Kitchen Hood Exhaust Design Guide

Introduction

Every commercial kitchen tells the same story: sizzling pans, rising smoke, and the constant battle to keep air clean and safe. 70% of restaurant fires start in the exhaust system—making proper hood design literally a matter of life and safety.

This guide cuts through the complexity of kitchen ventilation. Whether you're sizing a hood for a small café or a high-volume fast food operation, you'll learn the exact formulas, code requirements, and practical techniques used by HVAC professionals.

Why Kitchen Hood Design Matters

The Core Challenge

Kitchen hood design balances three competing demands:

1. Enough exhaust to capture all cooking effluent
2. Enough makeup air to prevent negative building pressure
3. Compliance with NFPA 96 fire codes and IMC ventilation requirements

Get any one wrong, and the system fails. This guide shows you how to get all three right.

What You'll Learn

By the end of this guide, you'll be able to:

• Calculate exhaust CFM using both simplified (CFM/linear foot) and detailed (capture velocity) methods
• Size makeup air systems to maintain proper building pressure
• Select the right hood type (Type I vs Type II) for your equipment
• Meet NFPA 96 duct velocity and fire suppression requirements
• Apply correction factors for hood height, multiple appliances, and cross-drafts

Quick Answer: How to Calculate Kitchen Hood Exhaust CFM

Size kitchen exhaust hoods based on linear footage and cooking type, then verify against NFPA 96 requirements.

Core Formula (Simplified Method)

CFM per Foot Guidelines

Additional Formulas

Worked Example

Reference Table

Key Standards

Kitchen exhaust hoods are the first line of defense in commercial and residential kitchens, serving multiple critical functions that extend far beyond simple airflow removal. Understanding their role in the complete kitchen ventilation system is essential for effective design.

Primary Functions

1. Contaminant Capture and Removal Kitchen hoods capture cooking effluent—a complex mixture of grease particles, smoke, steam, heat, and odors—before they can disperse into the occupied space. The hood's capture efficiency directly impacts indoor atmosphere quality, with poorly designed systems allowing 20-40% of contaminants to escape into the kitchen environment.

2. Fire Safety and Grease Management Type I hoods (grease-producing equipment) are specifically designed to prevent grease accumulation in ductwork, which is the leading cause of commercial kitchen fires. According to NFPA statistics, 70% of restaurant fires originate in kitchen exhaust systems, making proper hood design a critical life safety requirement.

3. Thermal Comfort Commercial cooking equipment generates substantial heat loads—typically 50,000-200,000 Btu/h per appliance. Without adequate exhaust, kitchen temperatures can exceed 100°F (38°C), creating uncomfortable working conditions and potential heat stress hazards.

4. Odor Control Cooking odors can migrate throughout buildings, affecting adjacent spaces and customer experience. Properly sized hoods with adequate capture velocity prevent odor migration and maintain acceptable indoor ventilation air quality.

5. Equipment Protection Excessive heat and grease accumulation can damage kitchen equipment, reduce efficiency, and increase maintenance costs. Effective hood systems protect equipment investment and extend service life.

System Components

A complete kitchen ventilation system consists of several integrated components:

1. Exhaust Hood - Captures contaminants at the source
2. Grease Filters - Removes grease particles (Type I hoods)
3. Exhaust Ductwork - Transports exhaust fresh air to outdoors
4. Exhaust Fan - Provides airflow and static pressure
5. Makeup Air supply System - Replaces exhausted airflow
6. Fire Suppression Arrangement - Automatic fire protection (Type I)
7. Controls - Variable speed, demand-control, or manual operation

Design Requirements

Proper kitchen hood design requires careful consideration of multiple factors:

1. Adequate Exhaust Movement

• Sufficient CFM to capture all cooking effluent
• Accounts for hood height, cooking type, and appliance diversity
• Meets minimum code requirements (NFPA 96: $\geq 200$ CFM/ft² for Type I)

2. Proper Capture Velocity

• Minimum velocity at hood edge to overcome thermal plumes
• Varies by cooking type: 0.25-0.75 m/s (50-150 fpm)
• Accounts for cross-drafts and atmosphere movement patterns

3. Makeup Ventilation air Supply

• 80-100% of exhaust volume per IMC Section 508
• Properly located to avoid disrupting capture patterns
• Conditioned to maintain thermal comfort

4. Fire Suppression Integration

• UL 300 listed systems for Type I hoods
• Proper nozzle placement and coverage
• Automatic fuel/power shutoff on activation

5. Energy Performance

• Demand-control fresh air circulation (DCKV) reduces energy consumption by 30-50%
• Variable frequency drive (VFD) fans match airflow to cooking activity
• Energy recovery in makeup air supply systems

6. Maintainability

• Accessible filters and ductwork for cleaning
• Clear maintenance schedules per NFPA 96
• Documented cleaning records for code compliance

Exhaust Flow Calculation

Exhaust circulation computation is the foundation of kitchen hood design. Multiple methods exist, each with specific applications and accuracy levels. The most accurate approach combines theoretical calculations with empirical correction factors based on decades of field testing and research.

