Ductwork Fabrication & Installation

Ductwork is the circulatory system of every HVAC installation. It carries conditioned air from the air handler to occupied spaces and returns spent air for reconditioning. Poorly designed, fabricated, or installed ductwork wastes energy, creates comfort complaints, generates noise, and shortens equipment life. As an HVAC sheet metal apprentice, you need to master duct fabrication techniques, fitting selection, sealing methods, support requirements, airflow calculations, and the code standards that govern duct installations. This guide covers the practical skills and knowledge you will use every day in the shop and on the job site.

Duct Types and Materials

Galvanized Sheet Metal

The industry standard for commercial and residential trunk lines, and the most durable duct material.

Gauge selection by duct size (SMACNA standards):

• Up to 12" width: 26 gauge
• 13" to 30": 24 gauge
• 31" to 60": 22 gauge
• 61" to 84": 20 gauge
• Over 84": 18 gauge

For round duct, gauge selection depends on diameter and pressure class. Higher-pressure systems require heavier gauge.

Advantages: Durable, fire-resistant, does not support mold growth, excellent for custom fabrication, long service life (50+ years).

Disadvantages: Heavier than alternatives, requires insulation for unconditioned spaces, can transmit noise (oil-canning, expansion pops).

Flexible Duct

Flexible duct (flex) consists of an inner liner of spiral wire and plastic film, covered with fiberglass insulation and an outer vapor barrier jacket. It is used for branch runs from the trunk line to individual supply registers.

Installation requirements per SMACNA and most codes:

• Pull flex tight - no more than 4% extra length beyond straight-line distance (a sagging flex duct has dramatic airflow loss)
• Support every 5 feet maximum, and within 3 feet of connections
• Maximum length varies by jurisdiction (typically 14-25 feet; shorter is better)
• Do not compress or kink - kinks severely restrict airflow
• Seal inner liner to the metal collar with mastic and a zip tie or drawband
• Available in insulated (R-4.2, R-6, R-8) and uninsulated versions

Pressure loss warning: A poorly installed flex duct (sagging, too long, kinked) can have 3-5 times the pressure loss of an equivalent rigid duct. This is one of the most common causes of airflow complaints.

Fiberglass Duct Board

Rigid fiberglass panels (typically 1" or 1.5" thick) fabricated into rectangular duct sections:

• Built-in insulation (R-4.3 for 1", R-6.5 for 1.5")
• Lighter than sheet metal
• Excellent sound attenuation
• Fabricated with a shiplap or v-groove closure
• Sealed with foil tape (UL 181) and staples
• Less durable than sheet metal - cannot withstand physical abuse
• Inner surface must have a smooth, sealed finish to resist mold and erosion
• Not suitable for kitchen exhaust, return air plenums in some codes, or high-velocity systems

Spiral (Round) Duct

Round sheet metal duct formed in a continuous spiral seam:

• Stronger than rectangular duct of equivalent size (round shape resists deflection)
• Lower friction loss per unit of airflow (round duct has the best perimeter-to-area ratio)
• Quieter than rectangular duct (less oil-canning and vibration)
• Lower leakage (fewer joints per linear foot)
• Less material per CFM delivered
• Harder to fit in tight spaces (rectangular duct has a lower profile for the same capacity)
• Joins with slip couplings, beaded fittings, or flanged connections

Sheet Metal Duct Fabrication

Reading the Plans

Before cutting any metal, study the HVAC plans:

• Duct layout - Shows the routing of trunk lines and branches
• Duct sizes - Width x height for rectangular, diameter for round
• CFM at each takeoff - The airflow requirement for each branch
• Fitting types - Elbows, transitions, tees, takeoffs specified by the engineer
• Pressure class - Determines gauge, reinforcement, and sealing requirements. Residential is typically 1" w.c. Commercial ranges from 2" to 10" w.c.
• Insulation requirements - R-value and whether internal or external

Layout and Pattern Development

Layout is the process of creating a flat pattern that, when cut and bent, produces the desired three-dimensional duct section or fitting.

