Ductwork specification is where ventilation projects most frequently encounter field problems — wrong clearance assumptions, undersized diameters, and non-listed components that fail inspection. NAKS factory-built round grease duct is UL listed under UL1978 and UL2221, requires no field welding, and is available in configurations covering clearance-to-combustibles requirements from 18 inches down to zero.
This guide covers what resellers and installers need to know to specify the right product for every project.
Why UL Listing Matters for Grease Duct
UL listed grease duct is not a premium option — it is the code baseline for commercial kitchen exhaust duct in most jurisdictions. Field-fabricated alternatives that do not carry UL listing create inspection risk and long-term liability.
- NAKS round ductwork is listed under UL1978 (single wall and double wall reduced clearance) and UL2221 (double wall zero clearance)
- UL listing verifies that the duct meets the material, construction, and performance standards required for grease-laden exhaust applications
- Non-listed duct may not be accepted by the AHJ regardless of material quality
- Factory-built UL listed duct eliminates the field welding requirement, reducing installation labor and inspection risk
Always confirm AHJ requirements for duct listing before specifying any grease duct product.
The Three NAKS Round Duct Configurations
NAKS round grease duct is available in three configurations, each designed for a specific clearance-to-combustibles condition.
Single Wall:
- Inner diameter range: 10 to 22 inches
- Clearance to combustibles: 18 inches required
- Clearance to non-combustibles: 0 inches
- Total footprint: OD plus clearance
Double Wall Reduced Clearance (10 to 14 inch ID):
- Clearance to combustibles: 2 inches required
- Clearance to non-combustibles: 0 inches
Double Wall Reduced Clearance (16 to 22 inch ID):
- Clearance to combustibles: 3 inches required
- Clearance to non-combustibles: 0 inches
Double Wall Zero Clearance:
- Inner diameter range: 10 to 22 inches
- Clearance to combustibles: 0 inches required
- Clearance to non-combustibles: 0 inches
- Total footprint: OD plus clearance
Zero clearance duct is the specification solution for retrofit and low-ceiling applications where standard clearance distances cannot be achieved.
Duct Diameter Sizing by Hood Length and CFM
Duct diameter must be sized to handle the calculated CFM volume without creating airflow restriction. NAKS provides sizing charts for both medium duty (200 CFM per linear foot) and heavy duty (250 CFM per linear foot) applications.
Medium duty sizing examples (200 CFM per linear foot):
- 3 to 5.5 foot hood (600 to 1,075 CFM): 10-inch duct diameter
- 7.5 to 9.11 foot hood (1,500 to 1,975 CFM): 14-inch duct diameter
- 15.6 to 20 foot hood (3,100 to 4,000 CFM): 20-inch duct diameter
Heavy duty sizing examples (250 CFM per linear foot):
- 3 to 5.5 foot hood (750 to 1,125 CFM): 10-inch duct diameter
- 8 to 9.11 foot hood (2,000 to 2,475 CFM): 16-inch duct diameter
- 15.6 to 20 foot hood (3,875 to 5,000 CFM): 22-inch duct diameter
Always refer to the full NAKS duct diameter sizing chart by hood size and CFM for complete specifications.
Common Ductwork Specification Errors
The most frequent ductwork errors are clearance misassumptions and diameter undersizing — both of which create field problems that are expensive to correct after installation.
- Specifying single wall duct in applications where 18 inches of clearance to combustibles is not available
- Undersizing duct diameter relative to calculated CFM, creating airflow restriction that degrades system performance
- Failing to account for duct connection diameter at the hood collar when sizing field connections
- Using non-listed duct in jurisdictions that require UL listing for grease duct applications
Verify available clearance in the duct path and confirm calculated CFM before finalizing duct configuration and diameter.
FAQ: Round Ductwork Specification
Q: Does NAKS round ductwork require field welding?
A: No. NAKS factory-built round grease duct requires no field welding, reducing installation labor and inspection complexity.
Q: What is the difference between reduced clearance and zero clearance duct?
A: Reduced clearance duct requires 2 to 3 inches of clearance to combustible materials depending on diameter. Zero clearance duct requires no clearance to combustibles or non-combustibles, making it the appropriate specification for space-constrained installations.
Q: How do I determine whether a single wall or double wall duct is required?
A: Measure available clearance between the duct centerline and any combustible material in the duct path. If 18 inches of clearance is not achievable, double wall reduced or zero clearance duct is required.
Q: Does the duct diameter affect the hood connection diameter?
A: Yes. Hood connection diameter is specified separately from duct diameter and must be confirmed against the hood collar size. Refer to the NAKS sizing chart for hood connection diameter by hood length and CFM.
Ductwork Specification Support from NAKS
NAKS supports resellers and installers with ductwork product selection, sizing guidance, and technical documentation across all project types and clearance conditions. Get a free quote today.
