HVAC Duct Fittings Types: A Contractor's Visual Guide to Every Ducting Fitting
Every HVAC system is built from two things: straight duct and fittings. The straight duct moves air in one direction; the fittings change its direction, split it, reduce it, and connect it to registers. Choosing the wrong fitting — or the wrong size — adds friction, noise, and static pressure that your system cannot afford. This guide covers every ducting fitting type you will encounter on a job site, with the specs and pressure drop data you need to make the right call.
1. Direction-Change Fittings
90-Degree Elbows
The workhorse of duct systems. A 90-degree elbow turns airflow by a full right angle. Available in two profiles:
- Long-radius (standard) — centerline radius = 1.5x duct diameter. Lower pressure drop, smoother airflow, fewer noise issues. This is the default for new installations.
- Short-radius (tight) — centerline radius = 1.0x duct diameter. Higher pressure drop (roughly 2x a long-radius), but fits in tighter spaces. Use only when ceiling void or wall cavity constraints prevent a long-radius.
| Duct Size | Long-Radius (1.5D) | Short-Radius (1.0D) | Equivalent Length |
|---|---|---|---|
| 6" (150 mm) | 0.04" W.G. | 0.09" W.G. | 10 ft / 22 ft |
| 8" (200 mm) | 0.05" W.G. | 0.11" W.G. | 12 ft / 25 ft |
| 10" (250 mm) | 0.07" W.G. | 0.15" W.G. | 15 ft / 30 ft |
| 12" (300 mm) | 0.08" W.G. | 0.17" W.G. | 18 ft / 35 ft |
| 14" (350 mm) | 0.09" W.G. | 0.19" W.G. | 20 ft / 40 ft |
Contractor tip: Every short-radius elbow you swap for a long-radius saves 10-20 feet of equivalent duct length in your static pressure calculation. On a system with four elbows, that is 40-80 feet of "free" duct length — often the difference between a system that meets its airflow targets and one that does not.
45-Degree Elbows
Half-turn elbows used when a 90-degree change is not needed or when space allows for a gentler turn. Pressure drop is approximately 60% of a same-size 90-degree long-radius elbow. Two 45-degree elbows with a straight section between them create a duct offset — used to route around beams, pipes, and structural obstacles without a sharp turn.
Adjustable Elbows
Multi-section elbows (typically 3-5 crimped sections) that can be bent to any angle from 0 to 90 degrees on site. Useful for awkward routing where the exact angle is not known until installation. Pressure drop is slightly higher than a smooth stamped elbow of the same angle due to the corrugated interior. Available in 4" to 12" diameters.
2. Size-Change Fittings
Reducers (Concentric and Eccentric)
Reducers transition between two different duct diameters. Used when the trunk size steps down after a branch take-off, or when connecting equipment with different port sizes.
- Concentric reducers — both ends are centered on the same axis. Creates a symmetrical taper. Best for vertical runs and suspended horizontal runs where centering does not matter.
- Eccentric (flat-bottom) reducers — one side is flat, the other tapers. Keeps the bottom of the duct level, which is essential for horizontal runs sitting on supports or when maintaining ceiling clearance above the duct.
Standard reducer length is 4-8 inches, with longer tapers (8-12 inches) producing less turbulence and lower pressure drop. Avoid reducing more than two sizes in a single fitting (e.g., 12" to 6") — the abrupt area change creates excessive turbulence. Use two sequential reducers instead: 12" to 10", then 10" to 8", then 8" to 6".
Increasers
The reverse of reducers — transitioning from a smaller diameter to a larger one. Increasers are less common but necessary when duct velocity is too high and you need to slow air down before a diffuser or branch connection. The same concentric/eccentric options apply.
3. Branch and Distribution Fittings
Branch Take-Offs (BTOs)
Branch take-offs connect a branch duct to a trunk duct. The BTO is mounted on the trunk surface, and a hole is cut in the trunk to match. Available in several styles:
- Conical take-off — a cone-shaped collar with a flange that mounts on the trunk. The taper provides a smooth airflow transition. Best for high-CFM branches.
- Saddle take-off — a curved saddle that wraps around the trunk with a straight collar rising from it. Quick to install, lower cost, adequate for residential and light commercial.
- 90-degree take-off — a BTO with a built-in 90-degree turn, used when the branch duct runs perpendicular to the trunk immediately after the connection.
Specification format: Trunk Size - Branch Size. For example, "12-08 BTO" means a take-off from a 12" trunk to an 8" branch.
Contractor tip: Always install a volume damper immediately downstream of a BTO (within 2 duct diameters). Without a damper, you cannot balance the branch airflow during commissioning.
Tees and Wyes
Tees split the duct into two branches at 90 degrees (T-shape). Wyes split at 45 degrees (Y-shape). Wyes have significantly lower pressure drop than tees because the air changes direction more gradually.
| Fitting | Pressure Drop | Equivalent Length | Noise Level |
|---|---|---|---|
| Standard Tee (90°) | 0.18" W.G. | 35 ft | Moderate-High |
| Wye (45°) | 0.08" W.G. | 15 ft | Low |
| Wye with turning vanes | 0.05" W.G. | 10 ft | Very Low |
Use wyes whenever possible, especially in low-static systems (residential, multi-family). Reserve tees for situations where space constraints prevent a 45-degree split.