Straight duct sections:

• Calculate the stretch-out (flat pattern width) = sum of all four sides + allowances for seams
• Pittsburgh lock seam: add approximately 1-1/4" for the pocket and 3/8" for the hook
• Mark the bend lines on the flat sheet with a scratch awl
• Cut to length (allow for connector flanges at each end)

Fittings (elbows, transitions, tees):

• More complex patterns require pattern development techniques:
  • Parallel line development - For rectangular elbows and transitions
  • Radial line development - For round tapered fittings (cones)
  • Triangulation - For irregular transitions (rectangular to round)
• Modern shops use CNC plasma cutters with pattern software, but understanding manual pattern development helps you verify CNC output and fabricate in the field

Cutting

• Aviation snips - Left (red), right (green), and straight (yellow) cutting. Use the correct snip for the cut direction to avoid distorting the metal.
• Hand notcher - For cutting corners and tabs at seam intersections
• Shear (foot-operated or electric) - For long straight cuts on light-gauge material
• Plasma cutter - For heavier gauge and complex shapes
• CNC plasma table - Production cutting with computer-generated patterns (shop fabrication)

Forming and Seaming

Brake (sheet metal brake):

• Makes straight bends in sheet metal
• Set the bend line, clamp, and bend to the desired angle
• Allow for spring-back (the metal springs back slightly from the bend angle)
• For long duct sections, a 10-foot brake is standard

Pittsburgh Lock Seam:
The most common longitudinal seam for rectangular duct:

1. Form a pocket (channel) on one edge using a Pittsburgh lock forming machine or hand seamer
2. Form a hook (90-degree flange) on the opposite edge
3. Insert the hook into the pocket
4. Hammer or roll the pocket flat to lock the seam
5. The result is a strong, airtight mechanical joint that does not require welding

Snap-Lock (Snap-Button) Seam:

• A simpler longitudinal seam formed by interlocking two folded edges and snapping them together
• Faster than Pittsburgh but not as strong
• Suitable for low-pressure residential ductwork

Transverse Joints (Connecting Duct Sections End to End)

S-Slip and Drive Cleat:

• S-slip connects two duct sides with an S-shaped strip
• Drive cleats fold over the remaining two sides to complete the connection
• Suitable for small to medium duct sizes in low-pressure systems
• Seal with mastic after assembly

TDC (Transverse Duct Connection) / TDF (Transverse Duct Flange):

• A formed flange on each duct section with bolt holes at corners
• Sections are bolted together with a gasket between flanges
• Standard for commercial ductwork - allows easy disassembly for maintenance
• TDC uses a factory-formed flange integral to the duct end
• TDF uses a separate roll-formed flange slipped over the duct end and crimped

Flanged and Companion Angle:

• Angle iron bolted around the duct end
• Sections bolt together with a gasket
• Used for high-pressure systems and large duct sizes

Common Fittings

Elbows

• Mitered elbow with turning vanes - Made from flat sheets with internal vanes that guide airflow through the turn. Compact but higher pressure drop without vanes. Turning vanes are critical for maintaining performance.
• Radius elbow - A smooth curved elbow with a centerline radius of 1.5 times the duct width (1.5W). Lower pressure drop and quieter than mitered elbows. Preferred where space allows.
• Round elbows - Available in adjustable (3-piece, 5-piece) or smooth radius. More pieces = smoother turn = lower pressure loss.

Important: Square-throat mitered elbows WITHOUT turning vanes have extremely high pressure loss and should not be used. The airflow separates from the inner wall and creates severe turbulence.

Transitions

• Change duct size or shape (rectangular to larger/smaller rectangular, rectangular to round, etc.)
• Keep the taper angle as gradual as possible - maximum 15 degrees per side for expansions, 30 degrees per side for reductions (SMACNA recommendations)
• Steep tapers cause turbulence, noise, and pressure loss
• Offset transitions (when the centerline shifts) are more complex to fabricate and create asymmetric flow

Tees, Wyes, and Takeoffs

• Tee - A 90-degree branch from the trunk line. Higher pressure loss due to abrupt direction change.
• Wye - A 45-degree branch from the trunk line. Lower pressure loss and preferred for efficient systems.
• Conical takeoff - Round branch connection with a spun metal fitting that mounts on the trunk line. Includes a built-in collar for flex duct connection.
• Volume dampers - Install a manual volume damper in every branch for airflow balancing. Accessible from the register end or through an access door.

Duct Sealing

Duct leakage is one of the largest sources of energy waste in HVAC systems. Studies show that typical residential duct systems leak 20-30% of conditioned air. Even "well-sealed" systems may lose 5-10% without intentional sealing.

