A duct run can look straightforward on a drawing, then lose performance, become difficult to clean or fail to fit once it reaches site. This spiral duct fitting guide explains how to select the fittings around your ductwork properly, whether you are planning a commercial kitchen extraction system, a workshop extraction line or an industrial ventilation installation.
Spiral duct is a practical choice for exposed and concealed runs alike. Its rigid, round profile supports efficient airflow and gives installers a consistent system to work with. But the pipe itself is only part of the job. Elbows, branches, reducers, caps and connection pieces determine the final route, resistance and serviceability of the system.
Start with the duct diameter, not the fitting
Every fitting must match the duct diameter and the required airflow route. It sounds obvious, but many installation problems start when fittings are selected by what is readily available rather than what the system design requires.
A 200mm spiral pipe needs a 200mm compatible fitting. Where the run changes size, use a correctly sized reducer rather than forcing a smaller spigot into a larger section or relying on excessive sealant. The connection should be secure, round and accessible for fixing.
Diameter selection is tied to airflow demand and fan duty. A small duct may be easier to conceal and less expensive to buy, but it can increase air velocity and resistance. An oversized duct can take up valuable space above a canopy or along a workshop wall. The right balance depends on the extraction rate, length of run, number of direction changes and the type of contaminants being extracted.
For commercial kitchens, ducting should be considered as part of the whole ventilation system, from canopy connection through to fan and discharge point. Kitchen extract ductwork should be planned in line with DW172 requirements, while the ductwork construction and installation approach should reflect DW144 principles.
The main spiral duct fittings and where they belong
Bends and elbows
Elbows change the direction of a duct run. They are often required where extraction ducting passes from a canopy riser into a horizontal run, turns around structural steelwork or routes towards an external discharge point.
The sharper the turn, the more resistance it creates. A tight 90-degree elbow may save space, but it is not always the best choice for system performance. Where layout permits, a larger-radius bend or two gentler changes of direction will generally give air a cleaner path. This matters most on longer systems, high-demand extraction runs and installations already containing several fittings.
Keep bends to a sensible minimum. A duct route with repeated tight turns can make a correctly sized fan work harder than expected, increasing noise and reducing effective extraction at the canopy or pickup point.
Tees, branches and junctions
Tees allow one duct run to split into two, or enable a secondary pickup point to join a main extract line. They are useful in workshops with more than one extraction area and in larger catering installations with separate canopy sections.
A tee is not simply a convenient joining point. The branch direction affects how air enters the main run. Poorly planned branches can disrupt airflow and create unequal extraction between points. A more gradual branch arrangement is often preferable where performance is the priority and space allows.
If two extract points require substantially different airflow volumes, the duct design may need more than a standard tee. In those cases, consider the fan position, balancing requirements and whether separate duct runs provide a more reliable result.
Reducers
Reducers connect two different duct diameters. They are commonly needed when a fan connection, canopy outlet or existing duct run does not match the main spiral duct size.
Use a purpose-made reducer with the correct end sizes. Avoid sudden, improvised reductions wherever possible. A gradual reduction is normally kinder to airflow than an abrupt step change, particularly where the reduction is close to a fan inlet or follows a bend.
Reducing the duct size does not create more extraction. It changes the air velocity and can increase resistance. If a system has poor capture at the canopy, the answer is not automatically to fit smaller ducting. Check the fan selection, route length, fitting count and overall design before changing the diameter.
Spigots, couplers and joining pieces
Spigots and couplers form the working connections between duct sections and equipment. They need to suit the component being connected, whether that is spiral pipe, a canopy plenum, fan connection, silencer or vent cap.
A secure mechanical connection matters. Duct joints should be aligned properly, fixed in accordance with the installation requirements and sealed appropriately for the application. A loose or poorly sealed joint can leak air, allow grease-laden vapour to escape in kitchen extraction systems and create unwanted noise.
Check which component is designed to fit inside or over the adjoining section before ordering. Assuming every item has the same connection arrangement is a common cause of site delays.
Silencers and vent caps
A silencer is fitted where controlling transmitted fan noise is a priority. It is not a replacement for correct fan selection or sensible duct routing, but it can be a useful part of a well-planned system. Allow sufficient straight duct and physical space for the silencer where required, rather than treating it as an afterthought.
A vent cap protects the external discharge termination while allowing extracted air to leave the system. Its location must be considered carefully. The discharge should not create nuisance, reintroduce extracted air into air intakes or conflict with the building layout. External termination requirements may also be affected by planning, landlord and local authority considerations.
Plan fittings around access and cleaning
In commercial kitchen extraction, grease management is a practical maintenance issue, not just a specification detail. Duct routes should support safe inspection and cleaning. Long sections with multiple bends, inaccessible junctions or no sensible access strategy can become costly to maintain.
Where grease-laden air is present, avoid creating unnecessary pockets where deposits can build up. Think about where access doors may be needed and whether the route can realistically be reached after ceilings, services and finishes are in place.
Workshop and industrial systems have different contaminants, but the same principle applies. Fine dust, debris or process residue can collect at changes in direction and low points. The fitting layout should suit the material being extracted and the maintenance regime the site can actually carry out.
Material and thickness: standard versus custom
For many standard ducting applications, galvanised spiral duct and fittings provide a practical, cost-effective solution. However, the right material depends on the environment, extracted air and required durability.
Stainless steel may be specified where corrosion resistance, hygiene or a visible finish is required. In commercial kitchens, surrounding fabrications such as canopies, wall cladding and prep tables may use stainless steel, but that does not mean every duct component must automatically be made from the same grade. Specify according to the duty of the part.
CanopyMan manufactures stainless steel products in-house and can accommodate made-to-order requirements where a standard component or material choice is not suitable. Standard stainless steel products use 430-grade steel at 0.7mm thickness, while 304-grade stainless and alternative thicknesses can be supplied for applications that justify the additional cost.
The trade-off is straightforward: standard products are usually faster and more economical, while custom sizes, grades and thicknesses give greater control where the installation has particular technical or site constraints.
Avoid the fitting mistakes that cost time on site
Before placing an order, check the full duct route rather than compiling a list one item at a time. Confirm duct diameters, centre-line dimensions, elevations, equipment connection sizes and the direction of every bend. If a fitting has an orientation, make sure it suits the direction of airflow and the available installation space.
Also allow for supports. Spiral ducting and its fittings need appropriate support along the run, particularly around heavier items such as fans, silencers and external termination sections. Do not expect a lightweight duct joint to carry equipment weight that should be independently supported.
Finally, separate what is standard from what needs fabrication. A standard elbow, tee or reducer can often keep cost and lead time under control. A non-standard transition, unusual offset or bespoke canopy connection may need to be made to suit the installation. Identifying this early avoids trying to solve a design issue with unsuitable off-the-shelf parts.
A good duct fitting schedule gives installers clear, compatible components and gives the finished system a better chance of performing as intended. If the route is complex, resolve the awkward connections on the drawing first, then order the ductwork that fits the job rather than making the job fit the ductwork.