HVAC Duct Fittings Guide: 12 Types, 206 SKUs, Galvanized | Airwise [2026]

1. What Are HVAC Duct Fittings?

HVAC duct fittings are the connectors, transitions, and junction pieces that link straight duct sections into a complete air distribution system. While straight duct carries air from point A to point B, ducting fittings for HVAC systems are what make the turns, branches, and size transitions that route conditioned air to every room in a building. They include elbows, branch take-offs, reducers, boots, Y-pieces, starting collars, and trunking — each serving a specific function in the ductwork layout.

A typical residential HVAC system uses 15-30 individual fittings. A commercial system can require hundreds. The quality and sizing of these fittings directly determines three critical performance factors: pressure drop (how hard the blower must work), noise level (turbulence creates audible air noise), and air leakage (poorly sealed joints waste energy).

According to the U.S. Department of Energy, duct leakage in typical residential systems wastes 20-30% of conditioned air. Properly selected and sealed fittings can reduce this to under 5%, saving hundreds of dollars annually in energy costs.

2. Types of Duct Fittings

The following seven fitting types cover the vast majority of residential and commercial HVAC installations. Each description includes sizing data from our production catalog.

2.1 Branch Take-Off (BTO)

A branch take-off creates a perpendicular connection from a main trunk duct to a branch duct. It is the single most common fitting in any ducted HVAC system. BTOs come in two configurations: single branch (one outlet) and double branch (two outlets on opposite sides of the trunk).

Our single BTO range covers 30 size combinations from 6"-04" to 14"-12", all in G60 galvanised steel. The double BTO range adds 16 configurations for symmetrical branch distribution.

2.2 Unit Boot & Side Boot

Boots are transition fittings that convert round duct to the rectangular opening of a floor, wall, or ceiling register. A unit boot transitions straight down (or up) from the duct to a register mounted in the floor or ceiling. A side boot makes a 90-degree turn so the register mounts on a wall.

Our unit boot range is the largest in our catalog at 36 SKUs, covering duct diameters from 6" to 14" with register openings from 10"x4" to 14"x10". The side boot range adds 17 configurations.

2.3 Elbow

Elbows redirect airflow around corners and obstacles. They are the second most-used fitting after BTOs in most duct systems. Elbows come in three standard angles: 90-degree (right-angle turn), 45-degree (gentle direction change), and adjustable (set to any angle from 0-90 degrees on site). Round elbows are further divided into smooth-radius (low pressure drop, 1.5D centerline radius) and sectional/gored (fabricated from flat segments, higher pressure drop but easier to manufacture in large sizes).

Pressure drop matters with elbows. A single 90-degree elbow adds the equivalent of 15-25 feet of straight duct in pressure loss. Using two 45-degree elbows instead of one 90-degree elbow can reduce pressure drop by 30-40% while achieving the same direction change — a technique experienced contractors use in tight mechanical spaces. Where possible, smooth-radius elbows should be specified over sectional elbows: they generate approximately 50% less pressure drop at the same velocity.

Our elbow range covers standard 90-degree and 45-degree elbows in diameters from 4" to 14", all in G60 galvanised steel with crimped ends for slip connection. Adjustable elbows are available from 6" to 12" diameter.

Source: ACCA Manual D, Table 7. Equivalent lengths are approximations for design velocity of 600-900 FPM. Actual values vary by manufacturer radius and surface roughness.

2.4 Y-Piece (Wye)

A Y-piece splits a single duct into two branches at a gradual angle (typically 45°), producing significantly less turbulence and pressure drop than a T-junction. Y-pieces are preferred over tees in HVAC because the angled branches maintain smoother airflow.

Our Y-piece range includes 17 configurations. Sizes range from 8"-06"-06" (8" main splitting into two 6" branches) up to 14"-12"-12".

2.4 Reducer

Reducers transition between two different duct diameters. They come in two types: concentric (centered, the outlet is in the middle of the inlet) and eccentric (one side stays flat, used when the duct must maintain a flat bottom or top line). Concentric reducers are standard for suspended ductwork; eccentric reducers are used when ductwork sits on a shelf or must maintain a uniform top line.

Our reducer catalog covers standard step-down combinations. Custom reductions available on request.

2.5 Starting Collar

A starting collar attaches to a plenum box or trunk duct to provide a crimped connection point for branch ductwork. It creates a clean, sealed transition from the flat wall of a plenum to a round duct. Starting collars are essential for connecting flexible duct to sheet metal plenums and trunk lines.

Our starting collar range is available in standard round sizes from 6" to 14" diameter.

2.6 Metal Joiner

Metal joiners (also called duct couplers or connectors) join two sections of round duct end-to-end. A standard joiner is a short sleeve that slides over both duct ends, secured with sheet metal screws and sealed with mastic or foil tape. Crimp joiners have one end crimped for a friction fit into the next duct section.

Our joiner range matches all standard duct diameters in our catalog.

2.7 Trunking

Metal and plastic trunking provides a finished, enclosed pathway for ductwork in exposed applications such as commercial ceilings, garages, and retrofits. Metal trunking is available in galvanised steel for HVAC applications; plastic trunking is used for lighter-duty condensate lines and mini-split installations.

