Duct fittings are the components that connect, redirect, branch, and transition ductwork in an HVAC system. A typical residential system uses 15-30 fittings; a commercial system may use hundreds. Every fitting creates some pressure drop — and the cumulative effect determines whether the system delivers its rated airflow or falls short.
Understanding fitting types and their pressure drop characteristics is essential for sizing duct systems correctly. This guide covers every common fitting, explains the pressure drop implications, and gives you practical rules for selecting the right fitting at each point in the system.
Elbows (Direction Changes)
Elbows change the direction of duct runs. They are the most common fitting and typically the largest source of pressure loss after the duct length itself.
90° Elbow
The standard direction change fitting. Available in three configurations:
Smooth radius (long-radius): The centerline radius equals 1.5 times the duct diameter (1.5D). This is the best-performing 90° elbow — smooth airflow, low turbulence, equivalent to 5-8 feet of straight duct. Use wherever space allows.
Short radius: Centerline radius equals 1.0D. More compact than long-radius but higher pressure drop — equivalent to 10-12 feet of straight duct. Use when ceiling clearance is limited.
Mitered (square throat): Sharp 90° turn made from flat sheet metal pieces. Highest pressure drop — equivalent to 15-20 feet. Generally avoided except in rectangular duct where curved elbows are impractical. Adding turning vanes inside a mitered elbow reduces the equivalent length to 5-8 feet.
45° Elbow
A gentler direction change. Equivalent to approximately 5-7 feet of straight duct for standard radius, 3-4 feet for long-radius. Two 45° elbows with a straight section between them are often better than a single 90° elbow because the total pressure drop is lower and the airflow pattern is smoother.
Adjustable Elbow
A multi-segment (gore) elbow that can be set to any angle from 0° to 90°. Extremely versatile for field-adjusted routing. Pressure drop is slightly higher than a smooth-radius elbow of the same angle because the segmented interior is not perfectly smooth.
Fitting Equivalent Length Table
| Fitting | Equiv. Length (ft) | Notes |
|---|---|---|
| 90° smooth radius elbow | 5-8 | Best 90° option |
| 90° short radius elbow | 10-12 | Compact spaces |
| 90° mitered (no vanes) | 15-20 | Avoid if possible |
| 90° mitered (with vanes) | 5-8 | Rectangular duct standard |
| 45° elbow | 3-7 | Preferred over 90° when possible |
| Tee (90° branch) | 15-25 | High loss — use wye instead |
| Wye (45° branch) | 8-12 | 40-60% less drop than tee |
| Concentric reducer | 2-5 | Keep angle ≤15°/side |
| Register boot | 10-15 | Round-to-rectangular transition |
| Start collar / saddle tap | 5-10 | Depends on entry angle |
| Flex duct gentle bend | 3-5 | Radius ≥ 1D, fully extended |
Branch Fittings (Tees, Wyes, Takeoffs)
Tee
A 90° branch connection where the branch exits perpendicular to the trunk. Air must make a sharp turn to enter the branch, causing high turbulence and pressure drop. Use tees only for return air where velocity is low, or where space prohibits a wye fitting.
Wye (Y-Fitting)
The preferred branch fitting. The branch exits at 30-45° in the direction of airflow, allowing a smoother transition from trunk to branch. Wyes produce 40-60% less pressure drop than tees and are standard for supply air distribution and dust collection systems.
Start Collar / Saddle Tap
A start collar is welded or riveted to the side of a trunk duct to provide a branch takeoff point. A saddle tap is a self-sealing collar that clamps onto the trunk without welding — faster to install but slightly higher air leakage. For a full overview of no-weld ducting connections, see our dedicated guide. Both connect round branch ducts (including flex duct) to a sheet metal trunk.
Reducers & Transitions
Concentric Reducer
Reduces duct diameter while keeping the centerline aligned. Used on vertical duct runs and wherever centering matters. The reduction angle should not exceed 15° per side (30° total) — this means a reducer from 12" to 8" should be at least 6 inches long.
Eccentric Reducer
Reduces diameter with one side flat. Used on horizontal duct runs where the bottom of the duct must remain level — important for condensation drainage in cooling systems and for maintaining clearance above the duct.
Round-to-Rectangular Transition
Converts between round duct (supply trunks, spiral duct) and rectangular openings (register boxes, equipment connections). The transition angle should not exceed 15° per side to minimize turbulence. See our rectangular duct guide for detailed transition rules.
Register Boots
Register boots connect round duct to rectangular wall or ceiling registers. They are the final fitting in each supply run, transitioning from the round branch duct to the rectangular register opening. Common types: straight boot (duct enters from above), 90° boot (duct enters from the side), and end boot (duct enters from behind).
Register boots are a significant source of pressure drop (10-15 equivalent feet) because they combine a shape change with a direction change. Oversizing the branch duct by one diameter (e.g., 7" instead of 6") compensates for the boot's restriction and ensures full airflow at the register.
Connecting Flex Duct to Fittings
Flexible duct connects to rigid fittings using a two-step process: the inner core is pulled over the collar and secured with a strap clamp, then the outer jacket (insulation + vapor barrier on insulated duct) is pulled over and sealed with foil tape or mastic. Both layers must be independently sealed for airtight, code-compliant connections.
The connection collar should extend at least 1 inch past the clamp for a secure grip. Use worm-gear duct connectors rather than zip ties — they provide a tighter, more uniform seal and are required by most building codes.
For a complete step-by-step, see our flex duct installation guide.