Sealing Methods

Mastic (water-based duct sealant):

• The most durable and effective duct sealant
• Applied with a brush, gloved hand, or caulk gun (for some formulations)
• Apply to all longitudinal seams, transverse joints, and takeoff connections
• Use fiberglass mesh tape embedded in the mastic for joints wider than 1/4"
• Mastic does not dry out, crack, or peel like tape can
• UL 181B-M rated mastic for fiberglass duct board, UL 181A-M for metal duct

Foil tape (UL 181):

• Pressure-sensitive aluminum tape rated specifically for duct sealing
• Apply over clean, dry surfaces with firm hand pressure
• Must carry the UL 181A-P (metal duct) or UL 181B-FX (flex duct) listing
• Do NOT use cloth "duct tape" (the gray tape sold in hardware stores). Despite its name, cloth duct tape is not rated for HVAC ductwork. It dries out and falls off within 1-5 years.

Aerosol duct sealing (Aeroseal):

• A newer technology for existing duct systems
• The system pressurizes the ducts and injects a sealant fog
• Sealant particles accumulate at leak points and seal them from the inside
• Can reduce duct leakage by 80-90% in existing systems where access is limited
• Requires specialized equipment and training

Duct Leakage Testing

Duct blaster test:

• A calibrated fan and pressure gauge measure total duct leakage
• The fan pressurizes the duct system to 25 Pa (0.1" w.c.)
• Leakage is measured in CFM at 25 Pa (CFM25)
• Energy codes (IECC) typically require total leakage less than 4 CFM per 100 square feet of conditioned floor area
• Some jurisdictions require testing to the outside only (leakage to unconditioned spaces)

Duct Insulation

All ductwork running through unconditioned spaces (attics, crawl spaces, garages, unconditioned basements) must be insulated to prevent energy loss and condensation.

Insulation Types

• Duct wrap (fiberglass blanket with foil facing): The most common external insulation. Wrap around the duct with the foil facing out, staple the longitudinal seam, and seal all seams and joints with foil tape. Available in R-4.2, R-6, and R-8.
• Rigid board (fiberglass or foam): Used for rectangular duct in commercial applications. Adhered with spray adhesive and sealed with tape.
• Spray-on insulation: Applied in the field for irregular shapes and existing ductwork.
• Internal liner (fiberglass): Installed inside the duct to provide both insulation and sound attenuation. The smooth inner surface must face the airflow. Used in commercial air handlers and first few feet of ductwork.

Code Requirements

• IECC (International Energy Conservation Code) specifies minimum R-values by climate zone:
  • Climate Zones 1-2: R-6 for supply, R-4.2 for return
  • Climate Zones 3-4: R-8 for supply, R-6 for return
  • Climate Zones 5-8: R-8 for supply, R-8 for return
• Supply ducts in unconditioned spaces generally need more insulation than return ducts
• Insulation vapor barrier must face the warm side (exterior in cooling climates, interior in heating climates) to prevent condensation

Duct Sizing and Airflow Calculations

The Basics of Airflow

• CFM (Cubic Feet per Minute) - The volume of air flowing through the duct per minute
• Velocity (FPM - Feet per Minute) - How fast the air is moving
• Static pressure (inches of water column, in. w.c.) - The resistance to airflow in the system
• Fundamental relationship: CFM = Area (sq ft) x Velocity (FPM)

Duct Sizing Methods

Equal friction method (most common for residential):

• Select a friction rate (pressure loss per 100 feet of duct), typically 0.08" w.c. per 100 ft for residential
• Use a duct calculator (slide rule, app, or chart) to find the duct size for the required CFM at the selected friction rate
• All sections of the duct system are sized to the same friction rate, simplifying the design

Static regain method (commercial):

• Sizes each duct section so that the static pressure remains approximately constant throughout the system
• As velocity decreases (larger duct), some velocity pressure converts back to static pressure (regain)
• More complex but produces more balanced systems for large commercial installations

Velocity Limits

Maintaining appropriate air velocity controls noise:

Location	Maximum Velocity (FPM)
Main trunk (residential)	700-900
Branch ducts (residential)	500-700
Supply registers	500-750
Return grilles	300-500
Main trunk (commercial)	1,200-2,000
Branch ducts (commercial)	800-1,200
NC-25 occupied spaces (office)	Size per ASHRAE noise criteria

Duct Size Calculation Example

Problem: Size a branch duct for 150 CFM using the equal friction method at 0.08" w.c./100 ft.

Using duct calculator: At 150 CFM and 0.08" friction rate:

• Round duct: 7" diameter
• Equivalent rectangular: 8" x 6" or 10" x 5"

Verify velocity: Area of 7" round = pi x (3.5/12)² = 0.267 sq ft. Velocity = 150 / 0.267 = 562 FPM. This is within the acceptable range for a branch duct.

