HVAC Duct Connection Types Explained: Slip, Drive, TDC, and Flanged

Every duct fitting has two ends, and how those ends connect to the next piece of duct determines the joint's strength, air tightness, ease of assembly, and whether you can take it apart later. The four main connection types for rectangular sheet metal duct are slip, drive cleat, TDC (transverse duct connector), and flanged. Each serves a different purpose, and choosing the wrong one costs time on the job site or creates callbacks from air leaks.

Quick Comparison

Slip Connections

A slip connection is the simplest joint in ductwork. One end of the duct is crimped (reduced slightly) so it slides into the uncrimped end of the next section. The overlap is typically 1 to 1.5 inches. The joint is secured with sheet metal screws and sealed with mastic or foil tape.

When to Use Slip Connections

• Small residential branch runs. 6" x 6" and 8" x 6" branch duct to individual registers.
• Low-pressure systems. Standard residential forced-air systems operating at 0.5" w.c. or less.
• Quick installs. Slip joints go together fast with minimal tools. Crimp, slide, screw, seal.

Limitations

Slip joints are not structurally strong. They rely on screws and friction, and they cannot handle significant positive or negative pressure without risking separation. They are also nearly impossible to disassemble without destroying the crimped end, which makes future service difficult.

On larger duct (over 12" wide), slip connections become impractical because the crimped end must be reduced on all four sides, and maintaining alignment on large rectangular sections is difficult without a brake.

Drive Cleat Connections

Drive cleats (also called S-cleats and drive strips) are the workhorse connection for residential and light commercial straight duct and fittings. The joint uses two different fasteners: S-cleats on the long sides and drive strips on the short sides.

Each duct end has a raw 1-inch flange bent at 90 degrees. The S-cleat hooks over the flanges of both pieces on the top and bottom. Drive strips are hammered over the flanges on the two short sides to lock the joint. The result is a rigid, permanent connection.

When to Use Drive Cleats

• Residential trunk lines. 8" x 8" through 14" x 12" trunk duct in basements and crawl spaces.
• Light commercial systems. Up to about 24" wide on systems under 2" w.c. static pressure.
• Permanent installations. Drive cleat joints are not designed to come apart. They are ideal for long-life installations that will not need disassembly.

Limitations

Drive cleats are permanent. Once hammered in place, you are not taking them apart without cutting. They are also labor-intensive on larger duct because the drive strips require significant force to seat on heavy-gauge flanges. Above 24" wide, most contractors switch to TDC or flanged connections.

TDC (Transverse Duct Connector)

TDC is a rail-and-cleat system that has become the dominant connection method for medium to large commercial ductwork. A formed metal rail is attached to each duct end, creating a flange with a built-in gasket channel. The two rails mate face-to-face and are locked together with TDC cleats (corner brackets) at each corner, then bolted tight.

When to Use TDC

• Commercial HVAC systems. TDC is the standard connection for commercial straight duct, elbows, tees, and transitions from about 10" wide and up.
• Medium-pressure systems. TDC connections are rated for up to 4" w.c. static pressure, covering the majority of commercial supply and return systems.
• Systems requiring disassembly. TDC joints can be unbolted and separated without destroying the duct. This matters in commercial buildings where ductwork may need to be reconfigured, inspected, or cleaned.
• Jobs requiring consistent air tightness. The built-in gasket channel accepts a closed-cell neoprene gasket that creates a reliable seal without mastic.

Limitations

TDC rails add cost and weight to each fitting. The corner cleats and bolts take more time to assemble than slip or drive connections. And while 4" w.c. handles most commercial systems, high-pressure applications (VAV systems, clean rooms) may exceed TDC ratings.

Flanged Connections

Flanged connections use an angle-iron or formed-steel flange bolted to each duct end with a gasket sandwiched between. This is the heaviest, strongest, and most expensive connection method, used when pressure ratings, structural loads, or code requirements exceed what TDC can deliver.

When to Use Flanged Connections

• High-pressure systems. VAV (variable air volume) systems, clean rooms, and industrial process duct operating above 4" w.c.
• Large duct sizes. On duct 30" wide and larger, flanged connections provide the structural rigidity needed to prevent the joint from flexing under its own weight or from pressure forces.
• Kitchen exhaust and grease duct. NFPA 96 requires liquid-tight joints on grease duct, and flanged connections with high-temperature gaskets meet this requirement.
• Stainless steel systems. Stainless duct in clean rooms and pharmaceutical facilities often uses flanged connections for the combination of strength, cleanliness, and disassembly capability.

Limitations

Flanged connections are the most expensive option in both material and labor. Each flange must be drilled and bolted, which takes significantly more time than any other connection method. The flanges themselves add weight and protrude from the duct profile, which can matter in tight ceiling spaces.

Raw Ends

In addition to the four main connection types, PMX Ductwork offers a "raw" end option. This is an unfinished duct end with no connection hardware. Raw ends are used when:

• You are connecting to existing ductwork that has its own connection type.
• You need to field-modify the connection (add your own cleats, slip it into a plenum, or weld it).
• The fitting connects directly to equipment (air handlers, plenums, diffuser boxes).

Choosing the Right Connection

The decision comes down to three factors: system pressure, duct size, and whether the joint needs to be removable.

• Residential, under 12" wide: Slip connections. Fast, cheap, and adequate for low-pressure systems.
• Residential trunk and light commercial, 8" to 24": Drive cleat. Strong permanent joint, no gaskets needed (seal with mastic).
• Commercial, 10" to 48": TDC. The industry standard. Built-in gasket, removable, rated for most commercial pressures.
• High-pressure, large, or code-driven: Flanged. Maximum strength and sealing for demanding applications.

Every fitting at PMX Ductwork can be ordered with any connection type on either end. Need slip on one side and TDC on the other? No problem. Specify your connections when you design your fitting, and we fabricate it exactly as ordered.