Sheet Metal and Fabrication Flashcards

What is the most common gauge of galvanized sheet metal used for residential ductwork?

Answer: 26 gauge (0.018")

26 gauge galvanized steel is the standard for most residential ductwork, providing adequate rigidity for typical residential pressure classes.

What is the purpose of 'seam' construction in sheet metal ductwork?

Answer: To join the longitudinal edges of sheet metal to form a duct shape, providing structural strength and airtight construction

Longitudinal seams join the two edges of flat sheet metal to form rectangular or round duct sections, providing structural rigidity and, when properly formed, airtight joints.

What is 'Pittsburgh lock' seam and when is it used in ductwork fabrication?

Answer: A longitudinal seam type using a rolled mechanical lock; standard for rectangular ductwork fabrication

The Pittsburgh lock is a longitudinal seam where one edge is formed into a 'Pittsburgh' channel profile and the other edge is inserted and crimped over—the standard seam for mechanically formed rectangular ductwork.

What is the difference between 'transverse' and 'longitudinal' joints in ductwork?

Answer: Transverse joints run across the duct (connecting duct sections end-to-end); longitudinal joints run along the length of a duct section (forming the duct shape from flat sheet)

Transverse joints connect individual duct sections end-to-end; longitudinal seams form the duct shape by joining edges along the duct's length.

What is 'TDF' (Transverse Duct Flange) connection?

Answer: A rolled flange formed on the duct end that accepts a flange connector (clip), providing airtight, rigid connections faster than slip-and-drive for commercial ductwork

TDF uses a formed flange rolled into the duct end, connected with corner clips. It provides faster installation, more airtight joints (with gasket), and greater structural rigidity than traditional slip-and-drive connections.

What is the difference between 'galvanized steel' and 'stainless steel' ductwork?

Answer: Galvanized is zinc-coated mild steel for standard applications; stainless steel is chromium-alloyed for corrosion-resistant applications like kitchen exhaust and chemical environments

Galvanized steel (hot-dipped zinc coating) serves standard HVAC applications. Stainless steel is required for grease exhaust (Type I hoods), chemical labs, marine environments, and other corrosive or high-humidity applications.

What is a 'radius elbow' versus a 'square throat elbow'?

Answer: A radius elbow has a curved inner turning surface; a square throat elbow has a sharp 90° inner corner; radius elbows have significantly lower pressure drop and turbulence

Radius elbows with smooth curved turning vanes significantly reduce pressure drop and turbulence compared to square-throat elbows, which create large areas of flow separation and recirculation.

What is 'duct leakage' and how does SMACNA classify acceptable leakage?

Answer: Airflow escaping through unsealed seams and joints; SMACNA specifies leakage classes (LC) of 3, 6, 12, 24, 48 CFM/100 sq ft at 1 inch WG, with lower LC numbers indicating tighter construction

Duct leakage wastes conditioned air and energy. SMACNA's leakage class (LC) system specifies allowable CFM leakage per 100 square feet of duct surface area at 1 inch WG test pressure.

What type of sealant is used for duct seams and joints to achieve airtight construction?

Answer: Water-based mastic duct sealant applied to all seams and joints, or pressure-sensitive foil tape meeting UL 181A or 181B

Water-based mastic sealant (duct mastic) or UL 181-listed foil tape are the only code-acceptable methods for sealing ductwork. Standard 'duct tape' (gray cloth tape) is not UL listed and fails within months.

What is a 'duct liner' and how does it differ from 'duct wrap'?

Answer: Duct liner is insulation installed inside the duct (thermal insulation + sound absorption); duct wrap is insulation applied to the outside of the duct (primarily thermal insulation for ducts in unconditioned spaces)

Duct liner (interior insulation) provides sound attenuation and thermal insulation inside the duct. Duct wrap (exterior insulation) wraps around the outside of ducts in unconditioned spaces for thermal insulation.

