Range Hood Duct Blocked? How to Check Your Ductwork Step by Step

To check if your range hood duct is blocked, run the hood on high and feel for strong airflow at the outside cap. If the fan sounds normal but little air comes out, something between the hood and the cap is in the way. The checks below narrow it down in order, starting from the cheapest and most common causes.

Before you blame the duct, clean the grease filters and confirm the fan is spinning at full speed. Filters cause most weak suction, and the full list of causes is in our weak suction guide. This page picks up once you suspect the duct itself.

Quick diagnosis

Test 1: Compare airflow with filters out

This tells you whether the restriction is in the filters or further up.

1. Clean the filters, reinstall them, and turn the hood on high.
2. Hold a tissue a few inches below the filters. It should pull up firmly and stick.
3. Remove the filters and repeat. With a clear duct, the pull gets somewhat stronger but not dramatically.
4. If the tissue barely moves even with filters out and the fan clearly spinning, the restriction is past the fan: in the damper, duct or cap.

Test 2: Check airflow at the exterior cap

1. Find where the duct exits. Wall caps are usually on the outside wall behind or near the kitchen; roof caps sit above the kitchen or near the chimney chase.
2. Have someone turn the hood on high.
3. At a wall cap, hold your hand or a tissue near the opening. You should feel a steady stream and see the flap open. A 300–400 CFM hood through a 6” duct produces a noticeable breeze.
4. For a roof cap, watch from the ground with binoculars if you can see the flap, rather than climbing. If you must go up, follow the ladder safety steps in our vent flap stuck guide.

Strong airflow at the cap means the duct is clear and the problem is at the hood. Weak or no airflow means keep going.

Test 3: Inspect the damper at the hood collar

Many hoods have a backdraft damper where the duct connects on top of the hood. Grease can glue it shut, or tape from installation can pin it down.

Turn off power at the breaker, not only at the hood’s switch.

1. Remove the filters and, if needed, the inner cover or blower housing so you can see up into the collar.
2. Shine a flashlight up. You should see one or two flaps that swing freely when you push them with a finger.
3. If a flap is stuck, wipe the hinge with hot, soapy water until it moves freely. Remove any leftover packing tape or foam.
4. Make sure the flap isn’t reversed; it should open upward, in the direction of airflow.

Test 4: Look up the duct

1. With filters and cover off, look up through the collar with a flashlight.
2. For a better view, feed an inspection camera up the duct, or hold your phone up in video mode with the flashlight on.
3. Look for:
   • Nesting material: twigs, grass, feathers, insulation fluff.
   • A brown or amber coating on the duct walls, which is grease.
   • A sharp kink, collapsed section, or daylight showing through a joint.

Short, straight runs can often be seen end to end. Runs with elbows usually can’t, so move on to the attic or exterior check.

Test 5: Follow the duct in the attic or soffit

If your duct goes through the attic, a short visual inspection there finds a lot of problems.

1. Take a flashlight and step only on joists or boarded floor.
2. Follow the duct from the kitchen ceiling to the roof or wall.
3. Check for:
   • Crushed or kinked flexible duct. Storage boxes, insulation, or a person stepping on it can flatten it.
   • A disconnected joint. The duct may have slipped off the collar or cap, blowing kitchen air into the attic. The IRC requires kitchen exhaust to go outdoors, not into the attic (M1503.3).
   • Sagging runs. Low spots collect grease and condensation.
   • Cloth duct tape that has dried and let go.

How to clear the most common blockages

Stuck damper at the cap

A painted-shut, iced or grease-glued exterior flap is common. The vent flap stuck guide covers wall and roof caps.

Nest or debris

1. From outside, remove the cap’s screws or guard if it has one.
2. If you see eggs or young birds, stop and call a wildlife removal pro; some nesting birds are protected.
3. Pull out nesting material with gloved hands or a grabber tool.
4. From inside, push a dryer vent brush up the duct to loosen anything further in, then pull debris out at the cap.
5. Reinstall the cap and fit a guard with openings around ½”, which keeps birds out without clogging as fast as fine screen.

Crushed or disconnected duct

1. Reshape lightly crushed rigid duct by hand if it isn’t creased. Replace creased sections.
2. Replace flexible foil duct with smooth rigid metal duct. The IRC calls for galvanized steel, stainless steel or copper ducts (M1503.4).
3. Reconnect joints so the upstream section slides inside the downstream one, in the direction of airflow.
4. Seal joints with foil tape. Don’t use screws that poke into the duct; they catch grease.

Grease buildup

Light film near the collar can be wiped with hot, soapy water. Anything thicker is a fire hazard and a job for a duct cleaning pro. See grease in ductwork for what’s involved.

When the “blockage” is the duct design

If nothing is physically blocking the duct but airflow has always been weak, the run itself may be the problem.

Example: 12 feet of straight duct plus two 90° elbows counts as roughly 32–42 equivalent feet, already past the target. Plug your cooktop and duct size into the CFM calculator to see whether the hood and duct are matched.

When to call a pro

• Grease buildup you can see beyond arm’s reach, or thick deposits anywhere.
• Roof caps on steep, high or wet roofs.
• Live animals, eggs, or young in the duct.
• Ducts inside finished walls or ceilings with no access.
• Rerouting or resizing the duct, which may need a permit; check with your local building department.

A duct cleaning or vent service call is typically $150–400 in the US; rerouting ductwork costs more depending on access.

Bottom line

Feel for air at the cap first. If it’s weak, check the collar damper, look up the duct, and follow it through the attic. Most blockages turn out to be a stuck flap, a nest, or a crushed section of flex duct, and all three are fixable in an afternoon.