Basic Formula

The fundamental equation for exhaust flow rate analysis is:

Where:

• $Q$ = exhaust discharge rate (m³/s or CFM)
• $A$ = effective hood opening area (m² or ft²)
• $V_{\text{capture}}$ = capture velocity at hood edge (m/s or fpm)

Important Notes:

• Hood area ($A$) refers to the effective capture area, not the physical hood dimensions
• For wall-mounted hoods, $A =$ hood width $\times$ hood depth
• For island hoods, $A$ includes perimeter capture area
• Capture velocity must be measured at the hood edge, where contaminants are captured

Capture Velocity

Capture velocity is the minimum airflow velocity required at the hood edge to overcome the thermal plume rising from cooking equipment and capture contaminants. This velocity must exceed the upward velocity of the thermal plume, which varies significantly with cooking type and equipment heat output.

Factors Affecting Capture Velocity:

• Equipment heat output: Higher heat = stronger thermal plume = higher capture velocity needed
• Cooking method: Frying and grilling produce more grease and smoke than baking
• Hood height: Higher hoods require higher capture velocity due to plume dispersion
• Cross-drafts: Atmosphere movement in kitchen can disrupt capture patterns
• Hood configuration: Island hoods need higher velocity than wall-mounted hoods

ASHRAE Simplified Method

For quick preliminary sizing, ASHRAE provides simplified rules based on linear footage:

Where:

• $L$ = hood length (linear feet)
• $R_{\text{CFM/ft}}$ = CFM per linear foot (varies by hood type and cooking duty)

CFM per Linear Foot Guidelines:

When to Use Simplified Method:

• Preliminary design and budgeting
• Standard kitchen layouts with typical equipment
• Quick field estimates
• When detailed equipment data unavailable

Limitations:

• Does not account for hood height variations
• Assumes standard overhang dimensions
• May oversize or undersize for non-standard configurations
• Should be verified with detailed determination

Detailed Calculation Method

For accurate design, use the detailed method with correction factors:

Where:

• $C_H$ = height correction factor
• $C_T$ = thermal plume correction factor
• $C_D$ = draft/interference correction factor

Correction Factors

1. Hood Height Correction

As hood height increases above the cooking surface, the thermal plume disperses and weakens, requiring higher exhaust stream to maintain capture:

Where:

• $H$ = hood height above cooking surface (m)
• For imperial units: $C_H = 1 + 0.04 \times H$ (where $H$ is in feet)

Example:

• Standard height (1.5 m): $C_H = 1 + 0.2 \times 1.5 = 1.30$ (30% increase)
• High ceiling (2.5 m): $C_H = 1 + 0.2 \times 2.5 = 1.50$ (50% increase)

2. Thermal Plume Correction

Accounts for multiple appliances and their combined heat output:

Where:

• $N$ = number of cooking appliances under hood

Example:

• Single appliance: $C_T = 1.0$
• Two appliances: $C_T = 1.1$ (10% increase)
• Three appliances: $C_T = 1.2$ (20% increase)

3. Draft and Interference Correction

Accounts for cross-drafts, ventilation air movement, and interference from adjacent equipment:

• $C_D = 1.1$: Protected location, minimal drafts
• $C_D = 1.2$: Typical commercial kitchen
• $C_D = 1.3$: High-traffic area, significant fresh air movement

Equipment Heat Load Method

For precise calculations, size exhaust based on equipment heat output:

Where:

• $Q_{\text{equipment}}$ = equipment heat output (Btu/h or kW)
• $F_{\text{exhaust}}$ = exhaust fraction (typically 0.3-0.5 for hoods)
• $C_p$ = specific heat of air supply (0.24 Btu/lb·°F)
• $\rho$ = airflow density (0.075 lb/ft³ at standard conditions)
• $\Delta T$ = temperature rise (typically 20-40°F)

Typical Exhaust Fractions by Equipment:

NFPA 96 Minimum Requirements

NFPA 96 establishes minimum exhaust requirements that must be met regardless of evaluation method:

Type I Hoods (Grease-Producing):

• Minimum: 200 CFM per ft² of hood face area
• Face area = hood length $\times$ hood depth
• This is a code minimum, not a design target

Type II Hoods (Non-Grease):

• Minimum: 50-100 CFM per ft² of hood opening area
• Lower requirements due to absence of grease

Verification: Always verify that calculated exhaust current meets or exceeds NFPA 96 minimums. If calculated movement is less than minimum, use the minimum requirement.

Makeup Air

Makeup atmosphere (also called replacement ventilation air or supply fresh air) is outdoor air supply introduced into a building to replace airflow exhausted by kitchen atmosphere exchange systems. Without adequate makeup ventilation air, kitchen exhaust creates negative building pressure, leading to numerous operational and safety problems.

