When a restaurant cooks with fryers, griddles, charbroilers, ranges, wok ranges, tandoors, and other commercial cooking equipment, the kitchen can produce large amounts of:
- Smoke
- Grease
- Heat
- Steam
- Cooking odors
A commercial kitchen hood is designed to capture these cooking emissions before they spread throughout the kitchen.
But how does it actually work?
The basic process is:
- Cooking creates a rising plume of hot air, smoke, and grease.
- The hood captures that plume above the equipment.
- Grease filters remove a portion of the airborne grease.
- An exhaust fan pulls the remaining air into the duct system.
- The exhaust air travels through the ductwork.
- The system discharges the air outside the building.
- Make-up air replaces air removed from the kitchen.
The hood itself is only one part of this process.
A complete restaurant exhaust system can include the hood canopy, filters, grease collection components, ductwork, exhaust fan, make-up air, controls, and fire suppression.
This guide explains how a restaurant hood captures smoke and grease, what grease filters do, how the exhaust fan works, and why proper airflow is so important in a commercial kitchen.
What Happens When Restaurant Equipment Cooks Food?
Commercial cooking equipment transfers large amounts of heat into food.
As cooking happens, hot air naturally rises from the equipment.
That rising air can carry:
- Smoke
- Grease particles
- Water vapor
- Heat
- Odors
This mixture is often referred to generally as the cooking plume.
What Is a Cooking Plume?
The cooking plume is the column of hot air and cooking emissions rising from an appliance.
You can often see this clearly above:
- Charbroilers
- Fryers
- Griddles
- Wok ranges
Even when you cannot visibly see smoke, hot air and airborne cooking material may still be rising toward the hood.
Why Does the Cooking Plume Rise?
Hot air is less dense than the cooler surrounding air, so it naturally rises.
This helps a canopy-style restaurant hood because the hood is positioned above the cooking equipment where the plume is already moving.
The exhaust system then helps capture and remove it.
Step 1: The Hood Captures the Cooking Plume
The first job of a commercial hood is:
capture.
The hood canopy is positioned above the cooking equipment to collect rising smoke, grease, heat, and steam.
A properly designed hood extends over the cooking area so the plume enters the hood instead of escaping into the kitchen.
Why Is the Hood Larger Than the Equipment?
Cooking emissions do not necessarily rise in a perfectly straight vertical column.
As hot air rises, the plume can spread outward.
This is why a commercial canopy hood commonly extends beyond the cooking surfaces underneath it.
This additional coverage is called:
hood overhang.
What Happens if the Hood Does Not Capture the Plume?
If cooking emissions escape around the hood, you may notice:
- Smoke entering the kitchen
- Strong cooking odors
- Grease accumulating on nearby surfaces
- More heat around the cook line
- Smoke traveling toward customer areas
This is often called smoke spillage or poor capture.
Step 2: Grease-Laden Air Enters the Hood
When foods containing fats and oils are cooked at high temperatures, some of that grease can become airborne.
This is especially common when using equipment such as:
- Commercial Fryers
- Griddles
- Charbroilers
- Commercial Ranges
- Wok Ranges
- Commercial Tandoors
The hood captures that grease-laden cooking air.
What Is Grease-Laden Vapor?
Grease-laden vapor is cooking exhaust that contains grease particles or grease-related material suspended in the air.
It can be created when:
- Oil is heated
- Fat drips onto hot surfaces
- Meat is grilled
- Foods are fried
- Grease reaches high temperatures
Why Is Grease in the Air a Problem?
If grease is not properly captured, it can settle onto:
- Walls
- Ceilings
- Equipment
- Floors
- Ductwork
Restaurant ventilation systems are designed to control this grease before it spreads throughout the building.
Step 3: Grease Filters Remove Grease From the Air
After grease-laden air enters a Type I hood, it typically passes through grease-removal filters.
These filters help separate grease from the exhaust stream.
Commercial hoods commonly use:
baffle filters.
What Is a Baffle Filter?
A baffle filter is a metal grease filter designed with overlapping channels or baffles.
Instead of letting air travel straight through, it forces the air to repeatedly change direction.
How Does a Baffle Filter Remove Grease?
Air can change direction relatively easily.
Heavier grease particles have more difficulty following those rapid changes.
As the grease-filled air moves through the filter:
- Air enters the filter.
- The baffles force it to change direction.
- Grease separates from the airflow.
- Grease collects on the filter surfaces.
