HVAC Sheet Metal and Fabrication — Questions and Answers

Question 1: What is the most common gauge of galvanized sheet metal used for residential ductwork?

30 gauge (0.012")
26 gauge (0.018") (Correct answer)
24 gauge (0.024")
20 gauge (0.036")

Correct answer: 26 gauge (0.018")

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

Sheet metal gauge is a counterintuitive numbering system—higher gauge numbers indicate thinner metal. For galvanized steel: 30 gauge = 0.012" (very thin), 26 gauge = 0.018" (standard residential), 24 gauge = 0.024" (commercial), 22 gauge = 0.030" (heavy-duty commercial). SMACNA standards specify minimum gauge requirements based on duct width/diameter and pressure class.

Question 2: What is the purpose of 'seam' construction in sheet metal ductwork?

To allow thermal expansion of the ductwork as it heats and cools
To join the longitudinal edges of sheet metal to form a duct shape, providing structural strength and airtight construction (Correct answer)
To create mounting points for hanging the ductwork from the structure
To allow sections to be separated for cleaning access

Correct 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.

Longitudinal seams run the length of a duct section, joining the two long edges to form the duct shape. Common seam types include: Pittsburgh lock (standard for rectangular ductwork), snap lock (simpler self-seaming for lighter applications), standing seam (for heavier gauge, higher-pressure applications), and drive slip (for round ductwork). Seam integrity is critical for airtightness.

Question 3: What is 'Pittsburgh lock' seam and when is it used in ductwork fabrication?

A high-security locking mechanism for equipment access panels
A longitudinal seam type using a rolled mechanical lock; standard for rectangular ductwork fabrication (Correct answer)
A transverse connection between duct sections using slip flanges
A corner seam used only at duct elbows

Correct 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.

The Pittsburgh lock seam is produced on automated duct forming machinery. The process: a flat sheet is run through a Pittsburgh machine that forms one edge into a folded channel. When the sheet is formed into a rectangular shape, the bent edge is inserted into the Pittsburgh channel, then the channel wings are crimped over the inserted edge, mechanically locking the seam. Modern sheet metal shops use CNC plasma cutting, automatic notching, and automated machines to produce duct sections rapidly.

Question 4: What is the difference between 'transverse' and 'longitudinal' joints in ductwork?

Transverse joints connect duct to equipment; longitudinal joints connect duct sections together
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) (Correct answer)
Transverse joints use flanges; longitudinal joints use slips
Transverse joints are for rectangular duct; longitudinal joints are for round duct

Correct 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.

Longitudinal joints (seams): run parallel to airflow direction, forming the duct shape from flat sheet metal (Pittsburgh lock, snap lock, standing seam). Transverse joints: connect separate duct sections end-to-end perpendicular to airflow; they include slip couplings, drive slips, S-cleat connections, and flange connections. Transverse joint quality significantly affects system airtightness.

Question 5: What is 'TDF' (Transverse Duct Flange) connection?

Thermal Duct Fitting; used only for insulated ductwork connections
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 (Correct answer)
A type of flexible duct connection for residential systems
A self-sealing tape used for commercial ductwork connections

Correct 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.

Transverse Duct Flange (TDF) systems have largely replaced slip-and-drive connections in commercial sheet metal work. The duct end is formed with a standing flange. Mating sections are connected by inserting corner clips into the flange corners and then snapping flange clips along the four sides. Foam gasket tape between the flanges provides the air seal. TDF connections are required for medium and high-pressure ductwork per SMACNA.

Question 6: What is the difference between 'galvanized steel' and 'stainless steel' ductwork?

Galvanized is zinc-coated mild steel for standard applications; stainless steel is chromium-alloyed for corrosion-resistant applications like kitchen exhaust and chemical environments (Correct answer)
Galvanized is for outdoor use; stainless is for indoor use only
Galvanized is standard for commercial; stainless is standard for residential
There is no functional difference; stainless is just cosmetically preferred

Correct 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.

Galvanized steel is mild steel coated with zinc through hot-dip galvanizing. Zinc provides sacrificial corrosion protection but can be attacked by acids and strong alkalis. Galvanized ductwork is appropriate for standard HVAC applications in non-corrosive environments. Stainless steel (typically 304 or 316 grade) is required for kitchen grease exhaust systems (Type I cooking hoods), chemical laboratory exhaust, coastal/marine environments, and food processing areas.

