How the Vent Hood & Makeup Air Calculator Works
Calculate the required total exhaust CFM (Cubic Feet per Minute) for your commercial kitchen hood system and the corresponding Makeup Air (MUA) needs, based on hood type and linear length following NFPA 96 and ASHRAE guidelines.
Formula:
Exhaust CFM = Hood Length (ft) × Duty Factor (CFM/ft)
Makeup Air (MUA) CFM = Total Exhaust CFM × MUA Percentage
Net Exhaust = Total Exhaust CFM - MUA CFMDuty Factors (CFM per linear foot)
| Duty Type | CFM/ft | Typical Equipment |
|---|---|---|
| Light Duty | 150 CFM/ft | Ovens, Steamers, Kettles |
| Medium Duty | 300 CFM/ft | Range, Fryer, Griddle |
| Heavy Duty | 450 CFM/ft | Charbroiler, Wok, Salamander |
Makeup Air
Makeup air replaces the air exhausted by the hood system. Typically designed at 80-90% of exhaust volume to maintain slight negative pressure in the kitchen. The remaining 10-20% is drawn from adjacent spaces (transfer air).
Formula:
Exhaust CFM = Hood Length (ft) × Duty Factor (CFM/ft)
Makeup Air (MUA) CFM = Total Exhaust CFM × MUA Percentage
Net Exhaust = Total Exhaust CFM - MUA CFMDuty Factors (CFM per linear foot)
| Duty Type | CFM/ft | Typical Equipment |
|---|---|---|
| Light Duty | 150 CFM/ft | Ovens, Steamers, Kettles |
| Medium Duty | 300 CFM/ft | Range, Fryer, Griddle |
| Heavy Duty | 450 CFM/ft | Charbroiler, Wok, Salamander |
Makeup Air
Makeup air replaces the air exhausted by the hood system. Typically designed at 80-90% of exhaust volume to maintain slight negative pressure in the kitchen. The remaining 10-20% is drawn from adjacent spaces (transfer air).
FAQ
Q: What is makeup air and why is it needed?
A: Makeup air replaces the air exhausted by the hood system. Without it, the kitchen would be under negative pressure, causing backdrafting and comfort issues.
Q: What is the difference between Type I and Type II hoods?
A: Type I hoods handle grease-laden vapors from cooking. Type II handles heat, steam, and odors only. This calculator is for Type I hoods.
Q: What CFM do I need for a 10-foot hood?
A: For medium-duty cooking (range/fryer), 10 ft × 300 CFM/ft = 3,000 CFM minimum exhaust.
References
- NFPA 96: Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations
- ASHRAE Standard 154: Ventilation for Commercial Cooking Operations
- International Mechanical Code (IMC) — Chapter 5: Exhaust Systems
- Local building and fire codes