Why Makeup Air is Critical

1. Prevents Negative Building Force When exhaust exceeds supply, buildings develop negative stress relative to outdoors. IMC Section 508 limits negative load to -0.02 inches w.g. to prevent:

• Backdrafting of combustion appliances (water heaters, furnaces)
• Difficulty opening exterior doors
• Uncomfortable drafts through cracks and openings
• Reduced exhaust fan performance (fan must overcome building pressure value)

2. Maintains Hood Capture Effectiveness Makeup fresh air must be properly located to avoid disrupting hood capture patterns. When makeup air supply is too close to the hood or improperly directed, it can:

• Reduce capture productivity by 30-50%
• Create cross-drafts that push contaminants away from hood
• Cause smoke and grease to escape into kitchen

3. Code Requirements IMC Section 508 mandates makeup airflow for exhaust systems exceeding 400 CFM (680 m³/h). Many jurisdictions require makeup atmosphere equal to 80-100% of exhaust volume.

4. Energy Output ratio Properly designed makeup ventilation air systems can:

• Recover energy from exhaust fresh air (energy recovery ventilators)
• Condition air supply to reduce heating/cooling loads
• Use demand-control to match supply to exhaust

Makeup Air Requirement

The makeup airflow circulation rate is calculated as a percentage of exhaust flow rate:

Where:

• $Q_{\text{makeup}}$ = makeup atmosphere discharge rate (m³/s or CFM)
• $Q_{\text{exhaust}}$ = exhaust ventilation air stream rate (m³/s or CFM)
• $F_{\text{makeup}}$ = makeup fresh air fraction (typically 0.80 to 1.00)

Makeup Air supply Fraction Guidelines:

Important Considerations:

• Never provide less than 80% of exhaust volume
• Balance makeup airflow with exhaust to maintain building power between -0.02 and +0.02 inches w.g.
• Account for other exhaust systems (restrooms, general exhaust)
• Consider infiltration and exfiltration in force calculations

Makeup Air Methods

1. Dedicated Makeup Atmosphere Units

Dedicated units provide the best control and performance:

Direct-Fired Makeup Ventilation air Units:

• Yield: 90-95% (combustion gases used for warming)
• Operation: Natural gas or propane burner heats outdoor fresh air directly
• Applications: Cold climates, high heat system loads
• Advantages: High performance, fast response, compact
• Disadvantages: Requires gas service, combustion products in air supply stream

Indirect-Fired Makeup Airflow Units:

• Effectiveness: 80-85% (separate heat exchanger)
• Operation: Hot water or steam coil heats outdoor atmosphere
• Applications: Buildings with central thermal system systems
• Advantages: No combustion products, can use waste heat
• Disadvantages: Lower productivity, requires hot water/steam source

Electric Makeup Ventilation air Units:

• Output ratio: 100% (all electricity converted to heat)
• Operation: Electric resistance heaters warm outdoor fresh air
• Applications: Small systems, areas without gas service
• Advantages: Simple installation, no combustion concerns
• Disadvantages: High operating costs, limited capacity

2. Transfer Air supply from Adjacent Spaces

Transfer airflow uses atmosphere from dining areas, corridors, or other conditioned spaces:

Advantages:

• No additional equipment cost
• No furnace system/air conditioning energy required
• Simple installation (grilles and ducts)

Disadvantages:

• Limited capacity (typically 20-30% of exhaust)
• May create drafts in source spaces
• Requires adequate source space volume
• Not suitable for large exhaust systems

Design Guidelines:

• Maximum transfer ventilation air: 30% of exhaust volume
• Source space must have adequate supply fresh air
• Use low-velocity grilles to minimize drafts
• Balance transfer air supply with source space requirements

3. Natural Airflow

Natural airflow movement uses operable windows, doors, or louvers:

Advantages:

• No mechanical equipment
• No energy consumption
• Simple and low-cost

Disadvantages:

• Limited control (weather-dependent)
• Inconsistent airflow
• Not suitable for large systems
• May cause comfort issues

Applications:

• Small residential kitchens
• Temporary installations
• Supplemental atmosphere supply only

4. Dedicated Outdoor Ventilation air Systems (DOAS) with Energy Recovery

DOAS systems condition outdoor fresh air and can recover energy from exhaust:

Energy Recovery Ventilators (ERV):

• Transfer sensible and latent heat between exhaust and supply
• Yield: 60-80% energy recovery
• Applications: Moderate climates, balanced systems

Heat Recovery Ventilators (HRV):

• Transfer sensible heat only
• Performance: 70-85% sensible recovery
• Applications: Cold climates, dry systems

Advantages:

• Significant energy savings (30-50% reduction in heater/AC)
• Improved indoor air supply quality
• Precise control of supply conditions

Disadvantages:

• Higher initial cost
• Requires maintenance
• More complex installation

Makeup Air Location and Distribution

Critical Rule: Minimum 10 ft Separation

Makeup airflow diffusers must be located minimum 10 ft (3 m) from hood edges to avoid disrupting capture patterns. Closer placement causes:

• 30-50% reduction in capture effectiveness
• Cross-drafts pushing contaminants away from hood
• Visible smoke and grease escape

