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Spiral, rectangular or flexible ductwork: which to use?

Updated 23 October 2026 · SEO Dons Editorial

What are the main types of commercial ductwork?

Sheet metal ductwork is made in rectangular, circular and flat oval sections, and BESA says makers choose between them according to the space available and the airflow the design needs. DW/144, BESA's specification, covers construction standards for all three shapes. Spiral ductwork is circular duct, and flexible duct sits outside a sheet metal specification: in England it is dealt with in Approved Document F instead.

TypeWhere the sources place itWatch for
RectangularTight ceiling voids, risers and plant rooms, and flush installationsMore friction than circular; keep the proportions near square
Circular (spiral)Covered by BS EN 12237 for strength and leakage; one maker describes a gasketed range certified to tightness class DDepth needed for the diameter; joint type and tested class
Flat ovalA third sheet metal shape within DW/144's construction standardsAsk the designer why it was chosen
FlexibleFinal connections only, with length kept to a minimumLength, sealing at each end, pressure loss

BESA says DW/144 is aimed at commercial and industrial air distribution running in normal conditions up to 70 degrees C. Approved Document L gives DW/144, or three British Standards, as the routes for ductwork, so a specification for commercial ductwork would be expected to cite one of them. The questions are the same in every city, and the Approved Documents cited here apply in England, so check the rules for Scotland, Wales and Northern Ireland separately. A line that says only commercial ductwork hides the choices above, so ask for the shape, pressure class and leakage class of each run.

How do they compare on space and installation?

BESA says rectangular duct makes efficient use of limited space, especially in ceiling voids, risers and plant rooms, and suits a flush installation. It is often used for main trunks feeding smaller branches, and its shape makes fire dampers, access panels and silencers easier to fit.

Circular duct trades that flexibility for a shape BESA describes as lower in friction. One maker, Lindab, says its gasketed circular range is quick and easy to mount and airtight. That is a manufacturer's description of its own product, not a finding that applies to every circular system, so ask any supplier for the tightness class and the test evidence.

Which is tighter, and which uses less fan energy?

Shape alone does not make a duct tight. BS EN 12237 sets strength and leakage requirements for circular ductwork and says its methods also apply to rectangular ductwork for leakage, while BS EN 1507 does the equivalent for rectangular sections. Approved Document L Volume 2 says ductwork should comply with DW/144 or with BS EN 1507, BS EN 12237 and BS EN 13403, so both shapes face the same test. BESA adds that poor sealing can cause excessive leakage. The DW/144 leakage guide explains the classes and the test.

Fan energy is where shape shows. BESA says rectangular ducts typically create more frictional resistance than circular ducts, although careful design can minimise pressure losses. Squeezing a rectangular duct into a shallow void can work against that. BESA says very wide or very narrow ducts have more surface area, which raises friction losses and material use, and that DW/144 recommends proportions close to square, an aspect ratio of about 1:1.

Support matters as well. BESA says ducts that are not reinforced or supported at the intervals DW/144 specifies may sag, twist or deflect beyond acceptable limits, and that supporting and sealing becomes more challenging in long rectangular runs.

Approved Document F Volume 2 asks for ductwork designed to keep overall pressure losses low, by limiting duct length and bends and sizing ducts for the airflow. Approved Document L Volume 2 then limits specific fan power by system type. Undersizing raises fan energy, and BESA says oversizing can create noise, which fits Approved Document F's call to size and join ducts correctly to minimise noise.

How do insulation, fire rules and access affect the choice?

Approved Document L Volume 2 says ductwork carrying warm or cold air should be insulated along its entire length. Insulation adds thickness to every run, which matters most in the tight voids where rectangular duct is chosen.

For fire, Approved Document B Volume 2 says ductwork should not become a route for fire and smoke to travel through the building. Where air handling ducts pass through fire-separating elements, it says their fire performance should be preserved using one or more of four methods, and a duct passing a cavity barrier should be fire resisting (minimum E 30) or fitted with a suitable fire damper. It also says two of those methods should not be used for kitchen extract, because grease can affect dampers. The HVAC dampers guide covers the damper types and their testing.

Approved Document F Volume 2 asks for reasonable access to maintain ventilation systems, including points for cleaning ductwork. Whatever the shape, the design or quote should show where the access panels sit, because damper testing and cleaning both depend on them. The TR19 Air duct cleaning guide explains what a cleaner then looks for.

What limits apply to flexible duct?

Approved Document F Volume 2 says rigid ducts should be used wherever possible. Flexible duct is acceptable only where necessary, for final connections, with the length kept as short as possible, and it should meet BSRIA's BG 43/2013. Connections should be mechanically secured and adequately sealed, with rigid connectors and jubilee clips used for flexible ducting.

The guidance gives no maximum length in metres. The figure comes from the designer's specification and the maker's data, so a quote that shows long flexible runs should state the length and the standard it was specified to. Approved Document F also asks designers to minimise overall pressure losses, so a long flexible run needs justifying.

What drives the cost of each option?

Prices come from quotes, so this page gives none. The drivers are the pressure class, which BESA says affects sheet thickness, jointing and leakage limits, and the leakage class and testing. Fittings, insulation, dampers and access panels follow, and so does the installation method: BESA warns that heavier metal or fussier fabrication than the job needs slows installation and adds expense. Fan energy over the system's life belongs in the comparison too. The commercial ventilation cost page sets out what moves a ventilation quote.

Questions to put to the designer or contractor:

  1. Which pressure class and leakage class is each duct run specified to, and will it be tested?
  2. Why was each shape chosen, and what does it do to the fan's specific fan power?
  3. What is the longest flexible run, and where does it sit?
  4. Which fire strategy applies where ducts cross fire-separating elements?

Ducts are one part of the wider commercial ventilation and MVHR system. You can send an enquiry through the form on this site.

Sources

Common questions

Is flexible duct allowed in commercial buildings?

Approved Document F Volume 2 says rigid ducts should be used wherever possible and that flexible ducts may be used for final connections, with their lengths kept to a minimum. It also says flexible ductwork should meet BSRIA's BG 43/2013. It gives no figure in metres, so ask the designer to state the limit in the specification.

Does circular duct leak less than rectangular?

Neither shape is tight by itself. BS EN 12237 sets strength and leakage requirements for circular ductwork and BS EN 1507 does so for rectangular, and DW/144 sets leakage classes for both. The result depends on the class specified, the sealing and a test, not the shape.

Do fire dampers fit any duct shape?

BESA says the rectangular shape makes it easier to fit fire dampers, access panels and silencers. Approved Document B Volume 2 asks for fire dampers to conform to BS EN 15650 and for access for inspection, testing and maintenance, whichever shape the duct is.

Who decides the duct type on a project?

DW/144 is written for system designers, contractors, manufacturers and clients, and BESA describes the designer's role as including pressure break points, damper types and material finishes. The shape, class and fittings should therefore be fixed in the design that every quote is priced against.

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