What is a 'standing seam' and when is it used in HVAC sheet metal work?

Answer: A longitudinal seam type where both edges are bent into standing flanges that are folded together; used for heavy gauge ductwork, outdoor casings, and higher-pressure applications

Standing seams are formed by bending both sheet metal edges into standing flanges (perpendicular to the sheet surface) that are then folded or locked together, providing strength and rigidity.

What is 'layout' in sheet metal fabrication?

Answer: The process of drawing the pattern (stretchout) on flat sheet metal before cutting and forming, using geometry to determine the exact flat dimensions of 3D shapes

Sheet metal layout (pattern development or stretchout) is the geometric process of drawing the exact flat pattern of a 3D fitting on flat sheet metal so that when cut and bent, it forms the correct 3D shape.

What is a 'slip coupling' or 'drive slip' in duct assembly?

Answer: A transverse connection system using S-shaped and Z-shaped sheet metal profiles that slide over duct section ends to connect rectangular duct sections

Drive slips (S-cleats) and slip connections are used at transverse joints of rectangular ductwork—S-shaped profiles slide over opposing duct ends and are secured with drive cleats hammered along the top and bottom.

What does 'gauge' indicate in sheet metal work, and is there a universal standard gauge system?

Answer: Gauge indicates metal thickness; there is no single universal standard—different gauge systems exist for steel, galvanized steel, aluminum, stainless steel, and copper

Sheet metal gauge indicates thickness, but different materials use different gauge systems—a '24 gauge' galvanized steel sheet, 24 gauge aluminum, and 24 gauge stainless steel are all different actual thicknesses.

What is a 'takeoff' or 'tap' in duct fabrication and what are common types?

Answer: A fitting that branches air from a main duct to a branch run, including starting collars, conical takeoffs, and stamped tees

Takeoffs (taps) connect branch ducts to main trunk ducts. Types include flat-boot, starting collar, spin-in collar, conical tap, and wye/tee fittings.

What is 'duct pressure testing' (duct leakage testing)?

Answer: Pressurizing a sealed duct system with a calibrated fan (duct blaster) and measuring airflow at a test pressure to calculate leakage rate

Duct leakage testing uses a calibrated blower (duct blaster) connected to the duct system after all registers are sealed. The fan pressurizes the ductwork to 25 Pa (0.1" WG), and the airflow required to maintain that pressure equals the leakage rate.

What are 'access doors' or 'access panels' in ductwork and where are they required?

Answer: Hinged or removable panels in ductwork that provide access for inspection, cleaning, and maintenance of internal components

Access panels provide service access to fire dampers, smoke detectors, coils, humidifiers, and other components located inside the ductwork—required by code wherever inspection, cleaning, or maintenance must occur.

What is the difference between a 'reducer' and a 'transition' fitting in sheet metal ductwork?

Answer: A reducer changes the cross-sectional size (area) within the same shape (rectangular to rectangular or round to round); a transition changes both size and shape (rectangular to round or vice versa)

A reducer changes duct size while maintaining the same shape (e.g., 24"×12" to 20"×12" rectangular; or 12" round to 10" round). A transition changes both size and shape, such as from a rectangular plenum to a round branch duct.

What is 'notching' in sheet metal fabrication?

Answer: Removing small squares or shapes from corners of sheet metal blanks before forming, allowing the metal to be bent into 3D shapes without overlapping material at corners

Notching removes material from the corners of flat sheet metal blanks so that when the sides are bent up, the corners can be folded without buckling or overlapping material.

What is a 'flexible duct connector' (canvas duct connector) and why is it used between equipment and rigid ductwork?

Answer: A short section of flexible material (canvas, neoprene, or similar) installed between the air handler/fan and rigid ductwork to isolate vibration and allow thermal expansion

Flexible connectors are installed between vibration-producing equipment (fans, air handlers) and rigid ductwork to absorb vibration and noise, preventing structure-borne vibration transmission through the duct system.