Distribution Strategies:

1. Perimeter Distribution

• Diffusers located around kitchen perimeter
• Directs atmosphere toward hood without disrupting capture
• Best for large kitchens with multiple hoods

2. Ceiling Distribution

• Diffusers in ceiling, directed away from hoods
• Creates general ventilation air movement toward exhaust
• Suitable for smaller kitchens

3. Floor-Level Distribution

• Low-velocity diffusers near floor
• Fresh air rises naturally, creating upward amperage
• Reduces drafts at cooking level

4. Combination Approach

• Mix of ceiling and perimeter distribution
• Optimizes airflow patterns
• Most effective for complex layouts

Makeup Air Conditioning

Makeup air supply must be conditioned to maintain thermal comfort:

Winter Warming:

• Heat makeup airflow to within 10°F (5.6°C) of space temperature
• Prevents cold drafts and discomfort
• Reduces heat system load on main HVAC installation

Summer Refrigeration:

• Cool makeup atmosphere to within 15°F (8.3°C) of space heat
• Prevents hot drafts
• May require dehumidification in humid climates

Energy Considerations:

• Use energy recovery when possible
• Consider demand-control to reduce conditioning loads
• Size thermal system/chilling capacity for peak exhaust rates

Hood Types

Kitchen hoods are classified by their mounting configuration, which directly affects capture productivity and exhaust requirements. Understanding hood types is essential for proper equipment design.

Wall-Mounted Hoods (Type A)

Wall-mounted hoods are attached to a wall with the cooking surface positioned against the wall. This configuration provides capture on three sides (front and two ends), with the wall blocking one side.

Characteristics:

• Mounting: Attached to wall, typically 30-36 inches above cooking surface
• Capture Area: 3 open sides (front, left, right)
• Exhaust Requirements: Lower than island hoods (200-400 CFM/ft)
• Applications: Small to medium commercial kitchens, residential kitchens
• Advantages: Lower exhaust requirements, easier installation, cost-effective
• Disadvantages: Limited to wall locations, less flexible layout

Design Considerations:

• Extend hood 6-12 inches beyond cooking surface on front and ends
• Minimum 6 inches overhang required, 12 inches preferred
• Height: 30-36 inches above gas equipment, 24-30 inches above electric
• Can use side panels or curtains to improve capture on ends

Exhaust Movement Assessment:

• Use ASHRAE simplified method: 200-400 CFM per linear foot
• Or detailed method with 3-sided capture area
• Account for wall effect (reduces required circulation by ~20% vs island)

Island Hoods (Type B)

Island hoods are suspended from the ceiling with cooking equipment positioned away from walls. This configuration requires capture on all four sides, resulting in higher exhaust requirements.

Characteristics:

• Mounting: Suspended from ceiling, typically 30-36 inches above cooking surface
• Capture Area: 4 open sides (all directions)
• Exhaust Requirements: Higher than wall hoods (300-500 CFM/ft)
• Applications: Large commercial kitchens, open kitchen concepts, display cooking
• Advantages: Flexible layout, maximum visibility, professional appearance
• Disadvantages: Higher exhaust requirements, more complex installation, higher cost

Design Considerations:

• Extend hood 12-18 inches beyond cooking surface on all sides
• Minimum 12 inches overhang required, 18 inches preferred for heavy cooking
• Use perimeter curtains or baffles to improve capture output ratio
• May require structural support for heavy hoods
• Consider ceiling height and clearance requirements

Exhaust Flow rate Solution:

• Use ASHRAE simplified method: 300-500 CFM per linear foot
• Or detailed method with 4-sided capture area
• Add 20-30% margin for cross-drafts and ventilation air movement

Canopy Hoods

Canopy hoods provide full coverage over cooking areas with maximum capture yield. They are typically used in heavy-duty commercial applications.

Types of Canopy Hoods:

1. Standard Canopy Hoods:

• Full rectangular canopy over cooking area
• Maximum capture performance
• Used for heavy-duty cooking (fryers, charbroilers, woks)
• Highest exhaust requirements

2. Proximity Hoods:

• Located closer to cooking surface (18-24 inches)
• Higher capture velocity at lower exhaust discharge
• Used for high-heat equipment (woks, salamanders)
• Reduces required CFM by 30-40% vs standard canopy

3. Backshelf Hoods:

• Low-profile hoods mounted at back of cooking equipment
• Used for countertop equipment (toasters, steam tables)
• Lower exhaust requirements (100-200 CFM/ft)
• Type II hoods (non-grease applications)

Characteristics:

• Capture Effectiveness: Highest of all hood types
• Exhaust Requirements: Highest (400-600 CFM/ft for heavy duty)
• Applications: Heavy-duty commercial kitchens, high-volume operations
• Advantages: Maximum capture, handles high heat loads, professional appearance
• Disadvantages: Highest cost, highest exhaust requirements, requires substantial makeup fresh air

Specialized Hood Types

1. Pass-Through Hoods:

• Used for equipment that extends through wall (pizza ovens, rotisseries)
• Capture on both sides of wall
• Requires coordination with wall construction