- The collected grease drains downward.
The filtered air then continues toward the exhaust duct.
Does the Grease Filter Remove All Grease?
No grease filter should be thought of as removing every trace of grease from restaurant exhaust.
The filter removes a portion of the grease before the exhaust reaches the duct system.
This helps reduce grease accumulation farther into the system.
Where Does the Grease Go?
Commercial hood filters are generally installed at an angle.
This allows captured grease to drain downward rather than collecting indefinitely on the filter.
The grease may flow toward:
- Grease trough
- Grease channel
- Grease cup
What Is the Grease Trough?
The grease trough is a channel inside the hood that helps collect grease draining from the filters.
From there, grease may collect in a removable grease cup.
What Is the Grease Cup?
The grease cup is a small removable container used to collect grease draining from the hood's filtration area.
Restaurant staff can empty and clean it as part of normal hood maintenance.
Step 4: The Exhaust Fan Pulls Air Through the Hood
The hood canopy and filters do not move air on their own.
The airflow is created by the:
exhaust fan.
The exhaust fan pulls air from the cooking area through:
- Hood
- Grease filters
- Exhaust plenum
- Ductwork
and eventually discharges that air outside.
Where Is the Restaurant Hood Fan?
In many restaurants, the exhaust fan is located on the roof.
A common restaurant exhaust fan design is an:
upblast exhaust fan.
What Is an Upblast Exhaust Fan?
An upblast fan pulls air through the kitchen exhaust system and discharges it upward away from the roof.
The fan is usually connected to the hood through the exhaust duct system.
Why Can't the Hood Work Without a Fan?
A stainless steel canopy can help collect rising air, but without proper exhaust airflow it cannot effectively remove that air from the building.
The fan creates the pressure difference that continuously pulls cooking exhaust through the system.
Step 5: The Exhaust Air Enters the Duct
After passing through the grease filters, the exhaust air enters the duct system.
For a Type I commercial kitchen hood, this is often referred to as:
grease ductwork.
What Does the Grease Duct Do?
The grease duct carries cooking exhaust from the hood toward the exterior discharge point.
Depending on the building, the duct may travel:
- Directly upward through the roof
- Through a shaft
- Through another approved route
Why Does the Duct Need Cleaning?
Even with grease filters, some grease can still accumulate in the exhaust system over time.
This is why commercial kitchen exhaust systems require appropriate inspection and cleaning.
The amount of grease buildup can depend on:
- Type of restaurant
- Cooking volume
- Equipment
- Maintenance practices
Step 6: Exhaust Air Leaves the Building
After traveling through the duct system, the cooking exhaust is discharged outside.
The goal is to remove contaminated kitchen air rather than allowing it to remain inside the restaurant.
Where Does the Smoke Go?
Smoke captured by the hood passes through the ventilation system and is exhausted outside.
The grease filters are primarily designed to remove grease, not to trap all smoke like a household air purifier.
The exhaust fan moves the remaining cooking air out of the building.
Does a Restaurant Hood Filter Smoke?
A commercial hood system primarily captures smoke and exhausts it outside.
The grease filters remove grease from the air stream, while the exhaust fan and ductwork move the remaining smoke and cooking vapors outside.
This is different from simply filtering indoor air and returning it to the kitchen.
What About Cooking Odors?
Many cooking odors travel with the exhaust air and are removed through the ventilation system.
However, odor control can depend on:
- Cooking process
- Exhaust location
- Ventilation design
- Special filtration if used
Step 7: Make-Up Air Replaces Exhausted Air
A restaurant hood can remove a large amount of air from the building.
That air has to be replaced.
The replacement air is called:
make-up air.
Why Is Make-Up Air Important?
Imagine an exhaust fan continuously pulling air out of a restaurant without allowing enough air back inside.
The building could develop negative pressure.
This can contribute to:
- Doors being difficult to open
- Strong drafts
- Poor hood capture
- HVAC problems
- Uncomfortable kitchen conditions
Where Does Make-Up Air Enter?
Replacement air can be supplied in different ways depending on the ventilation design.
It may enter:
- Near the hood
- Through a supply plenum
- Through the kitchen HVAC system
- From another dedicated supply location
Can Too Much Make-Up Air Cause Problems?
Improperly directed supply air can interfere with hood capture.
If air blows strongly across the cook line, it can push the cooking plume away from the hood.
This is why exhaust airflow and replacement air should be designed together.
