Calculating the correct kitchen duct size is the single most critical step in designing an effective ventilation system. An undersized duct creates excessive noise and restricts airflow, while an oversized duct leads to dangerously low airflow velocities that fail to capture grease. Getting this specification right ensures your range hood performs to its rated capacity, keeping your kitchen comfortable and protecting your cabinetry from grease buildup.
Understanding Airflow Velocity
The primary goal of kitchen duct size is to maintain optimal air velocity within the ductwork. Industry standards dictate that the ideal velocity for greasy kitchen air is between 600 and 900 feet per minute (FPM). If the air moves too slowly, grease particles drop out of the airstream and settle inside the duct, creating a severe fire hazard. Conversely, if the velocity is too high, the system becomes obnoxiously loud and can cause excessive duct turbulence, which reduces the hood’s capture efficiency.
The Role of the Hood and Duct
It is essential to distinguish between the capabilities of the range hood and the ducting that carries the air away. The hood is responsible for capturing the rising smoke and grease at the source, generating a specific Cubic Feet per Minute (CFM) rating. The duct system is the pathway that must transport that specific volume of air outside without creating excessive static pressure. Choosing a duct that is too small for the hood’s CFM rating starves the system, causing the hood to underperform and the motor to strain.
Calculating Total Equivalent Length (TEL)
Professional installers rarely rely on a simple measurement of the wall-to-roof distance. They calculate the Total Equivalent Length (TEL), which factors in the resistance caused by every inch of duct run. Every 90-degree elbow adds the equivalent length of several feet of straight duct, as do roof caps, louvers, and transitions. Ignoring TEL results in picking a duct that is physically the correct length but functionally too restrictive, leading to poor performance and excessive noise from the fan.
| Duct Diameter (inches) | Recommended Maximum CFM | Common Use Case |
|---|---|---|
| 4 | 200 – 300 | Small galley or under-counter units |
| 5 | 300 – 400 | Typical apartment or small kitchen |
| 6 | 400 – 600 | Standard home primary kitchen |
| 7 | 600 – 800 | Large kitchens or heavy commercial use |
| 8 | 800 – 1200+ | High-end restaurant or very large residential island |
Material Matters: Rigid vs. Flexible
The physical composition of the duct impacts the internal diameter and, consequently, the airflow. Rigid metal duct maintains its shape perfectly, ensuring the calculated diameter is exactly what the air travels through. Flexible foil ducting, while easier to install for complex runs, often crimps at the seams during installation. This crimping effectively reduces the internal diameter, creating an unseen bottleneck that restricts airflow and increases turbulence regardless of what the nominal size label claims.
The Vent Cap Factor
Once the duct reaches the exterior wall, the transition to the vent cap introduces another critical diameter consideration. The end cap, or wall flash, must be at least as large in diameter as the duct itself. A common mistake is installing a 6-inch duct that terminates in a 4-inch or 5-inch wall cap. This mismatch creates an immediate choke point that drastically reduces the system’s ability to exhaust air, regardless of how powerful the range hood motor is.
Sealing and Insulation
Beyond size, the method of joining duct sections is vital for maintaining velocity. Joints should be sealed with foil tape (not cloth duct tape, which deteriorates) to prevent air from leaking into the cavity. Furthermore, insulated duct is necessary in unconditionated spaces like attics to prevent condensation. However, the insulation lining adds a slight internal roughness; choosing insulation that is smooth-backed and maintaining the same effective diameter is crucial to preventing friction that slows the air down.
Final Recommendations
When selecting kitchen duct size, prioritize matching the duct diameter to the highest anticipated load of your appliances. If you plan to use a commercial-style range with high BTHs, opting for a 7-inch system even in a standard home future-proofs your investment. Always minimize bends, use smooth-wall metal whenever possible, and ensure the termination cap is an equal or larger diameter to guarantee the air you cook with is expelled safely and efficiently.
