A duct tee is a T-shaped fitting used in HVAC ductwork systems to create a branch connection from a main trunk line, splitting or combining airflow between three duct sections. The fitting allows conditioned air to be directed from a primary supply duct into secondary branch runs that serve individual zones, rooms, or terminal devices. Duct tees are among the most common fittings in forced-air distribution systems and play a critical role in maintaining proper airflow balance throughout a building.
Technical Details
A standard duct tee features a branch outlet set at 90 degrees to the main duct, though custom-angle configurations are available for specialized layouts. The fitting introduces a measurable pressure drop that varies based on the ratio of branch airflow to main duct airflow and the overall geometry of the fitting. Loss coefficients (C-values) for duct tees typically range from 0.2 to over 1.0, with lower values corresponding to more favorable velocity and airflow ratios.
Key specifications include:
- Branch angle: 90 degrees is standard; angled variants reduce turbulence and pressure loss.
- Materials: Galvanized steel (most common), aluminum, stainless steel, and fiberglass duct board.
- Sizing: Available in round, rectangular, and oval configurations, with branch diameters or dimensions typically equal to or smaller than the main duct.
- Connection types: Slip-fit, flanged (TDC/TDF), or raw-edge for field modification.
Turning vanes or radius-entry branch designs can be incorporated into duct tees to reduce turbulence and lower the associated pressure penalty, particularly at higher velocities.
Applications
Duct tees are used in supply air distribution, return air collection, and exhaust systems across residential, commercial, and industrial HVAC installations. In a typical trunk-and-branch layout, tees allow the main supply trunk to feed multiple branch ducts that deliver conditioned air to diffusers and registers. They are also used in return duct systems to merge airflow from multiple zones back to the air handling unit. Proper sizing and placement of duct tees directly affects CFM balancing, occupant comfort, and overall system efficiency.
Related Standards and Codes
Duct tee design, fabrication, and installation are governed by several industry standards:
- SMACNA (Sheet Metal and Air Conditioning Contractors’ National Association): Provides construction standards for duct fittings, including material gauges, seam types, and reinforcement requirements.
- ASHRAE Fundamentals Handbook: Publishes loss coefficient data for various tee configurations used in duct pressure loss calculations.
- NFPA 90A and 90B: Establish fire safety requirements for duct materials and installations in commercial and residential buildings.
- UL 181: Covers factory-made air ducts and connectors, including closure systems for duct fittings.
Practical Significance
Selecting the correct duct tee size and configuration is essential for maintaining design airflow rates and minimizing energy waste. An improperly sized or poorly installed tee can create excessive pressure drop, generate noise from turbulence, and cause uneven air distribution across zones. During system commissioning, technicians measure airflow at branch outlets downstream of tee fittings and adjust dampers as needed to achieve balanced delivery. Specifying low-loss tee designs, such as conical or radius-entry branches, can reduce fan energy consumption by 5% to 15% in systems with numerous branch takeoffs.