What Is Capture and Containment?
You may hear restaurant ventilation professionals use the phrase:
capture and containment.
These are two closely related ideas.
Capture
The hood captures the rising cooking plume.
Containment
The hood keeps that plume from spilling back into the kitchen.
A hood needs to do both effectively.
What Does Smoke Spillage Look Like?
Signs that cooking emissions are escaping the hood may include:
- Visible smoke drifting beyond the hood
- Grease accumulation on nearby walls
- Strong cooking odors throughout the restaurant
- Excessive heat around the cook line
These symptoms can have several causes, so the system should be evaluated rather than assuming one specific problem.
Does a Bigger Exhaust Fan Always Remove More Smoke?
A larger fan is not automatically better.
The hood system is designed around:
- Hood size
- Ductwork
- Cooking equipment
- Cooking duty
- Make-up air
Changing only the fan can disrupt the balance of the system.
What Does CFM Mean?
CFM stands for:
Cubic Feet per Minute.
It is a measurement of airflow.
Restaurant hood systems move a certain amount of air through the exhaust system.
The required airflow varies depending on factors such as:
- Hood configuration
- Hood dimensions
- Type of equipment
- Cooking duty
Does Every Restaurant Hood Use the Same Airflow?
No.
A small hood over relatively light cooking equipment may have very different airflow needs from a large hood serving:
- Multiple fryers
- Large charbroiler
- Wok range
- Tandoors
Why Does Cooking Equipment Matter?
Different appliances create very different cooking plumes.
For example, boiling a pot of water is not the same as charbroiling burgers.
A hood system should be designed around the actual cooking equipment and cooking processes.
How Does a Fryer Affect the Hood?
Commercial fryers heat large quantities of cooking oil.
During operation, they can generate:
- Grease-laden vapor
- Heat
- Cooking odors
This is why Commercial Fryers are commonly installed beneath Type I ventilation.
How Does a Griddle Affect the Hood?
A griddle cooking:
- Burgers
- Bacon
- Chicken
- Cheesesteaks
can produce grease and smoke that needs to be captured by the hood.
How Does a Charbroiler Affect the Hood?
Charbroilers can create substantial amounts of:
- Smoke
- Heat
- Grease
especially when fat drips onto very hot cooking surfaces.
This can make charbroiling one of the heavier cooking processes on a restaurant line.
How Does a Commercial Range Affect the Hood?
A Commercial Range can perform many cooking methods, including:
- Sautéing
- Pan frying
- Braising
- Boiling
- Sauce preparation
The amount of grease and smoke depends heavily on what is being cooked.
How Does a Wok Range Affect the Hood?
High-temperature wok cooking can create a strong cooking plume containing:
- Heat
- Oil vapor
- Smoke
A Commercial Wok Range should therefore be included in hood planning from the beginning.
How Does a Tandoor Affect the Hood?
A Commercial Tandoor produces intense heat.
When cooking proteins such as:
- Chicken tikka
- Tandoori chicken
- Seekh kebabs
- Lamb
- Fish
it can also produce grease and smoke.
The hood needs to capture emissions rising from the top opening while still leaving enough room for the cook to work with skewers and bread tools.
Why Does Hood Height Matter?
A hood that is positioned too far from the cooking surface can have a harder time capturing the plume before it spreads outward.
A hood that is installed too low can interfere with:
- Cooking
- Large pots
- Equipment access
The correct mounting height depends on the specific hood design and installation requirements.
Why Does Hood Overhang Matter?
The hood needs sufficient coverage beyond the cooking area because the plume spreads as it rises.
If equipment extends too close to the edge of the hood, smoke and grease may escape around the sides.
Why Equipment Depth Matters
A deep piece of cooking equipment can move the cooking surface closer to the front edge of the hood.
This can reduce the hood's effective front coverage.
When replacing equipment, compare both:
- Width
- Depth
Why Cross Drafts Can Cause Smoke to Escape
A restaurant kitchen contains air movement from:
- HVAC vents
- Doors
- Fans
- Make-up air
- Employee movement
If strong air blows across the cook line, it can push the plume away from the hood.
Why Opening a Door Can Affect a Hood
In some kitchens, opening an exterior door can create a strong draft.
If that draft reaches the cooking line, it can interfere with the normal path of the cooking plume.
Does Hood Style Affect Smoke Capture?
Yes.
Commercial hood styles include:
- Wall canopy hood
- Island hood
- Low-proximity hood
- Backshelf hood
Each style captures cooking emissions differently.