Total External Static Pressure (TESP)

The air handler's fan must overcome the total static pressure of the entire duct system. TESP is the sum of:

• Friction losses in all straight duct sections
• Dynamic losses in fittings (elbows, tees, transitions)
• Losses through filters, coils, grilles, and registers
• Losses through any accessories (dampers, silencers, heat recovery)

Typical design TESP for residential systems: 0.5" w.c.
Maximum TESP for most residential air handlers: 0.8" w.c.

If TESP exceeds the air handler's rated capacity, airflow will be reduced, efficiency drops, and the system may not deliver adequate heating or cooling.

Duct Support and Hanging

Properly supported ductwork maintains its shape, alignment, and seal integrity over decades.

Support Methods

• Strap hangers - Galvanized steel or poly strapping looped under the duct and attached to overhead structure. Most common for rectangular duct.
• Trapeze hangers - Angle iron or channel strut spanning between two hanger rods. Used for larger ducts and grouped runs.
• Saddle supports - Shaped supports that cradle round duct from below.

Support Spacing

Per SMACNA standards and most codes:

• Rectangular duct (horizontal): Maximum 10 feet apart (8 feet is better practice)
• Round duct (horizontal): Maximum 12 feet apart
• Flexible duct: Maximum 5 feet apart, within 3 feet of connections
• Vertical risers: Support at each floor and at intervals not exceeding 15 feet
• Additional support at heavy fittings, transitions, and equipment connections

Seismic Bracing

In seismic zones, ductwork must be braced to prevent damage during earthquakes:

• Lateral and longitudinal bracing per SMACNA seismic guidelines
• Bracing required at specified intervals based on duct size and seismic zone
• Flexible connections where ducts connect to equipment to accommodate differential movement

Safety Practices

Shop Safety

• Sheet metal cuts are extremely common and can be severe. Always wear work gloves when handling cut sheet metal. Even seemingly smooth edges can cut deeply.
• Eye protection is mandatory when cutting, drilling, or working overhead
• Hearing protection when operating shears, brakes, or plasma cutters
• Machine guards must be in place on all shop equipment. Never reach into a shear, brake, or roll former while it is operating.
• Proper lifting for sheet metal stock - sheets are heavy, awkward, and have sharp edges. Use two people for large sheets.

Job Site Safety

• Overhead work - When hanging duct from ladders or lifts, ensure stable footing and use fall protection when required
• Fire safety - Keep fire extinguisher nearby when brazing or welding duct connections
• Sharp edges - Cap or tape duct openings and sharp fitting edges to protect yourself and other trades walking through the work area
• Dust and fiberglass - Wear respiratory protection when cutting fiberglass duct board or insulation. Wear long sleeves to prevent skin irritation.
• OSHA scaffold and ladder requirements apply to all duct installation work

Common Mistakes in Duct Installation

1. Not sealing duct joints - "We'll seal it later" often means it never gets sealed. Seal as you go.
2. Sagging flex duct - Support every 5 feet and keep it pulled tight. Sagging flex dramatically reduces airflow.
3. Sharp bends in flex duct - The inner liner collapses, restricting airflow to a fraction of the rated capacity. Use a hard elbow or rigid duct for turns tighter than the duct's minimum bend radius.
4. Undersized return duct - Returns are often undersized because they are less visible. An undersized return restricts airflow across the entire system. The return duct area should be at least equal to the supply duct area.
5. Not installing volume dampers - Without dampers in branch runs, there is no way to balance the system. Install them during fabrication, not as an afterthought.
6. Ignoring duct leakage to outdoors - A supply duct leaking in an attic is delivering conditioned air to the outdoors. This is the most expensive type of duct leakage.
7. Running duct through fire-rated assemblies without fire dampers - Code requires fire dampers where ducts penetrate fire-rated walls and floors (with some exceptions per the mechanical code).

Key Takeaways

• Use galvanized sheet metal for trunk lines and keep flex duct short, straight, and well-supported
• Seal all duct joints and seams with mastic or UL 181-rated foil tape - never cloth "duct tape"
• Insulate ductwork in unconditioned spaces to the R-value required by the energy code
• Size ducts based on required CFM, acceptable velocity, and available static pressure
• Keep total external static pressure within the air handler's rated capacity
• Balance the system after installation using volume dampers and airflow measurement
• Support ductwork at proper intervals to maintain shape and seal integrity