Technical Foundation
What Is a Duct and Fitting?
Ductwork is the sheet-metal or flexible conduit that distributes conditioned or exhaust air through a building, and fittings are the transition pieces — elbows, reducers, tees, and dampers — that connect straight duct runs while managing airflow direction, static pressure loss and volume balancing. Round duct is more material-efficient and has lower friction loss per unit of airflow than rectangular duct of equivalent area, but rectangular duct fits more easily into shallow ceiling plenums and structural spaces, which is why commercial buildings often mix both depending on the run.
Gauge (sheet metal thickness) is specified per SMACNA standards based on duct dimension and static pressure class, with larger ducts and higher-pressure systems requiring heavier gauge to resist oil-canning and pressure deflection. Fittings must match not just the duct diameter but the connection method — slip-and-drive, flanged (TDC/TDF), or beaded — since mismatched connection types create air leakage points that erode system efficiency well beyond what the fitting's rated loss coefficient predicts.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Duct Shape | Round, rectangular, oval, flexible | Round has lowest friction loss per area; rectangular fits shallow plenums; flexible eases final connections |
| Gauge/Thickness | SMACNA-specified sheet metal gauge by size and pressure class | Heavier gauge resists oil-canning and deflection at larger diameters and higher static pressures |
| Material | Galvanized steel, stainless steel, aluminum | Stainless/aluminum used for corrosive, high-humidity, or weight-sensitive exhaust applications |
| Connection Type | Slip-and-drive, flanged (TDC/TDF), beaded | Must match across duct and fitting to maintain an airtight, code-compliant seam |
| Pressure Class | Low (up to 2" w.g.), medium, high static pressure | Determines required gauge, seam type, and reinforcement to prevent duct failure under system pressure |
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Applications
Where Duct and Fittings Get Used
Duct and fitting selection follows directly from the air being moved — conditioned supply air, general exhaust, or process/fume exhaust each drive different material and gauge choices.
Commercial HVAC Distribution
Rectangular and round galvanized duct runs distributing conditioned air through office and retail ceiling plenums.
Industrial Exhaust & Fume Extraction
Heavier-gauge or stainless duct rated for continuous exposure to particulate, moisture, or corrosive fumes.
Kitchen Exhaust Systems
Welded black-iron or stainless grease duct meeting fire-code clearance and cleanout access requirements.
Data Center & Cleanroom Air Handling
Sealed, low-leakage duct systems maintaining precise pressure differentials and airflow volumes.
Residential HVAC
Flexible insulated duct and lighter-gauge round duct for branch runs to individual supply registers.
Paint Booth & Spray Finishing Exhaust
Corrosion-resistant duct and dampers rated for solvent-laden exhaust airflow.
FAQ
Duct and Fittings Questions, Answered
Should I use round or rectangular duct for a given run?
Round duct has lower friction loss per unit of cross-sectional area and uses less sheet metal for the same airflow capacity, making it the more efficient choice where ceiling or chase space allows it. Rectangular duct is chosen mainly where vertical clearance is limited, since it can achieve the same airflow area in a shallower profile — the tradeoff is higher friction loss and generally more sheet metal and seams per linear foot.
What duct gauge do I need for a given size and pressure class?
SMACNA duct construction standards specify minimum gauge by duct dimension and pressure class (low, medium, high static pressure), with larger ducts and higher pressures requiring heavier gauge and, often, added reinforcement to prevent oil-canning or panel deflection. Always check gauge against both the duct's largest dimension and the system's design static pressure, not just one or the other.
What's the difference between slip-and-drive and flanged duct connections?
Slip-and-drive connections use a formed slip that one duct section slides into and is secured with drive cleats, common on smaller rectangular duct and lower-pressure systems. Flanged connections (TDC/TDF or similar) use a rolled or attached flange bolted or clipped between sections with a gasket, giving a more airtight, mechanically robust joint typically used on larger duct or medium/high-pressure systems.
When should stainless steel or aluminum duct be used instead of galvanized?
Stainless steel is specified for corrosive exhaust (chemical fumes, high-humidity or washdown environments) or where hygiene requirements rule out galvanized coatings, such as commercial kitchen grease duct. Aluminum is chosen where weight matters, such as duct supported on lightweight framing, or where mild corrosion resistance is needed without the cost of stainless.
How much air leakage is acceptable in a duct system?
SMACNA leakage classes (expressed in CFM per 100 sq ft of duct surface at a given test pressure) set the allowable leakage for a given pressure class and application, and low-leakage or 'sealed duct' construction is often specified for high-pressure and energy-code-driven projects. Leakage above the specified class both wastes conditioned-air energy and can throw off room-by-room airflow balancing even when the fan and equipment are correctly sized.
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