Our metal trunking comes in 18 SKUs covering widths from 60mm to 140mm with various depths. Plastic trunking is available in 2 standard profiles.

2.9 Fitting Pressure Drop Quick Reference

Every fitting adds resistance to the duct system, measured as equivalent length of straight duct. When designing a system, you must account for these equivalent lengths to ensure the blower can deliver the required CFM at the design static pressure. The table below summarises typical equivalent lengths for the most common fittings at standard HVAC velocities (600-900 FPM).

Rule of thumb: Total equivalent length of a residential duct system typically equals 1.5-2.5x the measured duct length. If your measured supply run is 60 feet, plan for 90-150 feet of total equivalent length when sizing the blower. For a detailed sizing calculator, see our HVAC CFM & Duct Sizing Calculator.

3. Material Comparison

The material of your duct fittings affects corrosion resistance, weight, cost, and suitability for specific environments. Here is a comparison of the three most common materials used in HVAC duct fittings:

Recommendation: For 90% of residential and commercial HVAC projects, G60 galvanised steel is the optimal choice — it balances cost, durability, and availability. Specify stainless steel only when the environment demands it (coastal areas within 5 km of saltwater, chemical processing, commercial kitchens with grease-laden exhaust).

4. How to Select the Right Fitting

Selecting duct fittings involves matching four parameters to your system design:

Step 1 — Determine the connection sizes. Measure the inside diameter of the existing duct or check the mechanical drawings. Round fittings are sized by nominal diameter (6", 8", 10", 12", 14"). For boots, you also need the register opening size (e.g. 10"x6", 12"x6").

Step 2 — Choose the fitting type. The system layout determines which fitting is needed: BTOs for branches, boots for register connections, reducers for diameter transitions, Y-pieces for splits. Refer to the types table in Section 2 above.

Step 3 — Specify the material and gauge. Standard residential fittings use 26-28 gauge galvanised steel. Commercial systems typically require 24-26 gauge. Industrial applications may need 22 gauge or heavier. Our standard production is G60 galvanised; stainless steel and aluminium are available on request.

Step 4 — Consider the connection method. The three connection methods are slip (one end slides inside the other, sealed with mastic and screws), crimp (one end is crimped smaller for friction fit), and flanged (TDC or slip-and-drive flanges for large commercial duct). Most residential fittings use slip or crimp connections.

4b. Commercial vs Residential Fitting Selection

The requirements for duct fittings differ significantly between residential and commercial HVAC systems. Understanding these differences prevents over-specifying (wasting money) or under-specifying (performance problems and callbacks).

Key takeaway for contractors: Residential systems tolerate lighter gauges and simpler connections because velocities and static pressures are lower. Commercial systems demand heavier gauge, flanged connections, and strict sealing because higher velocities amplify every leak and every fitting's pressure drop. When in doubt on a borderline project (large residential or small commercial), spec commercial-grade fittings — the cost difference per fitting is $0.50-2.00, but the performance and durability difference is substantial.

5. Installation Best Practices

Even the best fittings will underperform if installed incorrectly. These five practices, drawn from SMACNA guidelines and 25 years of manufacturer experience, ensure optimal system performance:

Seal every joint. Use mastic sealant (not duct tape) on all longitudinal and transverse joints. Mastic remains flexible and maintains its seal for 20+ years, while cloth duct tape dries out and peels within 1-3 years. Apply mastic with a brush to cover the full joint circumference, minimum 3" wide.

Support fittings independently. Every fitting should have its own hanger or support, separate from the duct support. Unsupported fittings create stress points that can open joints over time, especially in vertical runs. Use galvanised strap or perforated hanging band at maximum 1.5 m intervals.

Maintain minimum straight duct before and after fittings. Allow at least 2 duct diameters of straight duct upstream and downstream of any fitting. Placing fittings back-to-back increases pressure drop by 40-60% compared to properly spaced fittings.

Orient crimped ends in the direction of airflow. The crimped (smaller) end of a fitting should always point in the direction of airflow. This prevents condensate from dripping at joints in cooling systems and ensures smooth airflow transition.

Screw before sealing. Secure each joint with a minimum of 3 sheet metal screws (equally spaced) before applying mastic sealant. Screws provide mechanical connection; mastic provides the air seal. Both are required — neither alone is sufficient.

6. Sourcing from a One-Stop Manufacturer

Most HVAC contractors and distributors source fittings from multiple suppliers — one for BTOs, another for boots, a third for trunking. This multi-vendor approach creates three problems: inconsistent quality between suppliers, higher shipping costs from multiple origins, and administrative overhead managing multiple purchase orders and deliveries.

A one-stop approach consolidates all fitting types under one manufacturer with one quality standard. At Airwise, our Ducts & Metal Fittings catalog covers 19 product types and 206 individual SKUs — from BTOs (30 sizes) to unit boots (36 sizes) to trunking (18 profiles). Combined with our grilles and diffusers, dampers, plenum boxes, and copper line sets, a single order can cover every component of an HVAC installation.

Our China factory produces all fittings in-house on self-developed equipment, ensuring consistent gauge tolerances and crimp quality across the entire range. For US customers, spiral ducts and fittings are fabricated at our Houston, TX facility for faster delivery.

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