Transverse Duct Connector (TDC) and Transverse Duct Flange (TDF) are two industry-standard flange systems for joining rectangular sheet metal ductwork, critical for ensuring airtightness, structural integrity, and compliance with SMACNA (Sheet Metal and Air Conditioning Contractors’ National Association) standards. Unlike traditional slip-and-drive or riveted connections, TDC and TDF use engineered flange profiles to create uniform, repeatable joints—essential for commercial and industrial HVAC systems where pressure, airflow efficiency, and installation speed are priorities. This guide provides a technical breakdown of their design, performance, material differences, and application-specific selection criteria, empowering engineers, contractors, and facility managers to choose the optimal connection system for their project.  

 

 

1. Foundational Definitions & Design Standards  

Both TDC and TDF are governed by SMACNA’s HVAC Duct Construction Standards (Chapter 5: Duct Connections) and AMCA International’s Publication 210 (Air Duct Leakage Test Methods), ensuring consistency in dimensions, pressure ratings, and airtightness.  

 

1.1 TDC (Transverse Duct Connector)  

TDC is a two-piece flange system consisting of:  

- A pre-fabricated, U-shaped metal flange (typically galvanized steel, 24–22 ga) with integral mounting holes (3/8 in diameter, spaced 6–8 in on center).  

- A corresponding duct end that is “crimped” or “folded” to accept the flange, creating a mechanical interlock.  

 

Key design features:  

- Flange Profile: 1.25 in (height) × 0.75 in (width) for standard applications; heavy-duty versions (1.5 in height) for high-pressure systems.  

- Material Compatibility: Works with galvanized steel, stainless steel, or aluminum duct (flange material must match duct material to prevent galvanic corrosion).  

- Pressure Rating: Standard TDC handles up to 4 in. w.g. (static pressure); heavy-duty TDC (thicker flange, reinforced mounting holes) handles up to 6 in. w.g.—suitable for industrial exhaust or high-velocity HVAC systems.  

 

1.2 TDF (Transverse Duct Flange)  

TDF is a one-piece, integral flange system where the duct’s own edge is formed (via specialized machinery) into a flange profile—eliminating the need for a separate flange component.  

 

Key design features:  

- Flange Formation: The duct’s transverse edge (perpendicular to airflow) is folded using a TDF roll-forming machine to create a 1.25 in tall flange with a 0.5 in return lip (for gasket seating).  

- Mounting Holes: Punched directly into the formed flange (same spacing as TDC: 6–8 in on center) during fabrication.  

- Pressure Rating: Standard TDF handles up to 3 in. w.g.; enhanced TDF (with a double-folded lip) handles up to 5 in. w.g.—ideal for commercial HVAC (offices, retail, schools) but not extreme high-pressure applications.  

 

 

2. Technical Comparison: TDC vs. TDF  

The choice between TDC and TDF hinges on performance metrics like airtightness, structural strength, cost, and installation efficiency. Below is a head-to-head analysis aligned with SMACNA and ASHRAE standards:  

 

| Metric | TDC (Transverse Duct Connector) | TDF (Transverse Duct Flange) |  

|--------|----------------------------------|--------------------------------|  

| Airtightness | - Achieves SMACNA Class 3 airtightness (≤0.5 cfm/100 ft² at 1.0 in. w.g.) when paired with a closed-cell EPDM gasket.<br>- The separate flange creates a uniform sealing surface, reducing leakage at joint gaps. | - Achieves SMACNA Class 3 airtightness with the same gasket, but relies on precise flange formation (poorly formed edges can increase leakage to Class 2: ≤2.0 cfm/100 ft²).<br>- No gap between duct and flange (integral design) minimizes potential leak paths. |  

| Structural Strength | - Superior rigidity: The separate flange acts as a reinforcement, preventing duct edge deformation under pressure or vibration.<br>- Can support duct weights up to 25 lbs/linear ft (when paired with proper hangers). | - Moderate rigidity: Relies on the duct’s base gauge (e.g., 24 ga duct = weaker flange than 22 ga).<br>- Supports duct weights up to 18 lbs/linear ft—requires more frequent hangers for long runs. |  

| Material & Labor Costs | - Material Cost: 15–20% higher than TDF (due to separate flange components, ~$0.30–$0.50 per linear ft of duct).<br>- Labor Cost: Lower installation time than traditional riveted joints but 10–15% higher than TDF (requires aligning and securing two components: duct + flange). | - Material Cost: 10–15% lower than TDC (no separate flange—uses the duct’s existing material).<br>- Labor Cost: 20–25% lower than TDC (integral flange eliminates flange attachment steps; faster to assemble with gaskets and bolts). |  

| Fabrication Complexity | - Can be fabricated on-site with basic tools (flange crimpers, hole punches) for small projects.<br>- Pre-fabricated flanges are widely available, making it suitable for emergency repairs or custom duct sections. | - Requires specialized TDF roll-forming machinery (cost: $15,000–$50,000) for flange formation—limited to off-site fabrication or large contractors with on-site equipment.<br>- Not feasible for on-site adjustments (e.g., trimming duct length requires re-forming the flange, which needs machinery). |  

| Pressure & Temperature Range | - Pressure: Up to 6 in. w.g. (heavy-duty).<br>- Temperature: -40°F to 400°F (galvanized steel flange); up to 800°F (stainless steel flange)—suitable for industrial exhaust (e.g., kitchen grease ducts, manufacturing fumes). | - Pressure: Up to 5 in. w.g. (enhanced).<br>- Temperature: -20°F to 300°F (limited by flange fold integrity—high heat can weaken the folded metal bond).<br>- Not recommended for high-temperature applications (e.g., boiler exhaust). |  

| Compatibility with Fittings | - Works with all rectangular duct fittings (elbows, tees, reducers) via matching TDC flanges—fittings are easy to source or fabricate. | - Fittings require TDF-compatible edges (must be formed with the same machinery as straight duct).<br>- Custom fittings (e.g., odd-angle elbows) may require TDC flanges if TDF formation is impractical. |  