2. Eyebrow Hoods:

• Low-profile hoods for display cooking
• Aesthetic design with functional capture
• Lower exhaust requirements

3. Ventless Hoods (Recirculating):

• Type II hoods only (non-grease applications)
• Filter and recirculate air supply instead of exhausting
• Requires high-productivity filters (carbon, HEPA)
• Not permitted for grease-producing equipment per NFPA 96

4. Downdraft Hoods:

• Draw airflow downward through cooking surface
• Used for island installations where overhead hoods are not desired
• Lower capture output ratio than overhead hoods
• Higher exhaust requirements to compensate

Hood Selection Guidelines

Choose Wall-Mounted Hood When:

• Kitchen layout allows wall mounting
• Budget is constrained
• Lower exhaust requirements desired
• Small to medium kitchen

Choose Island Hood When:

• Open kitchen concept desired
• Cooking equipment away from walls
• Display cooking important
• Large commercial kitchen

Choose Canopy Hood When:

• Heavy-duty cooking equipment
• Maximum capture yield required
• High heat loads
• Professional commercial kitchen

Choose Proximity Hood When:

• Very high heat equipment (woks, salamanders)
• Space constraints limit hood height
• Energy performance important
• Capture at source critical

Exhaust Fan Selection and Sizing

Selecting the proper exhaust fan is critical for infrastructure performance. The fan must provide sufficient airflow at the required static stress while operating efficiently and reliably.

Fan Types

1. Centrifugal Fans (Squirrel Cage)

Characteristics:

• Performance: High static load capability (up to 10 inches w.g.)
• Effectiveness: 60-75% at design point
• Applications: Long duct runs, high static pressure value systems
• Advantages: Handles high static setup pressure, reliable, long service life
• Disadvantages: Higher initial cost, larger footprint, requires VFD for speed control

2. Axial Fans (Propeller)

Characteristics:

• Performance: Low to medium static power (up to 2 inches w.g.)
• Productivity: 40-60% at design point
• Applications: Short duct runs, low static force, roof-mounted
• Advantages: Lower cost, compact, direct drive
• Disadvantages: Limited static stress capability, higher noise levels

3. Mixed-Stream Fans

Characteristics:

• Performance: Medium static load (2-6 inches w.g.)
• Output ratio: 55-70% at design point
• Applications: Medium duct runs, balanced performance
• Advantages: Good balance of pressure value and electrical flow, compact
• Disadvantages: Higher cost than axial, lower arrangement pressure than centrifugal

4. Inline Fans

Characteristics:

• Performance: Medium static electrical power (2-4 inches w.g.)
• Yield: 50-65% at design point
• Applications: Duct-mounted installations, space-constrained
• Advantages: Compact, quiet operation, easy installation
• Disadvantages: Limited capacity, higher cost per CFM

Fan Sizing Calculations

1. Required Airflow

The fan must provide the calculated exhaust movement:

Where:

• $Q_{\text{fan}}$ = fan rated capacity (CFM or m³/h)
• $Q_{\text{exhaust}}$ = calculated exhaust circulation
• $F_{\text{safety}}$ = safety factor (typically 1.10-1.25)

2. Mechanism Static Force

Total static stress includes all installation losses:

Where:

• $SP_{\text{duct}}$ = duct friction losses
• $SP_{\text{filters}}$ = grease filter load drop (1.5-2.0 inches w.g.)
• $SP_{\text{fittings}}$ = fitting losses (elbows, transitions)
• $SP_{\text{hood}}$ = hood entry loss (0.1-0.3 inches w.g.)
• $SP_{\text{exit}}$ = exit velocity pressure value loss

Typical Equipment Static Pressures:

3. Fan Selection

Select fan from manufacturer performance curves:

• Fan must provide required CFM at calculated static wattage
• Select fan operating near peak performance point
• Avoid operating near fan stall region
• Consider VFD for variable speed control

Variable Frequency Drives (VFD)

VFDs allow fan speed adjustment to match actual exhaust requirements:

Benefits:

• Energy Savings: 30-50% reduction in energy consumption
• Demand Control: Match airflow to cooking activity
• Soft Start: Reduces motor wear and electrical demand
• Precise Control: Maintains desired airflow regardless of filter loading

Applications:

• Demand-control kitchen atmosphere circulation (DCKV)
• Multiple hood systems
• Energy-conscious designs
• Systems with varying loads

Control Strategies:

• Thermal value-Based: Adjust speed based on hood degree
• Timer-Based: Schedule-based operation
• Occupancy-Based: Operate only during occupied hours
• Combination: Multiple sensors for optimal control

Duct Design and Sizing

Proper duct design ensures efficient airflow transport while meeting code requirements for grease exhaust systems.