Question 7: What is a 'radius elbow' versus a 'square throat elbow'?

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 (Correct answer)
A radius elbow is used in circular ducts; square throat in rectangular ducts
A radius elbow has a larger radius but same pressure loss; square throat is preferred for space constraints only
The terms are interchangeable—both have the same pressure loss characteristics

Correct 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.

Square throat (hard 90°) elbows create significant flow separation as air cannot follow the sharp inner corner—a large recirculation zone forms, effectively reducing available duct area and causing high turbulence and pressure loss. Radius elbows with turning radius = 1.5× duct width dramatically reduce pressure loss. Single-thickness turning vanes in a square-throat elbow can reduce pressure loss by 60-70% compared to a bare square-throat elbow.

Question 8: What is 'duct leakage' and how does SMACNA classify acceptable leakage?

Air leaking from ductwork is called duct loss; SMACNA allows 100% leakage at low pressure
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 (Correct answer)
Condensate leaking from cooling coils; SMACNA classifies by volume per hour
Refrigerant leaking from split systems connected to ducts; SMACNA specifies ppm limits

Correct 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.

SMACNA's leakage classes (LC 3, 6, 12, 24, 48) specify maximum allowable leakage. For low-pressure residential systems (1/2" to 1" WG): LC 24 or LC 12 is typical. Commercial medium-pressure systems: LC 6 required. High-pressure systems: LC 3 required. ASHRAE 90.1 and energy codes increasingly require duct leakage testing to verify tightness.

Question 9: What type of sealant is used for duct seams and joints to achieve airtight construction?

Standard silicone caulk from a hardware store
Water-based mastic duct sealant applied to all seams and joints, or pressure-sensitive foil tape meeting UL 181A or 181B (Correct answer)
Duct tape (cloth-backed adhesive tape) is the industry standard
Fiberglass mesh tape embedded in joint compound

Correct 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.

Water-based mastic sealant is applied with a brush to all seams and joints, fills gaps, hardens, and is rated for high temperature and humidity. For gaps larger than 1/4", fiberglass mesh tape is embedded in the mastic for reinforcement. UL 181A and 181B listed pressure-sensitive foil tapes are also acceptable. Standard gray cloth 'duct tape' is explicitly NOT acceptable per energy codes—it dries out, loses adhesion, and falls off within 1-3 years.

Question 10: What is a 'duct liner' and how does it differ from 'duct wrap'?

Duct liner is fiberglass on the outside; duct wrap is fiberglass on the inside—they have opposite effects
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) (Correct answer)
They are identical products with different names used in different regions
Duct liner is rigid foam board; duct wrap is flexible fiberglass

Correct 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.

Duct liner (internal insulation): Typically 1" or 2" fiberglass board installed inside rectangular ductwork. Provides noise attenuation, thermal insulation, and vapor barrier properties. Reduces effective duct cross-section (must account for in duct sizing). Duct wrap (external insulation): Flexible fiberglass batts with vapor retarder facing, applied around the outside of ducts running through unconditioned spaces. Provides thermal protection without affecting duct cross-section or acoustics.

Question 11: What is a 'standing seam' and when is it used in HVAC sheet metal work?

A seam that extends vertically above the surface; used for roofing on HVAC equipment enclosures
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 (Correct answer)
A seam that resists standing water infiltration; used for floor-level ductwork
A seam produced without machinery, standing seam is a hand-fabricated joint

Correct 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.

Standing seams are strong longitudinal joints where both sheet metal edges are bent perpendicular to the sheet surface, then the two standing flanges are folded together. Types include single standing seam, double standing seam, and snap-lock standing seam. Standing seams are appropriate for heavy-gauge industrial ductwork, outdoor equipment enclosures, architectural sheet metal, and HVAC equipment access doors.

Question 12: What is 'layout' in sheet metal fabrication?

The arrangement of duct sections in the building drawing
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 (Correct answer)
The process of hanging and supporting installed ductwork
Arranging sheet metal pieces in the shop for efficient cutting to minimize waste

Correct 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.

Sheet metal layout is the craft of pattern development. Three fundamental layout methods: Parallel line development (for prisms—rectangular duct, square elbows). Radial line development (for conical shapes—round tapers, conical elbows). Triangulation (for complex shapes—square-to-round transitions). Information needed: fitting dimensions, allowances for seams, flanges, and bends. CAD systems and duct fitting software have largely replaced manual layout in modern shops.

