A kitchen hood vents out of the house by pushing smoke, grease, and odors through a metal pipe, called ductwork, which runs from the hood to an opening on an exterior wall or the roof.
Deciphering Kitchen Ventilation Exhaust
Your kitchen range hood is more than just a light. It plays a vital role in keeping your indoor air clean. When you cook, you create smoke, grease particles, steam, and odors. The hood’s job is to capture these things and move them outside. This process is called kitchen ventilation exhaust.
If your hood doesn’t vent outside, it just cleans the air with a filter and blows it back into your kitchen. This is called recirculating or ductless venting. While better than nothing, it cannot remove heat, moisture, or heavy smoke as effectively as venting outside. For powerful cooking, venting out is the best choice.
Why External Venting Matters
Venting your kitchen exhaust outside offers many benefits. It helps keep your home healthier and cleaner.
Benefits of Ducted Venting:
- Removes heat and moisture effectively.
- Keeps grease particles out of your cabinets and walls.
- Reduces cooking odors lingering in your home.
- Improves overall air quality while cooking.
The Path: Range Hood Ductwork Installation Basics
The core of effective venting is the ductwork. This is the pathway for the air to travel from under the hood to the outside world. A proper range hood ductwork installation ensures the air moves smoothly and quickly.
Duct Size is Crucial
The size of the duct matters a lot. Using a duct that is too small forces the fan to work harder. This makes it louder and less efficient. Most powerful range hoods require a larger duct size.
| Hood CFM Rating | Minimum Recommended Duct Size (Round) | Minimum Recommended Duct Size (Rectangular) |
|---|---|---|
| 200–400 CFM | 6 inches | 8” x 4” |
| 400–600 CFM | 8 inches | 10” x 6” |
| 600+ CFM | 10 inches or larger | 12” x 8” |
Key Point: Always check your hood manufacturer’s guide. It tells you the exact size needed for the best performance. Using a smaller duct than recommended can void your warranty in some cases.
Material Choices for Ducting
The material you use for the pipe affects airflow and fire safety.
- Rigid Metal Duct (Aluminum or Galvanized Steel): This is the gold standard. It is smooth inside, which means air moves easily. It is also fire resistant. This is best for almost all ducted range hood setup projects.
- Semi-Rigid Metal Duct: This is sometimes used for short, straight runs. It is slightly more flexible than rigid metal. Use it only if the run is very short and straight.
- Flexible Foil Duct (White Plastic or Foil): Avoid this if you can. It traps grease easily. Its ridges slow down the air, making the fan work much harder. Never use it for the main run of your kitchen exhaust venting options.
Mapping the Route: Best Kitchen Hood Exhaust Routing
Figuring out the best path for the duct is one of the hardest parts of installing a kitchen vent pipe. You want the shortest, straightest path possible. Every bend or elbow adds resistance, making the fan less effective.
Minimizing Bends and Turns
Think of the air as water flowing through a hose. A straight hose lets water flow best. Bends create friction and slow the flow.
When planning your route, try to limit the number of turns. A single 90-degree elbow is better than two 45-degree elbows, though both create resistance.
- Rule of Thumb: Every 90-degree elbow adds the resistance of several feet of straight pipe. Try to keep the total number of elbows to three or less.
Venting Through the Roof vs. The Wall
There are two main ways to get the air outside: through the roof or through a side wall. The best choice depends on your home’s layout.
Wall Venting
This method involves running the duct straight out through the nearest exterior wall.
- Pros: Often shorter distance, easier access for cleaning later.
- Cons: Requires cutting a hole in an exterior wall, might place the exit vent near a window or deck, which can push smoky air back toward the house.
Roof Venting
This method involves running the duct vertically through the attic space and out through the roof.
- Pros: Vents smoke high above the house, keeping it away from neighbors or patios. Generally looks cleaner from the outside.
- Cons: Requires going through the attic and cutting the roof, which needs careful sealing to prevent leaks. This is often the preferred route for vent hood external venting when the kitchen is in the center of the house.
How to Vent a Microwave Hood Outside
Many people have an over-the-range microwave that includes a built-in vent fan. A common question is how to vent a microwave hood outside. Many of these units are installed in recirculation mode from the factory.