Duct Sizing Principles

1. Velocity-Based Sizing

Size ducts to maintain proper velocity:

Where:

• $A_{\text{duct}}$ = duct cross-sectional area (ft² or m²)
• $Q$ = exhaust stream rate (CFM or m³/s)
• $V$ = duct velocity (fpm or m/s)

Velocity Requirements (NFPA 96):

• Minimum: 500 fpm (2.5 m/s) - prevents grease settling
• Optimal: 1000-1200 fpm (5-6 m/s) - balances effectiveness and noise
• Maximum: 1500 fpm (7.6 m/s) - limits noise and erosion

2. Diameter Computation

For round ducts:

For rectangular ducts, maintain aspect ratio $\leq$ 4:1 for productivity.

Duct Construction Requirements

1. Material

Type I Hoods (Grease-Producing):

• Minimum: 16 gauge (1.6 mm) carbon steel
• Large Ducts: 14 gauge (1.9 mm) for ducts >75 inches diameter
• Welded Joints: All joints welded or brazed (no screws!)
• Coating: Galvanized or painted for corrosion protection

Type II Hoods (Non-Grease):

• Lighter materials acceptable (18-20 gauge)
• May use aluminum or stainless steel
• Less stringent construction requirements

2. Slope Requirements

Horizontal Ducts (NFPA 96 Section 8.2.3.2):

• Minimum Slope: 1/4 inch per foot (2% slope) toward hood
• Purpose: Allows grease drainage back to hood
• Critical: Prevents grease accumulation and fire hazard

3. Clearances

From Combustibles:

• Minimum: 18 inches (450 mm) clearance
• Protected: May reduce with approved fire protection
• Critical: Prevents fire spread from duct to building

4. Access Panels

Requirements:

• Maximum Spacing: Every 12 ft (3.6 m) along duct
• At Changes: Required at all direction changes
• Size: Minimum $12\,\text{inches} \times 12\,\text{inches}$ ($300\,\text{mm} \times 300\,\text{mm}$)
• Purpose: Allows inspection and cleaning

Duct Routing Considerations

1. Minimize Length

• Shortest practical route to outdoors
• Reduces static force losses
• Lowers installation cost

2. Minimize Fittings

• Each elbow adds 0.1-0.3 inches w.g. stress loss
• Use long-radius elbows ($R/D \geq 1.5$) when possible
• Avoid unnecessary transitions and changes

3. Vertical vs Horizontal

• Vertical: Preferred for grease exhaust (gravity assists drainage)
• Horizontal: Must slope toward hood for drainage
• Combination: Minimize horizontal runs

4. Termination

• Height: Minimum 40 inches (1.0 m) above roof
• Distance: Minimum 10 ft (3 m) from ventilation air intakes
• Property Line: Minimum 10 ft (3 m) from property lines
• Weather Protection: Use proper cap or weatherhead

Worked Examples

Example 1: Restaurant Kitchen (Wall-Mounted Hood)

Example 2: Fast Food Kitchen (Island Hood)

Design Guidelines

Duct Velocity

Maintain proper duct velocity to balance output ratio, noise, and grease transport:

NFPA 96 Requirements:

• Minimum: 500 fpm (2.5 m/s) - prevents grease settling
• Maximum: 1500 fpm (7.6 m/s) - limits noise and erosion
• Optimal: 1000-1200 fpm (5-6 m/s) - best balance

Hood Sizing

Horizontal Dimensions:

• Width: Extend minimum 6 inches (15 cm) beyond cooking surface on each side
• Preferred: 12 inches (30 cm) for improved capture
• Island hoods: 12-18 inches (30-45 cm) on all sides
• Purpose: Captures thermal plumes before they disperse

Vertical Clearance:

• Gas equipment: 30-36 inches (75-90 cm) above cooking surface
• Electric equipment: 24-30 inches (60-75 cm) above surface
• Minimum: 18 inches (45 cm) for fire safety (check manufacturer specs)
• Lower heights: Improve capture but must maintain fire clearances

Performance Impact:

• 12-inch overhang reduces required CFM by 20-30% vs 6-inch overhang
• Lower hood height improves capture performance
• High ceilings (>15 ft): Consider side panels or proximity hoods

Fire Safety

1. Grease Filters

Requirements:

• UL 1046 Listed: All Type I hoods require listed filters
• Types: Baffle filters (preferred) or mesh filters
• Mechanism pressure Drop: 1.5-2.0 inches w.g. when clean
• Installation: 18 inches minimum above cooking surface
• Maintenance: Clean weekly or as needed

Filter Selection:

• Baffle Filters: Better grease capture, higher load drop
• Mesh Filters: Easier cleaning, lower force drop
• Angle: Install at 45-60^$\circ$ slope toward grease collection

2. Fire Suppression Systems

Requirements (NFPA 96):

• UL 300 Listed: Automatic fire suppression installation required
• Agents: Wet chemical (K-class) for cooking oil fires
• Activation: Fusible links at 165-286°F (74-141°C)
• Manual Pull: Station within 10-20 ft of hood
• Shutoff: Automatic fuel/capacity shutoff on activation

Nozzle Placement:

• Spacing: 4-6 ft on center (per manufacturer)
• Coverage: Protect all cooking equipment under hood
• Duct Protection: Required for ducts >20 ft length