Wall Canopy Hood
A wall canopy hood is positioned over equipment installed along a wall.
The wall helps contain the cooking plume on one side.
Island Hood
An island hood is positioned over cooking equipment located away from a wall.
The cooking plume is exposed to air movement from more directions, so hood coverage becomes especially important.
Low-Proximity Hood
A low-proximity hood sits closer to the equipment and captures emissions nearer to their source.
Its design is different from a traditional overhead canopy.
What Is a Type I Hood?
A Type I hood is used for commercial cooking processes that produce grease-laden vapor or smoke.
Its system typically includes grease-removal components and the appropriate grease exhaust arrangement.
What Is a Type II Hood?
A Type II hood is used primarily for:
- Heat
- Steam
- Moisture
- Condensation
from equipment that does not produce grease-laden cooking vapor.
It does not perform the same function as a Type I grease hood.
Does a Type II Hood Remove Grease?
A Type II hood is not intended to substitute for the grease-removal function of a Type I system.
If cooking produces grease-laden vapor, the appropriate Type I system should be considered.
How Do Hood Filters Get Dirty?
Grease filters sit directly in the path of cooking exhaust.
Over time, grease accumulates on the filter surfaces.
The amount of buildup depends on:
- Cooking volume
- Menu
- Equipment
- Cleaning schedule
What Happens if Hood Filters Are Dirty?
Heavy grease buildup can interfere with the airflow path through the hood.
Dirty filters can also make the hood harder to keep clean.
Restaurant hood filters should therefore be cleaned regularly according to the hood and filter manufacturer's recommendations.
Can You Run a Hood Without Filters?
A Type I hood should use the grease-removal components specified for the system.
Removing filters can allow substantially more grease to travel into the ductwork.
Do not operate the system with missing or incorrect filters.
Can You Use Any Filter That Fits?
No.
The filter type is part of the hood design.
Use filters that are appropriate for the specific hood rather than choosing a replacement solely because its dimensions fit the opening.
Does Hood Cleaning Improve Smoke Removal?
Keeping filters and accessible hood components clean helps maintain the airflow path the system was designed to use.
However, poor smoke capture can have many possible causes.
Cleaning alone may not solve an issue related to:
- Equipment changes
- Hood sizing
- Fan performance
- Make-up air
- Cross drafts
Why Does Restaurant Equipment Placement Matter?
The hood is designed around the cooking equipment underneath it.
Moving equipment can change where the cooking plume rises.
For example, moving a charbroiler to the very edge of the hood may make capture more difficult if the appliance is no longer properly within the hood coverage area.
Can You Add More Equipment and Still Remove Smoke Properly?
Possibly.
But adding equipment changes the cooking load beneath the hood.
Adding another:
- Fryer
- Griddle
- Charbroiler
- Range
- Tandoor
can increase the amount of heat, grease, and smoke the ventilation system needs to manage.
Why an Existing Hood May Not Work for a New Restaurant Concept
Imagine taking over a restaurant that previously had:
- One range
- One fryer
- Small griddle
You install:
- Two fryers
- Large charbroiler
- Wok range
The hood may look large enough, but the new cooking operation is substantially different.
Does Fire Suppression Remove Smoke or Grease?
No.
The fire-suppression system serves a different purpose.
It is designed to respond to a cooking fire in the protected hood and equipment area.
The ventilation system handles normal cooking exhaust.
How the Whole Hood System Works Together
A simplified commercial kitchen exhaust system works like this:
| Component | What It Does |
|---|---|
| Cooking Equipment | Creates heat, smoke, grease, and steam |
| Hood Canopy | Captures the rising cooking plume |
| Grease Filters | Separate grease from the exhaust air |
| Grease Trough/Cup | Collects grease draining from filters |
| Exhaust Duct | Carries exhaust away from the hood |
| Exhaust Fan | Creates airflow through the system |
| Make-Up Air | Replaces air removed from the restaurant |
What Happens if One Part Does Not Work Properly?
Because these components work together, a problem with one part can affect the entire system.
Examples include:
- Dirty grease filters restricting airflow
- Exhaust fan problems reducing airflow
- Strong make-up air disrupting capture
- Equipment extending outside the hood capture area
- New cooking equipment creating a heavier load
Why Restaurant Hoods Are Designed Around the Cook Line
The ventilation system should be based on the equipment actually operating underneath it.