Checking the Existing Setup
Before assuming you can vent it outside, check the microwave unit itself. Some lower-end models are only designed for recirculation. If yours is designed for external venting, you will need to:
- Locate the knockout plug or plate inside the microwave housing where the duct connects.
- Remove this plug.
- Connect the appropriate size ductwork (usually 3-1/4″ x 10″ rectangular or sometimes 6″ round) to the connector on the back or top of the unit.
- Run the duct following the path guidelines above, ensuring it leads directly outside.
This setup must comply with range hood venting requirements just like a dedicated hood.
Exterior Vent Hood Termination: The Final Piece
The part where the duct meets the outside of your house is called the termination point. This final piece is very important for efficiency and safety.
Choosing the Right Cap
You must use a cap designed for exhaust, not intake. A proper termination cap does several jobs:
- Allows air to exit freely.
- Prevents rain, snow, and debris from entering the duct.
- Keeps pests (like birds or squirrels) out of the ductwork.
For vertical (roof) venting, you need a specialized roof cap with a damper. For horizontal (wall) venting, you need a wall cap with a damper.
The Critical Role of Dampers
A damper is a small flap inside the cap that opens when the fan is on and closes when the fan is off. This prevents cold air from coming in during winter and keeps conditioned air from escaping.
When selecting your exterior vent hood termination, look for heavy-duty metal construction. Plastic caps can warp or break easily.
Important Note on Airflow: Ensure the damper moves freely. A sticky or heavy damper restricts airflow even when the fan is running, reducing the effectiveness of your kitchen ventilation exhaust.
Adhering to Range Hood Venting Requirements
Building codes and local regulations dictate how venting must be done for safety and energy efficiency. Ignoring range hood venting requirements can lead to problems during inspection or even fire hazards.
Duct Length Limits
Most codes have a limit on the total length of ductwork you can use. This is because longer ducts cause excessive air pressure loss (static pressure).
- A common guideline suggests keeping the total equivalent length (including bends) under 35 feet, though this varies by jurisdiction.
- If you must have a long run, you might need a more powerful hood fan (higher CFM) or you might need to increase the duct diameter to compensate for the loss.
Fire Safety and Materials
Fire safety is paramount. Grease is highly flammable.
- Use metal ductwork only (aluminum or steel). Never use flexible plastic or foil ducts for the main exhaust path.
- Ensure the duct runs do not pass through cold, unconditioned spaces (like crawlspaces) unless they are properly insulated and sealed to prevent condensation buildup, which can lead to moisture damage or rust.
Makeup Air Considerations
This is a complex topic, but it is part of modern range hood venting requirements, especially for high-CFM hoods (usually over 400 CFM).
When your powerful hood sucks air out of the house, that air has to be replaced. If it isn’t, it creates negative pressure. This negative pressure can cause back-drafting of combustion appliances (like water heaters or furnaces) or pull air in through cracks, reducing energy efficiency.
For high-CFM systems, local codes often require a dedicated Makeup Air System (MAS). This system brings fresh, tempered air back into the house when the hood is running.
Achieving Optimal Performance in Your Ducted Range Hood Setup
High performance isn’t just about buying an expensive hood; it’s about smart installation. The goal is to move the maximum amount of air (CFM) with the least amount of noise and effort.
Calculations: Static Pressure Matters
Professionals use a concept called Static Pressure (measured in inches of water gauge, or IWG). Every component—the hood fan itself, the duct pipe, the elbows, and the external cap—creates resistance, which adds to the static pressure.
A good fan rated for 600 CFM on paper might only move 450 CFM in reality if the duct run is very restrictive (high static pressure).
Steps to Calculate Total Resistance:
- Find the manufacturer’s rating for the fan’s performance curve.
- Add up the equivalent length added by every elbow (e.g., one 90-degree elbow might equal 10 feet of straight pipe).
- Compare the total effective length to the fan’s performance curve to see the actual CFM delivered outside.
This detailed process ensures your vent hood external venting system performs as advertised.