Question 13: What is a 'slip coupling' or 'drive slip' in duct assembly?

A flexible coupling allowing angular misalignment between duct and equipment
A transverse connection system using S-shaped and Z-shaped sheet metal profiles that slide over duct section ends to connect rectangular duct sections (Correct answer)
A slip joint allowing thermal expansion in long straight duct runs
A sliding damper built into a duct fitting

Correct 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.

Traditional rectangular duct transverse connections use 'slip and drive' construction. S-slips are slid over the top and bottom flanged edges of two mating duct sections, straddling both edges. Drive slips (J-cleats) are then hammered along the two sides, engaging with the flanged edges to lock the S-slips in place. Properly installed, slip and drive connections require sealant applied to all joints for airtightness.

Question 14: What does 'gauge' indicate in sheet metal work, and is there a universal standard gauge system?

Gauge measures the alloy composition of the metal; there is one universal standard
Gauge indicates metal thickness; there is no single universal standard—different gauge systems exist for steel, galvanized steel, aluminum, stainless steel, and copper (Correct answer)
Gauge measures the hardness of the metal; aluminum and steel use the same system
Gauge is the same as the imperial inch measurement divided by 100

Correct 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.

Multiple, non-interchangeable gauge systems exist: US Standard Gauge (for uncoated steel), Galvanized Steel Gauge, Manufacturers' Standard Gauge (MSG, for stainless steel), and Brown & Sharpe (B&S) or AWG for non-ferrous metals including aluminum and copper. Example: 24 gauge galvanized steel = 0.0276" but 24 gauge aluminum = 0.0201". Always include both gauge number AND decimal thickness for clarity when specifying sheet metal.

Question 15: What is a 'takeoff' or 'tap' in duct fabrication and what are common types?

The starting point for duct installation at the air handler
A fitting that branches air from a main duct to a branch run, including starting collars, conical takeoffs, and stamped tees (Correct answer)
The section of duct where the ductwork ends at a register or grille
A measurement used when cutting duct to account for fittings

Correct 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.

Takeoffs and taps are branch connections from main supply or return air trunk ducts. Types: Flat-boot: installed on the main duct, accepts a flexible duct connection to a supply register. Starting collar: installed in a hole cut in the main duct, accepts round or flex duct. Spin-in collar: a round collar with a built-in manual damper blade—very common for residential flex duct branches. Conical tap: a tapered conical fitting that extracts air with lower pressure loss; preferred for commercial ductwork.

Question 16: What is 'duct pressure testing' (duct leakage testing)?

Testing ductwork with a pressure gauge during normal system operation to check for blockages
Pressurizing a sealed duct system with a calibrated fan (duct blaster) and measuring airflow at a test pressure to calculate leakage rate (Correct answer)
Testing duct joint strength by applying mechanical force
Checking static pressure at various points in the duct system with a manometer

Correct 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.

Duct pressurization testing quantifies how airtight a duct system is. Procedure: all supply registers, return grilles, and air handler openings are sealed. A duct blaster connected to an access point pressurizes the ductwork to 25 Pa above ambient. The measured airflow through the fan equals the leakage flow. Results are expressed as CFM25. California Title 24, ASHRAE 90.1, and Energy Star require duct leakage testing. Total leakage ≤4 CFM25 per 100 square feet of conditioned floor area is a common target.

Question 17: What are 'access doors' or 'access panels' in ductwork and where are they required?

Decorative covers over supply registers for aesthetic purposes
Hinged or removable panels in ductwork that provide access for inspection, cleaning, and maintenance of internal components (Correct answer)
Doors in equipment rooms providing access to ductwork above ceilings
Protective covers installed where ductwork penetrates fire-rated assemblies

Correct 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.

Code-required access locations include: fire dampers and fire/smoke dampers (must be operable and inspectable per NFPA 90A—access within 18" of damper), smoke detectors installed in ductwork, VAV box actuators, duct-mounted coils for cleaning, humidifiers, and flexible connections for inspection. Access doors must maintain the duct's pressure classification and fire rating if in a fire-rated shaft.

Question 18: What is the difference between a 'reducer' and a 'transition' fitting in sheet metal ductwork?

They are interchangeable terms for the same fitting
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) (Correct answer)
A reducer is only used in round ductwork; a transition is only used in rectangular ductwork
A reducer increases duct size; a transition decreases duct size

Correct 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.

