Custom Spiral Duct Manufacturing: Galvanized Steel, Custom Lengths & Tapped Holes [2026]

What Is Custom Spiral Duct?

Custom spiral duct is round metal ductwork manufactured to non-standard specifications — custom lengths, custom diameters, heavier gauges, specialty materials, or additional fabrication such as tapped holes, access doors, and welded connections. It starts with the same helical lockseam manufacturing process as standard spiral duct, but each order is produced to the buyer's exact engineering requirements.

The demand for custom spiral duct comes from three primary buyer groups:

• HVAC contractors who need precise lengths to eliminate field cutting and reduce jobsite waste. A 22-inch section for a specific ceiling plenum run, for example, is far more efficient than cutting a 5-foot standard piece on site.
• Industrial OEMs who integrate spiral duct into manufactured equipment — dust collectors, paint booths, drying systems, air handling units — and need duct with pre-drilled or tapped holes for sensors, gauges, and access ports.
• Mechanical engineers specifying non-standard diameters, heavier gauges for high-pressure systems, or specialty materials (stainless steel, aluminum) for corrosive or high-temperature environments.

At USFlexDuct, we manufacture custom spiral duct at our facility in Brookshire, TX. Our parent company, Airwise Parts, has 30 years of sheet metal fabrication experience — meaning your custom order is produced by experienced metalworkers, not outsourced to a third-party shop.

Standard vs Custom Spiral Duct: When Do You Need Custom?

Standard spiral duct covers a wide range of HVAC applications. But there are specific situations where custom manufacturing saves you time, money, and installation headaches. Here is the honest comparison:

Rule of thumb: If you are cutting more than 30% of your standard-length duct on site, you are wasting material and labor. Custom lengths eliminate that waste. If you need any holes, non-standard connections, or specialty materials, custom is the only option.

Customization Options for Spiral Duct

1. Custom Lengths

This is the most common custom request. Standard spiral duct ships in 5-foot or 10-foot sections, but real-world HVAC runs rarely divide evenly into those lengths. Custom-length spiral duct is cut to your specified dimension (to the nearest 1/2 inch) during production — the cut is clean, square, and burr-free, unlike field cuts with a grinder or snips.

Real-world example: A Wisconsin-based metal fabricator recently ordered 1,500 pieces of 5-inch spiral duct at exactly 22 inches long with tapped holes for assembly into their manufactured equipment. Standard 5-foot sections would have required cutting every piece and discarding 38 inches of waste per unit — 4,750 feet of wasted material.

2. Custom Diameters

Standard spiral duct comes in even-inch increments (4", 6", 8", etc.). But engineering calculations sometimes call for odd diameters — 5", 7", 9", 11", 13", 15" — or metric sizes for equipment designed to international standards. Our spiral forming machines accept tooling for any diameter from 3 inches to 60 inches.

Odd-diameter duct is particularly common in industrial dust collection, fume exhaust, and OEM equipment where the duct must match an existing port size on a machine, blower, or collector housing.

3. Material Selection

Galvanized steel (ASTM A653 G90) is the standard material for spiral duct — and the right choice for 90% of HVAC applications. But specific environments demand different metals:

• 304 Stainless Steel — Chemical plants, food processing, pharmaceutical cleanrooms, coastal marine environments where chloride-laden air corrodes galvanized coatings within 2-3 years.
• 316 Stainless Steel — Offshore platforms, sulfuric acid fume exhaust, swimming pool dehumidification systems. The molybdenum content in 316 resists pitting corrosion from chlorides and acids that 304 cannot withstand.
• Aluminum (3003-H14) — Lightweight applications where duct must be suspended from bar joists that cannot carry the weight of steel, or corrosive atmospheres where aluminum's natural oxide layer provides sufficient protection. Also used for exposed architectural duct where a softer, non-sparking surface is desired.

4. Tapped Holes and Penetrations

Covered in detail in the tapped holes section below. This includes threaded holes for sensors, punched holes for branch takeoffs, and weld-on couplings for heavy instrumentation.

5. End Connections

How the duct connects to the next section or fitting matters for both air-tightness and installation speed:

• Crimped end — One end reduced by approximately 1/4" to slip inside the next section. Standard for residential and light commercial.
• Beaded end — A rolled bead at the duct end that locks into a clamp coupling. Common in industrial applications for easy disassembly and cleaning.
• Flanged (angle ring) — A steel angle ring welded or riveted to each end, with bolt holes for gasketted flange-to-flange connections. Required by SMACNA for ducts above 3" w.g. static pressure class. Standard in industrial and process ductwork.
• Vanstone flange — The duct end is rolled outward to form a flat lip that sits against a loose backup ring. Allows angular adjustment during installation.
• Grooved end — A circumferential groove rolled near the duct end for use with grooved mechanical couplings (Victaulic-style). Fast, tool-free assembly — popular in data center cooling and modular mechanical rooms.