3. Duct Construction

Material Requirements:

• Type I: 16 gauge steel minimum (14 gauge for >75 inches)
• Joints: Welded or brazed (no screws!)
• Coating: Galvanized or painted for corrosion protection
• Clearance: 18 inches from combustibles unless protected

4. Clearances and Access

Clearances:

• From Combustibles: 18 inches minimum
• Protected: May reduce with approved fire protection
• Critical: Prevents fire spread

Access Panels:

• Spacing: Every 12 ft maximum along duct
• At Changes: Required at all direction changes
• Size: Minimum $12\,\text{inches} \times 12\,\text{inches}$
• Purpose: Inspection and cleaning access

Energy Efficiency Considerations

1. Demand-Control Airflow exchange (DCKV)

DCKV systems adjust exhaust amp based on cooking activity:

Benefits:

• Energy Savings: 30-50% reduction in exhaust and makeup atmosphere energy
• Comfort: Maintains proper airflow only when needed
• Compliance: Meets energy codes (California Title 24, NYC Local Law)

Control Methods:

• Heat level-Based: Adjusts based on hood temp
• Timer-Based: Schedule-based operation
• Occupancy-Based: Operates during occupied hours only
• Combination: Multiple sensors for optimal control

2. Variable Frequency Drives (VFD)

VFDs provide variable speed control for exhaust fans:

Benefits:

• Energy Savings: Fan energy proportional to speed³ (30-50% savings)
• Soft Start: Reduces motor wear and electrical demand
• Precise Control: Maintains desired airflow regardless of filter loading

3. Energy Recovery

Heat Recovery Ventilators (HRV):

• Recovers sensible heat from exhaust ventilation air
• Effectiveness: 70-85% sensible recovery
• Applications: Cold climates, balanced systems

Energy Recovery Ventilators (ERV):

• Recovers sensible and latent heat
• Productivity: 60-80% total recovery
• Applications: Moderate climates, humidity control needed

4. Makeup Fresh air Optimization

• Condition makeup air supply to minimize furnace system/temperature control loads
• Use energy recovery when possible
• Locate diffusers properly to avoid disrupting capture
• Balance supply and exhaust to maintain building stress

Troubleshooting Common Problems

Problem: Insufficient Capture (Smoke/Grease Escape)

Symptoms:

• Visible smoke or grease escaping hood
• Poor indoor airflow quality
• Grease accumulation on walls and ceilings
• Odors migrating to adjacent spaces

Causes and Solutions:

1. Undersized Exhaust Movement

• Cause: Exhaust CFM too low for cooking load
• Solution: Increase exhaust circulation, verify against NFPA 96 minimums
• Check: Measure required CFM with all correction factors

2. Inadequate Capture Velocity

• Cause: Capture velocity too low at hood edge
• Solution: Increase exhaust flow rate or reduce hood height
• Check: Measure velocity at hood edge, compare to requirements

3. Hood Height Too High

• Cause: Hood mounted too far above cooking surface
• Solution: Lower hood height (maintain fire clearances) or increase exhaust
• Check: Verify height is within recommended range

4. Insufficient Overhang

• Cause: Hood doesn't extend far enough beyond cooking surface
• Solution: Increase overhang to minimum 6 inches (12 inches preferred)
• Check: Measure overhang on all open sides

5. Cross-Drafts

• Cause: Atmosphere movement disrupting capture patterns
• Solution: Relocate makeup ventilation air diffusers, add side panels, increase exhaust
• Check: Identify sources of fresh air movement (doors, windows, HVAC)

6. Makeup Air supply Too Close

• Cause: Makeup airflow diffusers <10 ft from hood
• Solution: Relocate diffusers minimum 10 ft from hood edges
• Check: Measure distance from hood to nearest diffuser

Problem: Excessive Energy Consumption

Symptoms:

• High utility bills
• Overheating or overcooling of kitchen
• Fan running continuously at full speed

Causes and Solutions:

1. Oversized Equipment

• Cause: Infrastructure sized larger than needed
• Solution: Implement demand-control atmosphere movement (DCKV)
• Check: Compare actual usage to design capacity

2. No Variable Speed Control

• Cause: Fan operates at fixed speed regardless of need
• Solution: Install VFD for variable speed control
• Check: Evaluate fan control strategy

3. Inefficient Makeup Ventilation air

• Cause: Makeup fresh air not conditioned or no energy recovery
• Solution: Add heater/air conditioning to makeup air supply, consider energy recovery
• Check: Evaluate makeup airflow conditioning setup

4. Poor Arrangement Balance

• Cause: Makeup atmosphere and exhaust not balanced
• Solution: Balance airflows, verify building load
• Check: Measure building pressure value, adjust supply/exhaust

Problem: High Noise Levels

Symptoms:

• Excessive fan noise
• Vibration in ductwork
• Complaints from occupants

Causes and Solutions:

