What is the maximum duct length for a ducted range hood?

The maximum duct length depends on the hood's CFM rating and duct diameter. As a general guideline, keep the total equivalent duct length under 25 feet for a 400 CFM hood, and increase to 8-inch duct for longer runs or higher CFM. Check the manufacturer's spec sheet for the specific limit, as some models can handle up to 50 feet with proper sizing.

Can I use a flexible duct for a ducted range hood?

Flexible metal duct can be used for short, straight runs, but it is not recommended because its corrugated interior traps grease and reduces airflow by up to 30%. Rigid metal duct is preferred for efficiency and fire safety. If you must use flex, keep the length under 5 feet and avoid bends.

Do I need a make-up air system with a ducted range hood?

If your hood's CFM exceeds the capacity of your home's natural air leakage, typically above 400 CFM for gas appliances, you may need a make-up air system. This brings in fresh air to replace the exhausted air, preventing negative pressure that can cause backdrafts from water heaters or furnaces. Consult a local HVAC professional to determine if your jurisdiction requires it.

Can a ducted range hood vent through the wall instead of the roof?

Yes, venting through an exterior wall is common and often easier than roof venting, especially for kitchens on the ground floor or above a slab. The duct runs horizontally from the hood to a wall cap, which must be installed with a slight downward slope to prevent rain from entering. Ensure the wall cavity is clear of obstructions before cutting.

Ducted Range Hood Installation: How to Route the Vent Outside

Why Ducted Range Hoods Need a Dedicated Vent Path

A ducted range hood captures smoke, grease, and odors at the cooking surface and pushes them through a duct to the outdoors. Unlike ductless models that recirculate air through a charcoal filter, a ducted system requires a continuous, sealed path from the hood's blower to an exterior wall or roof cap. This path is what makes ducted hoods more effective at removing airborne particles and moisture from the kitchen.

The venting path typically starts at the hood's exhaust outlet, transitions to a round or rectangular duct, runs vertically or horizontally through the ceiling, wall, or attic, and terminates at a weatherproof cap. Each section of this path must be sized correctly and kept as straight and short as possible to maintain airflow and prevent backdrafts. Planning the route before you buy the hood saves time and avoids costly mistakes during installation.

Checklist 1: Measure the CFM and Duct Diameter

The first step is matching the hood's airflow capacity, measured in cubic feet per minute (CFM), to the duct diameter. A common rule of thumb for gas stoves is 100 CFM per 10,000 BTUs of burner output, while electric stoves often use 100 CFM per linear foot of cooktop width. Check the manufacturer's specifications for the recommended duct size, typically 6, 8, or 10 inches round, or a rectangular equivalent.

Using a duct that is too small for the CFM rating increases static pressure, reduces airflow, and makes the hood noisier. Conversely, a duct that is too large may cause the air to slow down and allow grease to settle in the duct. Measure the actual distance from the hood to the exterior cap and account for every elbow and transition, as these add resistance. If the run exceeds 25 feet, consider increasing the duct diameter by one size, but confirm the hood's inlet size allows this without adapters that create turbulence.

Checklist 2: Choose the Right Duct Material and Type

Use rigid metal ducting—either galvanized steel or aluminum—for the entire vent path. Rigid duct has a smooth interior that minimizes airflow resistance and is easier to clean than flexible duct. Flexible metal duct, such as aluminum flex, should only be used in short, straight runs if at all, because its ribbed interior traps grease and restricts airflow. Avoid PVC or other plastic pipes, as they are not rated for kitchen exhaust and can melt or catch fire.

For rectangular duct, common sizes include 3-1/4 by 10 inches and 3-1/4 by 12 inches, which often adapt to round duct via a transition piece. Ensure all joints are sealed with metal foil tape, not duct tape, which degrades over time. Use sheet metal screws at joints, but keep them on the outside to avoid snagging grease. If the duct passes through unconditioned spaces like an attic, wrap it with insulation to prevent condensation and heat loss, but do not compress the insulation against the duct, as that reduces its effectiveness.

Checklist 3: Plan the Shortest, Straightest Route to the Outside

The venting path should be as short and direct as possible, with a minimum number of bends. Each 90-degree elbow adds roughly 5 feet of equivalent duct length, and each 45-degree elbow adds about 2.5 feet. A long, winding route forces the blower to work harder, reducing efficiency and increasing noise. Ideally, the exterior cap should be within 25 to 30 equivalent feet of the hood, depending on the hood's CFM rating.

Before cutting any holes, inspect the wall or ceiling cavity for obstructions such as studs, joists, electrical wires, or plumbing. Use a stud finder and a drill with a long bit to probe the planned path. If the route goes through a cabinet, measure the cabinet depth and ensure the duct can pass through without interfering with shelves. For a second-story kitchen, the vent may run horizontally through a soffit, but that often requires additional elbows. For a single-story house, a vertical run through the roof is often simplest, but it requires flashing and a roof cap with a backdraft damper.

Checklist 4: Install the Backdraft Damper and Exterior Cap

A backdraft damper is a critical component that prevents outside air, insects, and small animals from entering the house when the hood is off. It consists of a spring-loaded flap or a set of gravity-operated blades that open when the blower runs and close when it stops. Install the damper as close to the exterior cap as possible, ideally within the duct run near the cap, to minimize the volume of air that can flow backward.

The exterior cap should be installed on the wall or roof, with the opening facing downward or away from the prevailing wind to reduce the chance of rain or snow entering. For wall caps, position the vent at least 12 inches below the soffit and 12 inches above the ground. For roof caps, ensure the flashing seals around the vent pipe to prevent leaks. Use a high-quality silicone sealant around the cap's base and any penetrations. After installation, test the damper by turning on the hood and feeling for airflow at the exterior; a properly installed damper will open fully and close completely when the fan stops.