Available Sizes: Diameter, Gauge & Maximum Length

The following table shows our standard production capabilities for custom galvanized spiral duct. Stainless steel and aluminum are available in the same diameters but may have different gauge options — contact us for specific material-diameter combinations.

Gauge upgrade options: Any diameter can be specified in a heavier gauge than the SMACNA minimum shown above. Common reasons to upgrade include negative-pressure exhaust systems (to prevent duct collapse), outdoor installations subject to wind load, and environments where physical impact damage is likely. Specify the heavier gauge when requesting a quote — there is no additional lead time, only a material cost increase proportional to the weight difference.

Tapped Holes & Instrumentation Ports

Tapped holes in spiral duct allow you to thread sensors, probes, and test instruments directly into the duct wall without drilling on site. This is critical for applications where measurement accuracy, air-tightness, and repeatability matter.

Common Tapped Hole Applications

• Temperature sensors — RTDs and thermocouples threaded into 1/4" or 3/8" NPT tapped holes for monitoring supply/return air temperature in process HVAC and industrial drying systems.
• Static pressure taps — 1/4" NPT holes with 1/8" brass barb fittings for connecting to manometers or building automation system (BAS) pressure transducers. Placed upstream and downstream of filters, coils, and fans per ASHRAE 111 testing guidelines.
• Humidity probes — 1/2" or 3/4" tapped holes for insertion-type RH sensors in cleanroom, pharmaceutical, and data center applications.
• Pitot tube ports — 5/8" or 3/4" holes at specific traverse points for airflow measurement per ASHRAE 111 or AMCA 803. We can place multiple holes at the exact angular positions required by the traverse pattern.
• Access ports for cleaning — Larger tapped holes (1" or 2" NPT) with threaded plugs for inspection and cleaning access in dust collection and kitchen exhaust systems.
• Drain ports — 1/2" tapped holes at the 6 o'clock position for condensate drainage in cooling systems and outdoor duct runs.

Tapped Hole Specifications

We offer both UNC (Unified National Coarse) and NPT (National Pipe Thread) tapped holes:

• UNC threads: 1/4"-20, 5/16"-18, 3/8"-16, 1/2"-13 — for machine screws and bolted sensor mounts.
• NPT threads: 1/8" NPT, 1/4" NPT, 3/8" NPT, 1/2" NPT, 3/4" NPT, 1" NPT — for pipe-threaded sensors, fittings, and couplings. NPT holes are tapped to the correct taper (1/16" per inch) for leak-free threaded connections with Teflon tape or pipe dope.

Positioning accuracy: Holes are positioned to +/- 1/16" along the duct length and +/- 2 degrees around the circumference. For OEM production runs, we fixture the duct on a CNC-controlled punch/tap station to ensure hole-to-hole repeatability across every piece in the order.

When specifying tapped holes, provide: thread size, thread type (UNC or NPT), distance from one end of the duct, and clock position (12 o'clock = top of duct). We will confirm the specification on a shop drawing before production begins.

How Custom Spiral Duct Is Manufactured

Understanding the manufacturing process helps you specify custom duct correctly and set realistic expectations for lead time and cost. Here is what happens to your order from quote to shipment:

Step 1: Coil Stock Preparation

Galvanized steel arrives at our facility in coils — 48-inch-wide master coils that are slit to the correct strip width for the specified diameter. The strip width determines the duct diameter (wider strip = larger diameter). For a 5-inch spiral duct, the strip is approximately 4.5 inches wide; for a 24-inch duct, approximately 8.5 inches wide. The steel is ASTM A653 G90 — meaning 0.90 oz/ft2 of zinc coating on each side for corrosion resistance.

Step 2: Spiral Forming

The steel strip is fed into a spiral forming machine that simultaneously bends the strip into a helix and interlocks each revolution with the previous one through a continuous lockseam. The machine operator sets the diameter (via forming rolls), the seam pitch, and the feed speed. Our machines produce duct at 15-40 feet per minute depending on gauge and diameter. The lockseam is mechanically crimped — no welding, adhesive, or sealant is used in the seam itself.

Step 3: Cut to Length

A flying shear or rotary cutter cuts the duct to the specified length while the machine continues running. For custom lengths, the cut position is controlled by a programmable length counter accurate to +/- 1/8 inch. Each piece is verified with a tape measure before moving to the next station.

Step 4: End Treatment

Depending on the order specification, the duct ends receive one of the treatments described in the customization options section: crimping, beading, flanging, or left raw. Crimping and beading are done inline on the spiral machine. Flanging is a secondary operation performed on a separate flanging press.

Step 5: Hole Punching and Tapping

For orders requiring tapped holes, the finished duct sections move to a punch/tap station. Holes are first punched to the correct pilot diameter, then tapped with the specified thread. For production runs, a CNC-controlled fixture ensures consistent hole positioning across all pieces. Each tapped hole is checked with a go/no-go thread gauge.

Step 6: Inspection and Packaging

Every custom spiral duct section is inspected for diameter (measured at both ends with a pi tape), length, seam integrity (visual and tactile), and — for tapped holes — thread quality. Finished duct is nested (smaller diameters inside larger ones where possible) and banded for shipping. Flanged duct is palletized to prevent flange damage in transit.