Sealing the Ductwork
Air leaks in the duct system waste energy and can deposit grease into wall cavities or attics, creating fire risks.
- Use aluminum foil tape (UL listed) to seal every joint and connection. Do not use standard grey duct tape; it dries out and fails quickly.
- Seal the connection point where the duct meets the hood fan housing.
- Seal the connection point where the duct meets the exterior cap.
Proper sealing is a key part of a professional range hood ductwork installation.
Insulation for Roof Vents
If you run ductwork through an unheated attic, condensation can form inside the pipe, leading to dripping water or rust. Insulating the ductwork prevents this. Wrap the duct in thermal insulation wrap, securing it tightly with tape. This keeps the exhaust air hot until it leaves the home, minimizing condensation.
Considering Kitchen Exhaust Venting Options Beyond Standard Runs
While the straight-shot metal duct is standard, sometimes the layout of the house demands creative solutions for kitchen exhaust venting options.
Horizontal Run Challenges
A long horizontal run directly through an exterior wall seems easy, but it has drawbacks:
- Grease Accumulation: Grease tends to settle in horizontal runs unless the fan is very powerful. Sloping the duct slightly toward the outside can help the grease drain out the cap, but this is rarely practical or code-approved.
- Backdraft: If the external cap sits too close to the ground or near an air intake, dirty air can be pulled back in.
Utilizing Chimneys (Not Recommended)
Some older homes might have an unused masonry chimney nearby. While tempting, installing a kitchen vent pipe into an existing chimney flue is generally discouraged by modern building codes.
- Chimneys are designed for hot, dry exhaust (like a fireplace). Kitchen exhaust is greasy and humid.
- The grease can coat the chimney lining, creating a serious fire hazard.
- It can also lead to moisture damage inside the chimney structure.
Maintenance for Long-Lasting Venting
Even the best ducted range hood setup needs care. Regular maintenance keeps the system running efficiently and safely.
Cleaning the Ductwork
If you cook with a lot of grease (deep frying, searing), the ducts will accumulate residue over time.
- For accessible, short runs, you might be able to disconnect the ductwork and clean it with degreaser and brushes periodically.
- For complex or long runs, hire a professional duct cleaning service experienced with kitchen hood systems. They use specialized tools to scrub the inside of the pipe.
Checking the Fan and Filters
Always keep the hood filters clean (washable metal mesh filters should be cleaned monthly). Also, check the fan motor area where the duct connects. Ensure no debris is blocking the damper flap. A simple check of the exterior vent hood termination cap once a year to clear leaves or nests is also good practice.
Frequently Asked Questions (FAQ)
Can I convert my ductless range hood to a ducted one?
Yes, often you can. If your hood is rated as “convertible,” it means the fan motor is capable of pushing air outside. You must remove the charcoal filter (used for recirculating) and purchase the appropriate duct transition piece (usually found in the hood’s accessory kit or sold separately) to connect to metal ductwork. You will need to complete the range hood ductwork installation through the wall or roof.
What is the smallest size duct I can use for any range hood?
While codes specify minimums based on CFM, generally, the smallest standard size used for residential venting is a 6-inch round duct. Some very low-CFM hood inserts might only require a 4-inch duct, but this is rare, and 6 inches is much more common for effective ventilation. Check your range hood venting requirements carefully.
Does the duct need to slope in any direction?
If venting horizontally through a wall, it is often recommended to have a slight upward slope toward the exterior vent cap. This encourages any moisture or small grease droplets to exit rather than pool inside the pipe. However, the run must remain as straight as possible.
What happens if I don’t vent my range hood outside?
If you do not vent outside, the hood operates in recirculating (ductless) mode. The air passes through a charcoal filter which traps some odors and grease, but it blows the heat, steam, and moisture right back into your kitchen. This increases humidity and can lead to mold or peeling paint over time.
How do I seal the roof penetration for a vent hood?
When installing a kitchen vent pipe through the roof, you must use a proper roof flashing and cap kit designed for that pipe diameter. The flashing sits under the shingles above the pipe, and the entire penetration must be sealed thoroughly with roofing cement or sealant to prevent water leaks. This is essential for a successful vent hood external venting job.