4. Termination and Transition Fittings
Register Boots
Boots transition from round duct to rectangular register openings. They are the final fitting before conditioned air enters the room. Three types:
- Unit (straight) boot — round duct enters the top, register faces straight down. For ceiling and floor registers.
- Side boot — round duct enters the back, register faces to the side. For wall registers where duct runs horizontally in the wall cavity.
- End boot — round duct enters one end, register faces down at the opposite end. For duct runs that terminate at a wall-ceiling junction.
Standard register sizes: 10"x6", 12"x6", 14"x6", 10"x4", and 12"x4". Match the boot to both the duct diameter and the register opening. Common combinations: 6" duct + 10"x6" register, 7" duct + 12"x6" register, 8" duct + 14"x6" register.
Starting Collars
Starting collars mount on a rectangular trunk duct to create a round branch connection. They are the trunk-side component that a branch duct slides onto. Types include flange-mount (self-tapping screws through the trunk wall), tab-in (tabs fold inside the trunk opening), and spin-in (twist-lock for tool-free installation).
For larger projects, spin-in starting collars save significant labor time — an installer can mount 30-40 spin-ins per hour versus 15-20 flange-mount collars. At scale, this labor savings pays for the slight cost premium of spin-in fittings.
End Caps
Flat discs or plates that seal the end of a duct run. Used to terminate trunk ducts, cap unused branches, or seal test openings. Available as press-fit friction caps (quick but removable) or flanged caps (sealed permanently with screws and mastic).
5. Connection and Joining Fittings
Duct Connectors (Metal Joiners)
Short sleeves that connect two sections of the same-diameter duct. One end is crimped to fit inside the male end of one duct section, the other end receives the male end of the next section. Secured with sheet metal screws (3 minimum, evenly spaced) and sealed with foil tape or mastic.
Flex-to-Rigid Connectors
Adapters that join flexible duct to rigid metal duct. Critical for residential systems where flex duct runs from a trunk take-off to a register boot. The connector provides a rigid mounting surface for the flex duct inner liner, which is clamped with a zip tie or hose clamp and then sealed with mastic.
Rectangular-to-Round Transitions
Transition fittings that convert rectangular duct to round duct (or vice versa). Used at equipment connections (most furnaces and air handlers have rectangular openings but connect to round trunk ducts) and at plenum box connections.
6. Material and Gauge Reference
| Duct Size Range | Standard Gauge | Thickness | Galvanization | Weight (per 90° elbow) |
|---|---|---|---|---|
| 4"–8" | 26 gauge | 0.45 mm | G60 | 0.3–0.8 kg |
| 9"–14" | 24 gauge | 0.60 mm | G60 | 0.9–2.0 kg |
| 15"–24" | 22 gauge | 0.75 mm | G60/G90 | 2.5–5.5 kg |
| 26"–36" | 20 gauge | 0.90 mm | G90 | 6.0–12.0 kg |
All Airwise duct fittings use 26-gauge galvanized steel for standard residential sizes (4"–12") and 24-gauge for 14" and above. We use G60 galvanization as standard, with G90 available on request for coastal, marine, or high-humidity applications.
7. Fitting Count Estimation
For project planning and quoting, here are typical fitting counts by system type:
| System Type | Elbows | Reducers | Take-offs | Boots | Other | Total Fittings |
|---|---|---|---|---|---|---|
| 2,000 sq ft residential | 8-12 | 3-5 | 6-10 | 8-12 | 5-8 | 30-47 |
| 5,000 sq ft commercial office | 15-25 | 6-10 | 12-20 | — | 10-15 | 43-70 |
| 20,000 sq ft retail | 30-50 | 10-15 | 25-40 | — | 15-25 | 80-130 |
Contractor tip: Order 10-15% extra fittings beyond the takeoff count. Field conditions always require additional fittings for routing around unexpected obstacles (pipes, beams, electrical conduit). The cost of shipping a supplemental order is far higher than the cost of having a few extra fittings on site.
8. Installation Best Practices
Always install fittings with the crimped end facing the direction of airflow. This is the single most important rule for duct fitting installation. Air should flow over the lap joint, not into it. Reversed fittings create turbulence, increase pressure drop by 10-20%, and make joints harder to seal.
Use three sheet metal screws minimum per joint. Evenly space screws around the circumference — at 12, 4, and 8 o'clock positions for round fittings. More screws do not improve the connection; they just add labor and puncture points.
Seal every joint. Code requires all duct joints to be sealed. Use UL 181A-M mastic sealant (brush-on) or UL 181B-FX foil tape. Do not use cloth-backed "duct tape" — it dries out and falls off within 2-5 years. Unsealed joints leak 15-25% of system airflow in a typical residential system.
Support fittings independently. Do not let the weight of a fitting hang from the connected duct. Each fitting (especially heavy elbows and tees) should have its own hanger strap or support within 12 inches of the joint.
9. Sourcing Duct Fittings from Airwise
Our Ducts & Metal Fittings catalog includes 206 SKUs covering every fitting type in this guide — elbows, reducers, take-offs, boots, starting collars, end caps, connectors, and transitions. All fabricated from galvanized steel at our Dongyang, China facility with optional Houston, TX warehouse pickup for US customers.
For complete system packages, source fittings alongside plenums, diffusers, and dampers from Airwise. One PO, one quality standard, one shipment. See our plenum box sizing guide for CFM calculations and tonnage charts.
Standard fittings ship within 15-25 business days. Custom sizes, non-standard angles, and G90 galvanization are available with 30-45 day lead time.