Ordering rectangular ductwork may seem like a straightforward task, but small errors in sizing, material selection, or specification can lead to major issues during installation. The result? Cost overruns, rework, project delays, and poor system performance.
To help contractors avoid these pitfalls, here are the top 10 most common mistakes made when ordering rectangular ductwork and how to fix them.
Contact HVAC Duct Manufactures
1. Incorrect or Incomplete Measurements
One of the most frequent mistakes is sending duct dimensions without accounting for:
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Flange thickness
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Internal insulation
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Clearance for installation
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Transitions or offsets
Solution: Always measure wall-to-wall clearances, confirm duct routing drawings, and include space for fittings, tabs, and insulation.
2. Not Specifying the Pressure Class
SMACNA standards require ducts to be built to specific pressure classes. Ordering without stating the required pressure class results in:
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Wrong gauge
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Incorrect reinforcement spacing
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Leakage issues
Solution: Always specify:
Example: 2-inch WG, pressure class 1, 2, 3, 4, 6, or higher depending on the system.
3. Choosing the Wrong Sheet Metal Gauge
Incorrect gauge selection leads to:
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Excessive vibration
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Oil-canning
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Structural failure
Solution: Gauge must match pressure class + duct size.
Example: Larger ducts require heavier gauge and proper reinforcement.
4. Forgetting to Mention the Connector System (TDF/TDC/Angle Iron/Slip & Drive)
Different projects require different joint systems. Failing to specify leads to mismatched components at site.
Solution: Confirm with the manufacturer whether you need:
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TDF / TDC
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Angle iron flanges
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Slip and drive
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Welded joints
5. Missing Insulation Requirements
If insulation isn’t mentioned, manufacturers may supply bare ducts—and modifying later increases cost.
Solution: Specify clearly:
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Internal / external insulation
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Thickness (e.g., 25 mm, 50 mm)
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Material (glass wool, PUF, elastomeric, etc.)
6. Ignoring Reinforcement Requirements for Large Ducts
Large rectangular ducts require reinforcements like:
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Tie rods
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Angle iron frames
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Cross breaking
Skipping these leads to duct deformation.
Solution: Use SMACNA tables or consult your fabricator for proper reinforcement spacing.
7. Missing Accessories and Fittings
Commonly forgotten items include:
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Access doors
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Dampers (VCD, FD, MSFD)
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Turning vanes
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Corners, cleats, gaskets
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Canvas connectors
Solution: Prepare a complete BOQ with all fittings included.
Rectangular ducts often clash with:
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RCC beams
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Electrical conduits
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Fire sprinkler lines
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Plumbing services
This causes redesign and rework onsite.
Solution: Coordinate drawings with MEP teams and conduct a site walk-through before ordering.
Very large pieces may not fit:
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Through elevators
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Staircases
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Tight shafts
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Doorways
Solution: Split oversized ducts into transportable sections and specify connector positions for easy assembly on site.
10. Not Reviewing the Fabrication Drawing (Shop Drawing)
Contractors sometimes approve orders without reviewing the manufacturer’s shop drawings. This leads to fabrication errors.
Solution: Always review and approve:
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Dimensions
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Fittings
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Reinforcements
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Material specs
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Joint types
before production begins.
Rectangular ductwork is a core component of any HVAC system but the smallest ordering mistake can create expensive and time-consuming site challenges. By avoiding these 10 mistakes, contractors can ensure faster installation, fewer revisions, improved airflow performance, and overall smoother project execution.