1. Excessive Duct Velocity

• Cause: Duct velocity >1500 fpm
• Solution: Increase duct size to reduce velocity
• Check: Assess actual duct velocity

2. Fan Selection

• Cause: Wrong fan type or size
• Solution: Select quieter fan (centrifugal vs axial), verify fan selection
• Check: Review fan performance curves and noise ratings

3. Duct Vibration

• Cause: Loose connections, inadequate support
• Solution: Secure all connections, add vibration isolators
• Check: Inspect ductwork for loose connections

4. No Vibration Isolation

• Cause: Fan directly connected to ductwork
• Solution: Install vibration isolators between fan and duct
• Check: Verify vibration isolation installation

Problem: Grease Accumulation

Symptoms:

• Grease buildup in ductwork
• Clogged filters
• Fire hazard concerns

Causes and Solutions:

1. Insufficient Duct Velocity

• Cause: Duct velocity <500 fpm
• Solution: Increase exhaust discharge or reduce duct size
• Check: Verify duct velocity meets NFPA 96 minimum

2. No Duct Slope

• Cause: Horizontal ducts not sloped toward hood
• Solution: Install ducts with 1/4 inch per foot slope
• Check: Verify slope on all horizontal runs

3. Infrequent Cleaning

• Cause: Cleaning schedule not followed
• Solution: Establish cleaning schedule per NFPA 96, maintain records
• Check: Review cleaning records, verify compliance

4. Inadequate Filters

• Cause: Filters not capturing grease effectively
• Solution: Replace with proper filters (baffle type preferred)
• Check: Verify filter type and condition

Problem: Negative Building Pressure

Symptoms:

• Difficulty opening exterior doors
• Backdrafting of combustion appliances
• Uncomfortable drafts
• Reduced exhaust fan performance

Causes and Solutions:

1. Inadequate Makeup Ventilation air

• Cause: Makeup fresh air <80% of exhaust
• Solution: Increase makeup air supply to 80-100% of exhaust
• Check: Measure makeup airflow stream, compare to exhaust

2. Makeup Atmosphere Not Operating

• Cause: Makeup ventilation air mechanism not running or failed
• Solution: Verify makeup fresh air operation, repair as needed
• Check: Test makeup air supply installation operation

3. Other Exhaust Systems

• Cause: Other exhaust systems (restrooms, general) not accounted for
• Solution: Balance all exhaust with total supply
• Check: Inventory all exhaust systems in building

4. Building Envelope Leakage

• Cause: Excessive infiltration/exfiltration
• Solution: Seal building envelope, balance mechanical systems
• Check: Measure building equipment pressure, identify leakage points

Standards and References

• DIN 18869: Airflow supply systems for commercial kitchens
• NFPA 96: Standard for Atmosphere circulation Control and Fire Protection
• ASHRAE Handbook - Applications: Chapter 33 - Kitchen Ventilation air exchange

Conclusion

Proper design of kitchen hood exhaust systems is essential for safety, comfort, and compliance. By calculating exhaust flow rates and makeup air requirements accurately, engineers can design effective systems that meet all standards while ensuring fire safety and maintaining acceptable indoor air quality.

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Key Takeaways

Further Learning

• Fresh Air Flow Guide - ASHRAE 62.1 ventilation rate calculations
• Duct Sizing Guide - Sizing exhaust ductwork
• Duct Pressure Loss Guide - Calculating fan static pressure
• Kitchen Hood Calculator - Interactive calculator for kitchen hood sizing

References & Standards

Primary Standards

NFPA 96 Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations. Requires Type I hoods (grease-producing) to have minimum 200 CFM/ft², duct velocity 500-1500 fpm, grease filters, and fire suppression systems. Specifies duct construction, slope requirements, and maintenance schedules.

IMC Section 508 International Mechanical Code requirements for makeup air. Mandates makeup air for exhaust systems exceeding 400 CFM, typically 80-100% of exhaust volume to prevent negative building pressure.

ASHRAE Applications Handbook Chapter 33: Kitchen Ventilation. Provides comprehensive guidance on kitchen ventilation design, exhaust flow calculations, capture velocity requirements, and system design principles.

Supporting Standards & Guidelines

DIN 18869 Ventilation systems for commercial kitchens. Provides European standards for kitchen ventilation system design and performance requirements.

SMACNA HVAC Systems Duct Design Manual Industry-standard duct design manual for sizing and construction practices. Provides detailed guidance on duct sizing, pressure loss calculations, and construction requirements.

Further Reading

• ASHRAE Technical Resources - American Society of Heating, Refrigerating and Air-Conditioning Engineers resources
• UL 300 - Fire suppression system requirements for commercial cooking equipment

Note: Standards and codes are regularly updated. Always verify you're using the current adopted edition applicable to your project's location. Consult with local authorities having jurisdiction (AHJ) for specific requirements.

Disclaimer: This guide provides general technical information based on international ventilation standards. Always verify calculations with applicable local codes and consult licensed professionals for actual installations. Ventilation system design should only be performed by qualified professionals. Component ratings and specifications may vary by manufacturer.