Kitchen Hood Calculator

Commercial kitchen exhaust hood systems are critical life-safety and environmental control systems capturing and removing heat, smoke, grease-laden vapors, and combustion products from cooking appliances. These systems protect building occupants from fire hazards, maintain acceptable indoor air quality, prevent grease accumulation in ductwork (which can ignite causing catastrophic fires), and provide thermal comfort for kitchen workers. Proper design, installation, and maintenance are mandated by International Mechanical Code (IMC) and NFPA 96 "Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations."

Hood Classification and Types: Type I hoods serve grease-producing cooking appliances (ranges, griddles, broilers, fryers) and must include grease filters (baffle or mesh style), automatic fire suppression systems (UL 300 wet chemical systems), and stainless steel or black steel ductwork with continuous welded seams. Type II hoods serve non-grease-producing appliances generating only heat and moisture (steamers, dishwashers, pasta cookers), requiring no grease filters or fire suppression with galvanized steel ductwork acceptable. Misclassifying hoods (using Type II for grease-producing equipment) dramatically increases fire risk and violates codes.

Hood Configurations: Wall-mounted canopy hoods (most common and economical) attach to walls with appliances in line. Island canopy hoods suspend from ceiling over island cooking stations requiring 20-25% higher exhaust rates (thermal plumes less contained). Backshelf/proximity hoods mount very close to cooking surfaces (150-300mm above) using lower exhaust rates suitable for low-ceiling applications. Proper sizing requires overhanging appliances minimum 150mm (6 inches) on all open sides per NFPA 96—undersized hoods allow grease-laden vapors to escape, depositing grease on ceilings and walls.

Exhaust Flow Rate Determination: Three methodologies codified in IMC and ASHRAE 154: (1) Area method (IMC Table 507.2.2) specifies CFM/ft² based on appliance duty—light duty 200 CFM/ft² wall (300 island), medium duty 300 wall (400 island), heavy duty 400 wall (500 island), extra-heavy duty 550 wall (700 island). (2) Perimeter method calculates 150-250 CFM per linear foot of hood perimeter. (3) Heat input method uses 100-150 CFM per 10,000 BTU/hr for gas appliances. Area method most commonly used for code compliance and permits.

Makeup Air Requirements: IMC Section 508 requires makeup air for exhaust systems exceeding 400 CFM, with capacity equal to exhaust minus lesser of 10% exhaust or 300 CFM. Example: 5,000 CFM exhaust requires 4,700 CFM minimum makeup air. Insufficient makeup air causes difficulty opening doors (negative pressure >12.5 Pa), combustion appliance backdrafting (carbon monoxide hazard), inadequate hood performance (grease spillage), and uncontrolled infiltration. Makeup air provided via direct-fired units (adds moisture), indirect gas/electric units (conditioned air, higher efficiency), or transfer air from adjacent spaces. ASHRAE 90.1 encourages energy recovery achieving 35-55% energy recovery.

Fire Suppression and Grease Removal: UL-listed automatic fire suppression systems (UL 300 wet chemical) mandatory for Type I hoods per NFPA 96 Section 10, consisting of fusible link detection (79-82°C), pressurized agent storage (potassium carbonate/acetate), distribution piping and nozzles, manual pull stations, and automatic fuel shutoff. System requires one nozzle per appliance or per 1.2m hood length. Baffle grease filters (standard method) capture 70-80% grease at 200-300 FPM face velocity requiring regular cleaning (daily to weekly). Electrostatic precipitators achieve 85-95% grease removal efficiency, extending cleaning intervals from semi-annual to annual.

Energy Efficiency and Code Compliance: ASHRAE 90.1 Section 6.5.7 requires demand-controlled kitchen ventilation (DCKV) using variable-speed exhaust fans modulating based on cooking activity (temperature sensors, optical sensors, time-based controls), saving 30-50% fan energy. Additional strategies include transfer air from dining areas, energy recovery on makeup air, low-velocity displacement makeup air, and high-efficiency EC motor fans reducing consumption 20-40%. NFPA 96 Section 11 requires professional cleaning on schedules based on cooking volume (monthly for high-volume heavy-duty, quarterly moderate, semi-annually light). Cleaning removes accumulated grease (fire risk), with NFPA 96 requiring cleaning when accumulation exceeds 1.6mm.

Standards Reference: NFPA 96 governs commercial cooking ventilation control and fire protection. IMC Sections 507-508 specify hood classification, exhaust rates, and makeup air requirements. ASHRAE 154 provides ventilation design methodology. UL 300 establishes fire suppression system testing standards. ASHRAE 90.1 Section 6.5.7 mandates demand-controlled ventilation for energy efficiency.