SMACNA Standards

SMACNA (Sheet Metal and Air Conditioning Contractors’ National Association) standards establish the foundational design criteria, fabrication methods, and quality benchmarks for HVAC ductwork systems. These consensus-based documents synthesize engineering principles with practical construction knowledge to provide comprehensive technical guidance for the sheet metal industry.

HVAC Duct Construction Standards - Metal and Flexible

The HVAC Duct Construction Standards - Metal and Flexible represents the primary reference for commercial and residential ductwork design and construction. This document correlates duct pressure classifications, material gauges, reinforcement requirements, and joint specifications into systematic selection tables.

Pressure-based design tables eliminate repetitive structural calculations by providing pre-engineered solutions for common duct configurations. Engineers select appropriate construction class based on system operating pressure, duct dimensions, and application requirements, with the tables specifying gauge, reinforcement spacing, and joint methods.

Seal class and leakage class specifications establish systematic sealing requirements correlated with performance objectives. The standards define three seal classes (A, B, C) and four leakage classes (3, 6, 12, 30 CFM per 100 sq ft), providing clear criteria for specification and verification.

Hangers and support spacing requirements ensure structural adequacy while preventing excessive duct deflection. Maximum support spacing varies with duct size, gauge, and orientation, typically ranging from 8 to 12 feet for horizontal runs and 12 to 16 feet for vertical risers.

Flexible duct construction standards address installation practices that significantly impact system performance. Proper installation requires stretched-tight configuration, support at maximum 4-foot spacing, and limited run lengths typically not exceeding 5 to 10 feet per manufacturer recommendations.

Rectangular Industrial Duct Construction Standard

Industrial duct construction standards address heavy-duty applications including process ventilation, industrial exhaust, and material handling systems. These systems typically operate at higher pressures, temperatures, and material loading compared to commercial HVAC applications.

Material thickness requirements increase substantially for industrial applications, with minimum gauge often starting at 20 or 18 gauge even for moderate-sized ducts. Reinforcement spacing decreases to accommodate higher pressures and provide resistance to mechanical abuse common in industrial environments.

Welded construction frequently replaces mechanical joints in industrial systems, providing maximum strength and sealing integrity for high-pressure or high-temperature applications. Welding procedures must follow AWS (American Welding Society) standards to ensure structural adequacy and quality control.

Access door sizing and frequency increase for industrial systems to facilitate inspection and cleaning. Doors typically incorporate heavy-duty latches, continuous gaskets, and reinforced construction to withstand system operating pressures and mechanical loading.

Round Industrial Duct Construction Standard

Round industrial ductwork leverages circular geometry to achieve superior strength-to-weight ratios for high-pressure and large-diameter applications. Spiral lockseam construction provides economical fabrication while maintaining structural integrity up to 10 inches water column.

Diameter limitations increase for industrial round duct compared to commercial applications, with standard spiral duct available in diameters up to 72 inches or larger through specialized fabrication. Large-diameter ducts may incorporate thicker gauge material or circumferential reinforcement rings to maintain roundness under operating loads.

Flanged connections for industrial round duct utilize heavier angle sections and closer bolt spacing compared to commercial construction. Van Stone flanges (rotating loose flanges) facilitate field alignment while providing robust connections for large-diameter, high-pressure systems.

Fibrous Glass Duct Construction Standard

Fibrous glass duct board construction standards establish fabrication methods, closure systems, and performance limitations for rigid fiberglass ductwork. This material provides combined thermal and acoustic insulation while serving as the structural duct element.

Maximum velocity limitations typically range from 2,500 to 4,000 feet per minute (fpm) depending on product density and reinforcement configuration. Higher velocities risk erosion of the glass fiber matrix, particularly at elbows and other high-turbulence locations.

Maximum static pressure ratings generally limit to 2 to 4 inches water column for standard duct board products. Higher pressure applications require upgraded products with reinforced facings or transition to alternative duct materials.

Closure systems utilize outward-clinching staples that penetrate the duct board without creating interior protrusions that could erode or dislodge. Closure staples must fully engage the duct board thickness while maintaining facing integrity. Pressure-sensitive tape applied over all joints and seams provides air sealing and additional structural reinforcement.

Thermoplastic Duct Construction Standard

Thermoplastic duct standards address PVC, CPVC, polypropylene, and other plastic duct materials suited for corrosive exhaust applications. These materials resist chemical attack from acids, bases, and organic solvents that rapidly degrade metallic ductwork.

Temperature limitations constrain thermoplastic duct applications, with maximum continuous operating temperatures ranging from 140°F for PVC to 210°F for polypropylene. Specialized high-temperature plastics extend these limits but at substantial material cost premiums.

Solvent welding provides the primary joining method for thermoplastic duct, creating homogeneous joints that achieve strength equivalent to the parent material. Proper solvent selection, surface preparation, and cure time prove critical for reliable joint performance.

Support requirements for thermoplastic duct account for reduced material stiffness and potential creep deformation under sustained loading. Support spacing typically ranges from 3 to 6 feet depending on diameter, material, and operating temperature.

Duct System Leakage Test Manual

The Leakage Test Manual establishes standardized procedures for quantifying installed duct system air leakage. Test methods include pressure decay testing and direct airflow measurement, each suited to specific system configurations and verification objectives.

Pressure decay testing involves pressurizing isolated duct sections and monitoring static pressure reduction over a specified time period. The pressure decay rate correlates with system leakage, allowing calculation of equivalent leakage area or airflow at standard pressure.

Test pressure selection typically equals 1.25 times the maximum design operating pressure, ensuring verification under conditions exceeding normal operation. Duct sections must be structurally adequate to withstand test pressure without permanent deformation.

IAQ Guidelines for Occupied Buildings Under Construction

Indoor air quality guidelines for construction projects address contamination prevention during HVAC system installation and building construction. These practices prevent construction-related contaminants from entering ductwork and subsequently impacting building occupants.

Duct protection during construction includes sealing all duct openings with approved barriers that prevent entry of construction dust, moisture, and debris. End caps, plastic sheeting, or temporary closure plates provide effective protection when properly installed and maintained.

Filter replacement protocols require changing all HVAC filters immediately before building occupancy to remove construction-generated particulate accumulated during commissioning and construction completion phases. Some specifications mandate multiple filter changes during extended construction periods.

Duct cleaning procedures address contamination that occurs despite protective measures. Post-construction cleaning typically employs mechanical brushing, negative pressure collection, or specialized equipment certified under NADCA (National Air Duct Cleaners Association) standards.