Reducers change the cross-sectional area while maintaining the same cross-sectional shape. Transitions change both size and shape, typically from rectangular to round or round to rectangular. Transitions are particularly common at air handling unit connections (typically rectangular) that connect to round ductwork. Proper transition angle (gradual taper, typically 15° or less) minimizes turbulence and pressure drop.

Question 19: What is 'notching' in sheet metal fabrication?

Creating decorative patterns on sheet metal for aesthetic HVAC panels
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 (Correct answer)
A method of creating seam connections using small cuts
Cutting holes in sheet metal for duct penetrations through walls or floors

Correct 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.

When bending flat sheet metal into a 3D shape such as a rectangular duct section or fitting body, the corners of the flat blank would overlap if not addressed. Notching removes the precise amount of material needed so that when all four sides are bent up to 90°, the corners meet cleanly. The notch depth equals the bend radius plus metal thickness. Automated duct fabrication lines include plasma or mechanical notching stations integrated into the production flow.

Question 20: What is a 'flexible duct connector' (canvas duct connector) and why is it used between equipment and rigid ductwork?

A flexible elbow used to navigate tight spaces in the building structure
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 (Correct answer)
A type of flexible duct used for all residential branch runs
A connector that allows easy disassembly of ductwork sections for cleaning

Correct 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.

Flexible duct connectors (also called vibration isolators or canvas connections) are short sections of flexible material installed between an air handling unit or fan housing and the rigid supply and return ductwork. They serve three purposes: vibration isolation (prevents compressor and fan vibration from transmitting into the rigid duct system and building structure), thermal expansion accommodation (allows relative movement between the equipment and building structure as temperatures change), and misalignment absorption (compensates for minor alignment discrepancies during installation). Materials include coated fiberglass fabric, neoprene-coated fabric, or silicone-coated fabric. They must be UL listed for the application and pressure class. Improperly installed flexible connectors (too taut or too long) defeat their purpose. SMACNA specifies maximum length (typically 10-12 inches) and installation requirements.

Question 21: What is a 'fire damper' and what is its function in HVAC ductwork?

A motorized damper that controls airflow to a zone for comfort control
A passive device installed where ductwork penetrates fire-rated barriers, designed to close automatically when exposed to heat to prevent fire from spreading through ductwork (Correct answer)
A damper that reduces noise from the air handler by restricting airflow velocity
A balancing damper installed to equalize airflow between duct branches

Correct answer: A passive device installed where ductwork penetrates fire-rated barriers, designed to close automatically when exposed to heat to prevent fire from spreading through ductwork

Fire dampers are passive safety devices installed at penetrations of fire-rated walls, floors, and ceilings. They contain a fusible link that melts at elevated temperatures (typically 165°F), releasing a spring-loaded blade that closes to stop fire and hot gases from passing through the duct opening.

Fire dampers are required by the International Mechanical Code (IMC) and NFPA 90A wherever HVAC ductwork penetrates fire-rated barriers (1-hour and 2-hour fire-rated walls, floors, and ceilings). A fusible link rated at 165°F (standard) or 212°F (high-temperature) holds the damper blade open during normal operation. When the link reaches its rated temperature, it melts and releases the spring-loaded damper blade, which closes instantly. The damper must meet UL 555 requirements for fire dampers. After a fire event, the damper must be manually reset (fusible link replaced) and inspected per NFPA 80. Fire/smoke combination dampers (UL 555 + UL 555S) are motorized to close for smoke control and also have a fusible link for thermal closure. Access doors must be provided within 18 inches of every fire damper for inspection and testing.

Question 22: What is a 'flexible duct' (flex duct) and what are the installation requirements for proper performance?

Flexible duct can be installed in any configuration without restrictions; it is simply a convenient alternative to rigid ductwork in all applications
Flexible duct is a pre-insulated duct with a flexible inner liner; it must be fully extended (not compressed or kinked), properly supported, and installed with gradual bends to achieve rated airflow (Correct answer)
Flexible duct must only be used for short connections under 2 feet; longer runs require rigid ductwork
Flexible duct has identical pressure drop to rigid ductwork of the same diameter

Correct answer: Flexible duct is a pre-insulated duct with a flexible inner liner; it must be fully extended (not compressed or kinked), properly supported, and installed with gradual bends to achieve rated airflow

Flexible duct is convenient for residential branch runs but has higher friction losses than rigid duct when compressed, kinked, or poorly supported—proper installation requires full extension, support every 4 feet, and gentle bends of at least 1 duct-diameter radius.

