A range hood that roars on its highest setting but leaves the kitchen smoky is a common complaint, and the fan is often not the problem. Air has to go somewhere, and if the duct is too small, too long, full of turns or blocked at the end, even a powerful fan cannot move much of it.
This troubleshooting guide walks through the duct-related causes of poor hood performance, starting with the easiest to check.
How a hood moves air
The fan pulls air and grease up through the filters, then pushes it into a duct that runs to an exterior wall or roof cap. Each part of that path adds resistance: the filters, the duct's size and length, every elbow, the damper and the exterior cap. A fan's rated airflow is measured under ideal conditions. In a real kitchen, actual airflow is lower, sometimes much lower, depending on how much resistance the duct adds.
Problem 1: the duct is smaller than the hood's outlet
Every ducted range hood has an outlet of a particular size and shape, round or rectangular. The installation guide will specify the duct size to connect, and usually says not to reduce it. Reducing to a smaller duct is one of the most common installation shortcuts, often because the old hood had a smaller duct or because a larger one would not fit through a joist bay.
Signs: loud whistling or rushing at high speed, weak airflow at the hood, more noise than air.
What to check: Look at the duct where it leaves the hood, usually visible inside the cabinet above or behind the chimney cover. If there is a reducer fitting or the duct is visibly smaller than the outlet, that is likely your problem.
Problem 2: the run is too long or has too many turns
Like a dryer vent, a hood duct loses airflow with every foot and every elbow. The installation guide often gives a maximum equivalent length, with each elbow and transition counting as a certain length of straight duct.
Signs: weak airflow even with the right duct size; noise.
What to check: Trace the duct from the hood to the outside. In townhouses and attached homes, the kitchen often has no outside wall of its own, so ducts may run through the ceiling to a rear wall or up through the floors above to the roof. In a renovated home, the duct may snake through soffits and joist bays. Count elbows and estimate the total length.
Problem 3: the wrong duct material
Hood ducts should be smooth-walled metal. Flexible foil or plastic duct creates a lot of friction, collects grease in its ridges and can be a fire hazard. Many hood manuals specifically call for rigid metal duct.
Signs: weak flow, grease dripping back into the hood, a duct that sags or is crushed.
Problem 4: a stuck damper or a bad exterior cap
Most hoods have a backdraft damper, a flap that opens when the fan runs and closes to keep outside air out. Exterior wall and roof caps often have their own dampers too.
Signs: the fan runs but very little air comes out of the exterior cap; cold drafts from the hood in winter; a rattling flap.
What to check: With the hood on high, go outside and see whether the cap's flap opens. Grease buildup, paint, bird nests or ice can hold it shut. Caps meant for bathroom fans are sometimes used on kitchen ducts and are too small or too restrictive.
Problem 5: the duct ends in the wrong place
A hood duct must terminate outdoors. It should not end in an attic, a soffit, a ceiling cavity, a chimney or the space between floors. Grease buildup in those spaces is a fire hazard and causes moisture damage.
In older homes, it is not unusual to find a duct that was never connected to anything, or one that ends in a soffit above the cabinets. If your hood seems to do nothing at all, this may be why.
Problem 6: no makeup air
A hood pulls air out of the house. That air has to be replaced from somewhere. In a well-sealed home, a strong hood can create negative pressure that limits its own airflow and can affect other appliances that vent naturally, such as a gas water heater or furnace. Some jurisdictions require makeup air for larger hoods.
Signs: the hood works noticeably better with a window open; doors are hard to close when the hood runs; other appliances' flues seem to backdraft.
Questions about makeup air and combustion safety should go to a licensed HVAC contractor, and anything involving permits to your township or borough code office (in Pennsylvania) or your town's construction official (in New Jersey).
Problem 7: clogged filters
This one is not the duct, but it causes the same symptoms. Grease filters that have not been cleaned restrict airflow badly. Charcoal filters in recirculating hoods need replacement on a schedule.
Quick troubleshooting table
| Symptom | Most likely cause |
|---|---|
| Loud, little airflow | Reducer, undersized duct, long run |
| Fan runs, nothing at exterior cap | Stuck damper, blocked cap, disconnected duct |
| Grease dripping from hood | Flex duct, saturated filters, condensation in a cold duct |
| Cold drafts in winter | Damper not closing |
| Works better with window open | Makeup air |
| Hood does nothing | Duct not connected, fan or switch failure |
Townhouse and condo considerations
Shared walls make exterior penetrations harder in a townhouse or twin, so the duct route may need to go to the rear wall or up to the roof. That means longer runs, which is why sizing matters more here. In condo buildings, exterior penetrations are typically controlled by the association, and some buildings only allow recirculating hoods. Check first.
Hood placement also interacts with duct routing. If you are choosing a new hood, read our guide on range hood mounting height. If you are considering an over-the-range microwave instead, see our over-the-range microwave measuring guide.
When to call a technician
If the duct is the right size, the run is reasonable, the filters are clean and the damper opens but the hood still underperforms, the fan motor, blower wheel or control may be failing. A motor that hums but spins slowly, a blower wheel caked with grease, or a speed control that only works on one setting are repair problems. Our range repair page covers what we handle; book a visit online and a technician can help sort out whether the problem is the duct or the hood.