Quality Standards: SMACNA, ASHRAE & UL

Custom spiral duct for commercial and industrial HVAC must comply with several industry standards. Here is what we build to and what you should specify:

SMACNA Duct Construction Standards

The Sheet Metal and Air Conditioning Contractors' National Association (SMACNA) publishes the definitive guide to duct construction in North America. Our custom spiral duct meets SMACNA Round Duct Construction Standards for:

• Gauge selection — Minimum steel thickness for each diameter and pressure class (see our spiral duct sizing guide for the full SMACNA gauge chart).
• Seam classification — Our spiral lockseam is a Class A seam per SMACNA, rated for all pressure classes.
• Joint types — Slip joints, flanged joints, and mechanical couplings per SMACNA Table 3-7.
• Reinforcement — For diameters above 24 inches and negative pressure above 2" w.g., we add external reinforcing angles per SMACNA spacing tables.

ASHRAE Standards

ASHRAE Standard 90.1 (Energy Standard for Buildings) requires duct systems to meet specific leakage rates. Spiral duct with a sealed lockseam inherently meets ASHRAE Leakage Class 6 (CL 6) — meaning less than 6 CFM of leakage per 100 square feet of duct surface at 1" w.g. test pressure. This is significantly better than rectangular duct (CL 12-24) and is one of the primary reasons engineers specify spiral for energy-efficient buildings.

Material Standards

• ASTM A653/A653M — Hot-dip galvanized steel sheet. Our standard G90 coating provides 0.90 oz/ft2 zinc per side.
• ASTM A240 — Stainless steel sheet (for 304 and 316 custom orders).
• ASTM B209 — Aluminum sheet (for 3003-H14 custom orders).

MOQ, Pricing & Lead Time

Custom spiral duct pricing depends on four factors: material, diameter/gauge, custom fabrication (holes, flanges), and quantity. Here is what to expect:

Minimum Order Quantities

• Custom length only (standard diameter, galvanized steel): 10 pieces minimum
• Custom length + tapped holes: 20 pieces minimum
• Non-standard diameter (odd sizes, metric): 50 pieces minimum (tooling setup required)
• Stainless steel or aluminum: 50 pieces minimum
• Flanged ends: 20 pieces minimum per flange specification

Typical Lead Times

• Galvanized, standard diameters, custom length only: 5-7 business days
• Galvanized, with tapped holes or flanges: 7-10 business days
• Non-standard diameters: 10-15 business days
• Stainless steel or aluminum: 10-15 business days
• Complex custom (multiple operations): 15-20 business days

Pricing Guidance

We quote every custom order individually because specifications vary widely. As a general reference for galvanized steel spiral duct:

• 3"-12" diameter, custom length: $3-8 per linear foot (quantity dependent)
• 14"-24" diameter, custom length: $8-18 per linear foot
• 26"-48" diameter, custom length: $18-45 per linear foot
• Tapped holes: Add $1.50-3.00 per hole (setup fee may apply for fewer than 20 pieces)
• Flanged ends: Add $8-25 per end depending on flange type and diameter

Stainless steel is typically 2.5-4x the galvanized price. Aluminum is 1.5-2.5x. Volume discounts apply — orders of 500+ pieces receive significant per-unit reductions. Request a quote with your exact specifications for firm pricing.

How to Order Custom Spiral Duct

Getting a quote for custom spiral duct is straightforward. Send us the following information and we will return a firm quote within 24 hours:

1. Diameter — Inside diameter in inches (specify if OD is required instead).
2. Length — Specify in inches or feet + inches. Tolerance is +/- 1/8".
3. Gauge / Material — Galvanized (standard), 304 SS, 316 SS, or aluminum. Specify gauge if different from SMACNA minimum.
4. End treatment — Crimped, beaded, flanged, grooved, or raw. Specify which end(s).
5. Holes — Thread size, thread type (UNC or NPT), distance from reference end, clock position. A sketch or drawing is ideal.
6. Quantity — Number of pieces per specification.
7. Delivery location — For freight quote.

We accept specifications via email, phone, or our online quote form. For complex orders, send a PDF or CAD drawing and our engineering team will review it, create a shop drawing for your approval, and confirm pricing and lead time.

Related Spiral Duct & Metal Duct Guides

• Spiral Duct Sizes & Cost Per Foot — complete diameter, gauge chart, and pricing reference for standard spiral duct
• Custom Air Ducts & Custom Ducting Solutions — custom flexible duct manufacturing for non-standard diameters, materials, and insulation
• Duct Fittings: Elbows, Tees, Reducers & Transitions — every HVAC duct fitting type with pressure drop equivalents and sizing rules
• Rigid Galvanized Steel Dryer Duct — 4" dryer duct sizing, gauge selection, and IRC code requirements
• Rectangular Duct Guide — when rectangular is the right choice and how it compares to spiral round