Flexible duct (flex duct) consists of a helical wire core covered with an inner film, fiberglass insulation, and a vapor-retarder outer jacket. When installed correctly (fully extended, supported every 4 feet maximum, bends ≥1 duct diameter radius, no sags), it has approximately 25-50% higher friction loss per foot than equivalent rigid smooth metal duct. When compressed or kinked, friction loss increases dramatically—compressed flex duct can have 10-20× higher resistance than rated. ASHRAE 90.1 limits flex duct to 5-foot maximum run lengths in many applications. Installation requirements per SMACNA and manufacturers: pull duct straight and connect at both ends before cutting to length (prevents compression), support with broad straps (not wire) every 4 feet, maintain minimum bend radius, no bends exceeding 90°, and verify correct duct size (undersized flex duct is a very common cause of poor airflow).

Question 23: What is 'duct static pressure' and how is it measured during HVAC system testing and balancing?

The dynamic pressure created by air velocity in the duct, measured with a pitot tube connected to a manometer
The pressure difference between the inside of the duct and the surrounding atmospheric pressure, measured with a magnehelic gauge or digital manometer using a static pressure tip inserted through the duct wall (Correct answer)
The pressure at the fan outlet only, used to determine fan blade angle
The friction pressure drop across the entire duct system from fan to registers

Correct answer: The pressure difference between the inside of the duct and the surrounding atmospheric pressure, measured with a magnehelic gauge or digital manometer using a static pressure tip inserted through the duct wall

Static pressure is the 'pushing' pressure of air against the duct walls, measured in inches of water column (WG) using a static pressure probe and manometer. It indicates available pressure to push air through the distribution system.

Static pressure in ductwork represents the potential energy available to move air through the distribution system against friction and fitting losses. It's measured in inches of water column (inches WG) using a static pressure tip (a tube with a hole perpendicular to airflow) connected to a magnehelic gauge, inclined manometer, or digital pressure gauge. Typical measurement locations: external static pressure (across the air handler—supply outlet to return inlet), duct static pressure (at various points in the supply main to verify pressure available at VAV boxes or dampers), and across components (filters, coils, heat exchangers) to detect excessive pressure drop indicating clogging. ACCA Manual D and SMACNA specify design static pressure values; exceeding them indicates undersized ducts or excessive fittings. Total external static pressure for residential systems is typically designed for 0.1 to 0.8 inches WG.

Question 24: What is 'spiral duct' (spiral wound duct) and what are its advantages over rectangular ductwork for commercial applications?

Spiral duct is rectangular ductwork with a spiral reinforcing rib pattern on the outside; used only for aesthetic purposes
Spiral duct is round or oval ductwork formed by spirally winding sheet metal strip, creating a rigid helical seam; advantages include lower air leakage, less turbulence, easier installation, and better acoustics than rectangular duct (Correct answer)
Spiral duct uses a spiral airflow pattern to increase air mixing in the occupied space
Spiral duct is flexible ductwork that spirals around obstacles in tight mechanical spaces

Correct answer: Spiral duct is round or oval ductwork formed by spirally winding sheet metal strip, creating a rigid helical seam; advantages include lower air leakage, less turbulence, easier installation, and better acoustics than rectangular duct

Spiral wound round ductwork has a continuous helical seam formed during manufacturing, providing superior airtightness compared to rectangular ductwork's multiple longitudinal and transverse joints—while also offering reduced pressure drop due to the aerodynamically efficient round cross-section.

Spiral duct (also called spiral round or oval duct, or Spiro duct after a major manufacturer) is produced by a spiral winding machine that forms metal strip into a round tube with a continuous helical lock seam. Advantages over rectangular ductwork: Lower air leakage—the continuous factory-formed spiral seam is inherently more airtight than the many joints of rectangular duct fabrication; typical leakage is 2-5× lower. Lower friction loss—round cross-section has the most favorable surface area-to-volume ratio, reducing friction per unit of airflow. Easier installation—fittings snap together, fewer pieces to fabricate. Better acoustics—round shape and smooth interior reduce turbulence-generated noise. Disadvantages: Less space-efficient (round duct requires more overhead clearance than flat rectangular duct of similar capacity). Used extensively in commercial and industrial HVAC, laboratories, and cleanrooms where airtightness and low turbulence are priorities.

HVAC Sheet Metal and Fabrication

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

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