 

 

3. Application-Specific Selection Guidelines  

TDC and TDF are not interchangeable—their performance limits align with distinct HVAC project types. Use the following criteria to select the right system:  

 

3.1 When to Choose TDC  

Prioritize TDC for projects requiring strength, high-pressure tolerance, or on-site flexibility:  

- Industrial HVAC Systems: Factories, power plants, or warehouses with static pressures >3 in. w.g. (e.g., high-velocity exhaust for dust collection).  

- High-Temperature Applications: Kitchen grease ducts (NFPA 96 compliant) or boiler exhaust systems (stainless steel TDC resists corrosion and heat).  

- On-Site Fabrication/Repairs: Small projects or emergency fixes where TDF machinery is unavailable (pre-fabricated TDC flanges can be attached with hand tools).  

- Heavy-Duty Duct Runs: Ducts larger than 24×18 in (rectangular) or with external loads (e.g., insulation, auxiliary sensors)—TDC’s reinforced flange prevents sagging.  

 

3.2 When to Choose TDF  

Prioritize TDF for commercial projects focused on cost efficiency and fast installation:  

- Commercial Buildings: Offices, retail stores, schools, or multi-family apartments with static pressures ≤3 in. w.g.—TDF’s lower material/labor costs reduce total project expenses by 10–15%.  

- Large-Scale Projects: New construction with hundreds of linear ft of duct (e.g., a 10-story office building)—TDF’s faster installation cuts labor time by 2–3 days per floor.  

- Standardized Duct Sizes: Projects using off-the-shelf rectangular duct (e.g., 12×8 in, 18×12 in) where TDF formation is consistent and efficient.  

- Low-Maintenance Environments: HVAC systems with minimal vibration or pressure fluctuations (e.g., comfort cooling in offices)—TDF’s integral design requires less long-term joint inspection.  

 

 

4. Installation Best Practices (SMACNA Compliant)  

Proper installation is critical to maximizing TDC/TDF performance—poorly executed joints can negate airtightness and structural integrity. Follow these guidelines:  

 

4.1 Pre-Installation Preparation  

1. Gasket Selection: Use closed-cell EPDM gaskets (3/16 in thick, 1 in wide) for both systems—avoid foam gaskets (they compress over time, reducing airtightness). Gaskets must be rated for the system’s temperature range (e.g., silicone gaskets for high-heat TDC applications).  

2. Flange Inspection:  

   - For TDC: Verify flanges are straight (no warping) and mounting holes are aligned (±1/16 in tolerance). Reject flanges with rust or bent edges.  

   - For TDF: Check that the formed flange has a uniform 1.25 in height and no cracks in the folded lip (use a straightedge to confirm flatness).  

3. Duct Alignment: Ensure duct sections are square (90° corners) and parallel—misaligned ducts force flanges to bend, creating leak paths. Use a framing square to verify duct squareness before joining.  

 

4.2 Installation Steps (TDC & TDF)  

1. Gasket Application: Apply a continuous bead of gasket adhesive to the flange’s sealing surface, then press the EPDM gasket into place (align with the flange’s outer edge to avoid blocking mounting holes).  

2. Joint Assembly:  

   - Align the duct sections so mounting holes are perfectly matched (use clamps to hold sections together temporarily).  

   - Insert 1/4-in stainless steel bolts through the holes (avoid galvanized bolts with TDF—they can seize in the formed flange).  

   - Tighten bolts to 8–10 ft-lbs of torque (over-tightening crushes the gasket; under-tightening leaves gaps). Space washers between bolts and flanges to distribute pressure.  

3. Seal Reinforcement: For critical applications (e.g., cleanrooms, laboratories), apply a bead of high-temperature mastic (ASTM C913) around the joint perimeter (over the gasket) to seal minor gaps.  

 

4.3 Post-Installation Testing  

- Leak Test: Perform a SMACNA Class 3 leakage test using a manometer and fan tester. For a 100 ft² duct section, leakage must be ≤0.5 cfm at 1.0 in. w.g.  

- Structural Test: For high-pressure systems, apply 125% of the design static pressure (e.g., 5 in. w.g. for a 4 in. w.g. design) and inspect joints for deformation—TDC joints should show no visible bending; TDF joints may have minor lip deflection (≤1/16 in) but no cracks.  

 

 

5. Long-Term Maintenance & Durability  

Both TDC and TDF joints require minimal maintenance, but proactive care extends their service life (15–20 years for galvanized steel systems):  

- Quarterly Inspections: Check for loose bolts (retighten to 8–10 ft-lbs) and gasket degradation (replace if cracked or compressed).  

- Corrosion Prevention: For humid environments (e.g., coastal buildings), apply a zinc-rich primer to TDC flanges (recoat every 5 years) or TDF formed edges (recoat every 7 years).  

- Leak Repair: For small leaks, inject aerosol duct sealant (UL 181B rated) into gaps—for large leaks, replace the gasket and re-tighten bolts.