a ladder or roof yourself.

5. Blocked, crushed or disconnected duct

If the filters are clean and both flaps open, but you still feel little air outside, the problem is inside the duct run.

How to check

1. Turn off power at the breaker, not only at the hood’s switch.
2. Remove the filters and, if you can, the blower cover. Look up the duct with a flashlight.
3. In an attic or basement, trace the duct. Look for joints that have pulled apart, a duct that ends in the attic, crushed flexible sections or heavy grease buildup.

How to fix

1. Reconnect separated joints and seal them with foil tape, not cloth duct tape.
2. Replace crushed or flexible foil duct with smooth rigid metal duct.
3. For a step-by-step inspection, see how to check if your ductwork is blocked. If the duct is coated in grease, don’t run the hood hard until it has been cleaned.

The International Residential Code (IRC 2021, M1503.3) requires kitchen exhaust to discharge outdoors, not into an attic, crawl space or inside the building. A duct that ends in the attic is both a performance and a code problem; check with your local building department.

6. Duct too small, too long or too twisty

A hood can only move its rated airflow through a duct that’s sized for it. Every added foot and bend eats into that.

How to check

1. Measure the hood’s collar and the duct diameter. The duct should never be smaller than the collar.
2. Count the elbows. Each 90° elbow costs roughly the same as 10–15 feet of straight duct.
3. Add it up. Ideally the total stays under about 30 equivalent feet.

As a rough guide, 6” round duct is typical up to about 400 CFM, 7–8” for 600 CFM and 8–10” above that. Low-CFM under-cabinet hoods often use 3¼” × 10” rectangular duct.

How to fix

1. Replace any reducer that necks the duct down below the collar size.
2. Replace sharp 90° elbows with two 45° fittings where you can, and shorten the run if possible.
3. Rerouting through walls or ceilings is usually a job for a contractor.

7. Not enough makeup air

Every cubic foot of air the hood pushes outside has to be replaced. In a newer, tightly sealed home, air can’t get back in fast enough, so the fan spins but moves little air.

How to check

1. Run the tissue test with all windows and doors closed.
2. Crack a nearby window 1–2 inches and test again.
3. If suction improves clearly, makeup air is part of your problem. Doors that whistle or slam when the hood runs are another sign.

How to fix

1. Crack a window while cooking as a short-term fix.
2. The IRC (M1503.6) requires makeup air for hood systems able to exhaust more than 400 CFM. Makeup-air dampers and systems need a contractor and a permit in most areas; check with your local building department.

Makeup air also matters for safety. A strong hood in a tight house can pull exhaust back down a gas water heater or furnace flue.

8. Hood undersized or mounted too high

If smoke escapes past the edges even when everything is clean and open, the hood may be too small for your cooking.

How to check

1. Compare the hood width to the cooktop. It should be at least as wide, and 3” wider on each side is better for gas.
2. Measure the height from cooktop to hood bottom. Typical ranges are 24–30” over electric and 30–36” over gas; your manual governs.
3. Estimate what you need: about 100 CFM per 10,000 BTU of total burner output for gas, or about 10 CFM per inch of width for electric. Our CFM calculator does the math for you.

How to fix

1. If the hood is mounted well above the manual’s maximum, lowering it can help noticeably.
2. If it’s far below the CFM you need, compare 300 vs 400 vs 600 CFM hoods and plan an upgrade, including duct size and makeup air.

9. Ductless hood, or a tired blower

Ductless hoods recirculate air through a charcoal filter and blow it back into the kitchen. They never remove heat, moisture or combustion byproducts. If yours has no outside vent, replace the charcoal filter (typically every 3–6 months) and ask whether ducting to the outside is an option in your kitchen.

A slow blower is the last common cause. Grease on the blower wheel blades reduces airflow, and a failing motor or capacitor makes the fan turn slowly.

1. Turn off power at the breaker, not only at the hood’s switch.
2. Remove the filters and blower cover, and look at the wheel. Thick grease on the blades means it needs cleaning with degreaser.
3. Spin the wheel by hand. It should turn freely and coast. Stiff or gritty rotation points to worn bearings.
4. If the fan hums or starts slowly, see our humming noise guide.

When to call a pro

Stop and call an HVAC contractor or appliance technician when:

• The duct runs through a finished ceiling, a roof, or an attic you can’t safely reach.
• The duct discharges into an attic or wall cavity instead of outdoors.
• You need to upsize, reroute or replace the duct, or add a makeup-air system.
• The motor hums, runs slowly or smells hot after you’ve cleaned the blower.
• Heavy grease coats the inside of the duct. That’s a fire risk and needs professional cleaning.

The bottom line

Start with the filters and the two flaps, and retest with a tissue after each fix. If suction is still weak with a clean, open path, the issue is the duct, makeup air or the hood’s size, and that’s the point where measuring beats guessing.