This is why restaurant owners should finalize their major cooking equipment before the hood design is completed.
What Information Should Be Used for Hood Planning?
For each piece of cooking equipment, identify:
- Manufacturer
- Model number
- Width
- Depth
- Height
- Fuel type
- Cooking method
This helps the ventilation system reflect the actual kitchen rather than a rough estimate.
Frequently Asked Questions
How does a restaurant hood remove smoke?
The hood captures the smoke rising from cooking equipment, and the exhaust fan pulls that smoke through the hood and duct system before discharging it outside.
How does a restaurant hood remove grease?
Grease-laden cooking air passes through grease filters inside a Type I hood. The filters separate a portion of the grease before the exhaust continues into the duct system.
What catches grease in a commercial hood?
Type I commercial hoods commonly use grease-removal filters such as baffle filters.
Where does the grease from hood filters go?
Captured grease can drain down the filters into a grease trough, channel, or removable grease cup.
Where does restaurant smoke go?
Captured smoke is pulled through the hood and exhaust duct by the exhaust fan and discharged outside the building.
Does the hood filter all smoke?
No. The hood primarily captures smoke and exhausts it outside. Grease filters are designed mainly to remove grease from the exhaust stream.
What does the exhaust fan do?
It creates the airflow that pulls cooking emissions through the hood, filters, and duct system.
Where is the restaurant hood exhaust fan?
Many restaurant exhaust fans are located on the roof or another appropriate exterior location.
What is a baffle filter?
A baffle filter uses metal channels that force air to change direction, helping separate grease particles from the exhaust stream.
Why are hood filters installed at an angle?
The angled position helps collected grease drain downward toward the hood's grease collection area.
Why does a restaurant need make-up air?
Make-up air replaces air removed by the exhaust system and helps maintain proper building air balance.
Can make-up air affect hood performance?
Yes. Poorly directed replacement air can create cross drafts that interfere with the cooking plume.
Why is smoke escaping from my hood?
Smoke spillage can have several possible causes, including equipment position, cooking load, dirty filters, airflow issues, hood coverage, or cross drafts. The complete system should be evaluated rather than assuming a single cause.
Will cleaning the filters stop smoke from escaping?
Clean filters help maintain the airflow path, but smoke capture problems can also involve other parts of the ventilation system.
Does a bigger fan always improve the hood?
No. The exhaust fan, ductwork, hood, cooking equipment, and make-up air should work together as a balanced system.
Does a fryer produce grease-laden vapor?
Commercial frying commonly produces grease-laden cooking emissions and is generally associated with Type I ventilation.
Does a charbroiler produce more smoke than other equipment?
Charbroilers can produce substantial smoke, grease, and heat, especially when cooking fatty proteins.
Does an electric appliance still produce grease?
It can. The energy source does not prevent food or oil from producing grease and smoke during cooking.
Can I add another fryer under my hood?
Possibly, but adding equipment can change the grease load, heat load, hood coverage, fire-suppression needs, and utility requirements.
Can a Type II hood remove fryer grease?
A Type II hood is intended primarily for heat, steam, and moisture rather than grease-producing commercial cooking. Frying is generally associated with Type I ventilation.
Final Thoughts: How Does a Restaurant Hood Remove Smoke and Grease?
A commercial restaurant hood removes smoke and grease through a combination of capture, filtration, and exhaust airflow.
The process begins when cooking equipment creates a rising plume containing:
- Heat
- Smoke
- Grease
- Steam
- Odors
The hood captures that plume.
Grease filters remove a portion of the airborne grease.
The exhaust fan then pulls the remaining cooking air through the duct system and discharges it outside.
At the same time, the restaurant's make-up air system helps replace the air being removed.
The easiest way to picture the process is:
Cooking Equipment → Hood → Grease Filters → Exhaust Duct → Exhaust Fan → Outside
with:
Make-Up Air → Back Into the Restaurant
Each part of the system matters.
A properly designed hood should be matched to the actual Commercial Cooking Equipment underneath it, including Commercial Fryers, Commercial Ranges, Wok Ranges, Stock Pot Ranges, and Commercial Tandoors.
If you are planning a new restaurant kitchen or changing an existing cook line, you can also learn more about Nishi Enterprise's Restaurant Design Consultation.
The simplest way to remember it is this: the hood captures the cooking plume, the filters separate grease, and the exhaust fan carries the remaining smoke and cooking air outside.
