Specifying spiral duct in 2026 is a diameter, gauge, and lining decision before it is a price decision: standard stock galvanised round covers 3"–28" at most regional fabricators [S3], heavy-duty spiral is routinely produced up to 60" for industrial trim and dust systems [S2], and custom round or flat-oval runs reach 72" on ovalisers [S3]. The build is a continuously helically-formed lock-seam (or button-punch on heavier gauges), which is why spiral duct typically beats rectangular on leakage class and pressure loss per foot when the run is straight and accessible.
Use this guide when the run is round or oval, when the system pressure class is above +/- 2 in. w.g., or when the owner wants fewer field joints. For tight plenum ceilings, low-pressure residential HVAC, or cost-driven residential supply, rectangular galvanised sheet still wins on material cost per CFM, and a control valve selection on the upstream trim loop should be re-validated before the duct gauge is fixed, because the duct and the trim valve share the same static budget.
Diameter Range and Stock vs Custom Sourcing
Stock spiral duct at regional US fabricators is concentrated in 3"–28" round [S3]; above 28" most shops move to make-to-order. G.F. Puhl publishes an in-house spiral range of 3"–60" out of its Gallatin, Tennessee facility, with custom fittings fabricated alongside the pipe [S2]. DC Duct & Sheet Metal in Mount Rainier, Maryland, extends custom round and flat-oval to 72" using an on-site ovaliser that converts round spiral into the flat-oval profile the job needs [S3]. Buyers should treat 3"–28" as off-the-shelf, 30"–60" as short custom lead time, and above 60" as engineered.
For low-pressure dust and trim collection, diameters 6"–24" dominate because they pair with standard 110V/220V workshop blowers and cyclone inlets. Above 30", expect a step-change in gauge and stiffener requirements. The forming machines that produce these diameters are sourced heavily from China: Made-in-China lists spiral tube forming machines alongside general tube-making and roll-forming lines as a top-ranking product class [S1]. A typical small electric shearing machine used in the duct-fab cell lists at US$1,000–50,000 per piece, MOQ 1, with 1,000 units/month capacity out of Maanshan [S4].
Gauge, Material, and Lining Choice
Galvanised steel (G60/G90) is the default for HVAC and dust systems and is what DC Duct stocks in standard sizes [S3]. For paper trim and dust lines the upgrade is heavy-gauge galvanised plus sealed seams; G.F. Puhl markets the build as heavy-duty spiral duct with the fittings fabricated in-house to keep the system matched [S2]. Stainless (typically 304 or 316) is selected for food, pharmaceutical, and corrosive dust; the price step is roughly 3–5x galvanised in coil form, and lead time stretches because mills schedule stainless differently. Aluminium spiral is used where weight matters (rooftop, modular skids) and accepts anodising but is soft, so gauge is bumped two steps vs steel for the same diameter.
Antimicrobial-coated galvanised duct and flat sheet are now stocked lines, not specials, at regional fabricators like DC Duct [S3]. For high-temperature applications (above ~400 °F continuous), galvanise burns off; aluminised steel or stainless is required, and the spiral lock-seam is replaced with a welded or button-punch seam to maintain joint strength at temperature. When the duty is abrasive dust (wood, paper trim, mineral), the standard move is 16–18 gauge with a heavier lock-seam and 45° lateral branches, not a lighter gauge with internal liners, because liners wear and become a maintenance liability.
SMACNA Compliance, Leakage Class, and Fittings
SMACNA HVAC Duct Construction Standards is the default reference for galvanised spiral in commercial and industrial work, and DC Duct states that its round spiral, flat oval, and fittings adhere to current SMACNA guidelines [S3]. The 2026 practical consequence: spiral lock-seam with factory gasket at the coupling is the lowest-leakage factory option for round, and is the reason engineers spec round spiral over rectangular when the leakage budget is tight. The fittings inventory at a competent regional shop includes male and female couplers, end caps, reducers, 90° and 45° elbows, stamped elbows, saddle taps with and without dampers, and bell mouths with and without dampers [S3] — each fitting is a leakage and pressure-loss node, so the spec should fix the fitting class, not just the pipe.
