Whether you're roughing in a new residential system or retrofitting commercial ductwork, choosing the right fittings determines airflow performance, installation speed, and system longevity. This guide covers every major fitting type, when to use each one, and how to size them correctly.
The backbone of every system. Rectangular or round, straight duct carries conditioned air from the air handler to branch takeoffs. Available in lengths from 6” to 120” (10 feet), with widths and heights from 2” to 48”.
Pro tip: For runs over 10 feet, use slip connections on one end and drive or TDC on the other for easy field assembly with minimal leakage.
Change direction by 90°. The throat radius is the single most important dimension—a tight throat creates turbulence and pressure drop, while a generous radius maintains smooth airflow. SMACNA recommends a throat radius of at least the duct width.
Typical use: Turning trunk lines around corners, dropping from ceiling plenums to wall cavities.
Change duct size while maintaining rectangular cross-section. Transitions connect a larger trunk to a smaller continuation after a branch takeoff. The taper angle should not exceed 15° per side to avoid separation and turbulence.
Typical use: Trunk line step-downs after each branch, air handler connections.
Similar to transitions but optimized specifically for narrowing—larger input to smaller output. The key difference is that reducers can have asymmetric tapers (one side flat, one side angled) for space-constrained installations.
Typical use: Stepping down trunk lines, connecting to equipment with fixed collar sizes.
Branch fittings that split airflow from a trunk line into a perpendicular branch. The trunk continues straight through while the branch exits at 90°. Size the branch based on CFM requirements for the zone it serves.
Typical use: Every branch takeoff in a trunk-and-branch system.
Like tees, but the branch exits at 30-45° instead of 90°. This angled takeoff significantly reduces pressure drop—a wye fitting can have 40-60% less pressure loss than a tee at the same airflow. The tradeoff is they require more vertical clearance.
Typical use: High-performance systems, commercial applications where energy efficiency matters.
Four-way distribution with branches on opposite sides of the trunk. Less common than tees but invaluable when the layout calls for branches in both directions from a central trunk.
Typical use: Symmetrical layouts, corridor distribution in commercial buildings.
S-shaped fittings that shift the duct path laterally while maintaining the same cross-section. Essential for routing around structural elements—beams, pipes, electrical conduit—without changing duct size.
Typical use: Navigating around obstructions in retrofits and tight mechanical rooms.
Convert from rectangular trunk lines to round branch duct. Critical for systems that use rectangular trunks with round flexible or spiral branch runs.
Typical use: Connecting rectangular trunk lines to round flex duct runs in residential systems.
Spiral or snap-lock round sections. Round duct is aerodynamically superior to rectangular—less surface area for the same cross-sectional area means less friction, less leakage, and less material cost.
Typical use: Branch runs, exposed architectural ductwork, industrial exhaust.
Connect floor or wall return grilles to the return trunk. Available with optional filter slots (for point-of-return filtration) and turning vanes (to reduce turbulence at the 90° turn).
Typical use: Every return air location in a residential forced-air system.
Seal the end of a trunk line or unused duct opening. Simple but essential—an uncapped duct is a direct conditioned air leak into unconditioned space.
Typical use: Trunk line terminations, abandoned branch connections during renovations.
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