G.F. Puhl publishes lead-time behaviour on fittings as a differentiator: spiral duct and fittings are fabricated in the same facility, which lets them support emergency replacement runs without sourcing a coupling from a third party [S2]. DC Duct publishes a 48-hour lag between duct delivery and fitting availability on custom work — useful for installers who batch duct and then forget to call in the elbow count [S3]. For balance, see a parallel decision on hydraulic power unit sizing — both decisions follow the same logic: lock the duty envelope first, then pick components that share a delivery batch.
Cost Tiers and 2026 Price Bands
Inputs in 2026 keep galvanised coil relatively stable but the duty mix has shifted up: more 22–24 gauge residential stock, more 18–20 gauge commercial, and a clear step up to 16 gauge for industrial dust. Reference price points for adjacent forming equipment on Made-in-China put a small electric shearing machine at US$1,000–50,000 per piece, MOQ 1, with monthly capacity of 1,000 units out of Maanshan [S4]. Round spiral fittings on the same marketplace show entry-level offers at US$0.70 per piece, MOQ 100 pieces, from Zhejiang-based Diamond Member suppliers — illustrative of the commodity floor for stamped, mass-produced round components [S5].
Spiral duct itself, per foot, scales non-linearly with diameter: a 6" galvanised 24-gauge stock run sits in the low single-digit USD per foot, 12"–18" lands in the mid single-digit, and 30"+ custom gauges often cross into the low double-digit per foot before fittings. These are qualitative bands, not quotes — the shipper, the gauge, the seam type, and the gasket class move the number more than the steel index. For comparison, the same diameter-vs-cost trade-off drives inline pipeline pump selection and vacuum generator choice: as size rises, the spec sheet matters more than the headline price.
Use Cases and Where NOT to Pick Spiral
Spec spiral round or oval when the system is: dust collection (paper trim, wood, plastic, mineral), industrial ventilation with high humidity, commercial HVAC where leakage class matters, food-grade exhaust, and any run with more than two 90° direction changes. The aesthetic is cleaner, the joint count is lower, and the leakage class beats rectangular at the same gauge [S3]. For trim and dust specifically, G.F. Puhl positions spiral duct as the load-bearing element of the system: "without the network of ductwork and fittings, trim collection systems are nothing more than a collection of spare parts" [S2] — meaning the duct is the system, not a commodity input.
Do NOT pick spiral when: the plenum height is constrained below the round duct diameter, the run is short and rectangular will do, the budget is for a low-pressure residential retrofit where galvanised rectangular stock is cheaper per CFM, or the duty is so corrosive (chlorine, strong acids) that even 316L has a short life and welded stainless with PTFE lining is required. Do NOT pick spiral for temporary HVAC on construction sites where reusable flexible duct is the economic answer. And do NOT specify spiral without committing to the fitting class — a spiral pipe with rectangular-style stamped elbows is the most common field failure I see, because the two systems have different leakage assumptions.
Sourcing, Standards, and Closing Checks
For 2026 procurement, run a four-point check before issuing the PO: (1) diameter fits the stock range or is committed to a custom line with a written lead time — 3"–28" stock, 30"–60" short custom, 60"+ engineered [S3][S2]; (2) gauge and material match the duty — 16–18 gauge for abrasive dust, 18–20 for commercial HVAC, 22–24 for residential, stainless or aluminised above 400 °F; (3) seam and coupling class match the leakage budget — lock-seam plus gasket for low-leak, button-punch or welded for high-temperature and high-pressure dust; (4) fittings batch is called in with the duct, not after — regional shops quote 48 hours for fittings behind duct delivery on custom runs [S3], and forgetting this delays the startup by a week. Sourcing paths split between domestic regional fabricators (DC Duct in Maryland, G.F. Puhl in Tennessee) for short lead-time and engineered work, and import channels for commodity fittings and forming equipment [S1][S5].
Trackable signals into the next buying window: coil price moves on the G90 galvanised index, any SMACNA revision on leakage class for spiral, and forming-machine lead times out of Maanshan and Zhejiang — a 2–3 week stretch on spiral tube forming machines [S1] usually leads a 4–6 week stretch on regional fabricator lead times two months later.
Spec-level background on the components involved: linear guide, and wiring duct.