A poorly routed duct run can turn a premium extraction system into an expensive source of noise, heat, odours and maintenance problems. The fan may be correctly specified and the canopy may capture contaminants effectively, but airflow is only as good as the route between the extract point and the discharge outlet. Knowing how to route exhaust ducting properly protects performance from day one and makes the system easier to clean, inspect and operate.
For commercial kitchens, workshops and industrial facilities, duct routing should be decided early - before ceilings are closed, services are fixed in place or expensive alterations are needed. The best route is rarely the one that simply looks shortest on a drawing. It must balance airflow resistance, fire safety, access, building structure, discharge location and the realities of installation.
Start with the air volume and duct size
Ducting is not a generic add-on. Its diameter or rectangular dimensions must suit the air volume required by the canopy, hood or extraction point. If the duct is undersized, air velocity rises, pressure losses increase and the fan has to work harder. That can create excessive noise, wasted energy and poor capture at the source.
Oversizing is not automatically the answer either. In grease-laden kitchen extraction, for example, velocity needs to remain sufficient to carry contaminants through the system rather than allowing deposits to build up. The correct size depends on the intended airflow, the type of extract, the duct material, the number of fittings and the total route length.
This is why routing and fan selection must be considered together. A fan selected only by airflow volume, without calculating the system resistance, may not deliver the duty required once bends, filters, attenuators and discharge grilles are installed.
Keep the route short, straight and purposeful
Every metre of ductwork and every change in direction adds resistance. The practical aim is to use the shortest viable route with the fewest possible bends. A direct vertical rise to roof level is often effective where the building layout allows it, particularly for high-temperature or grease-laden extract. In other sites, a carefully planned horizontal run to an external wall may be more practical.
Avoid sending ductwork around obstacles simply because there is space above a ceiling. Long, winding routes cost more to fabricate and install, increase fan duty and make cleaning more difficult. They also create more joints, supports and potential points of air leakage.
Where a direction change is unavoidable, use long-radius bends where space permits. Tight elbows create more turbulence and pressure loss. Two closely spaced bends are especially inefficient, so leave a straight section between fittings wherever practical. Good routing is about preserving smooth airflow, not just getting from point A to point B.
Do not reduce the duct size mid-run
A reduction may seem like an easy way to pass through a restricted void, but it can undermine the entire extraction design. If a constrained area cannot accommodate the required duct size, assess an alternative route, revise the duct shape or consider whether a different plant arrangement will provide a better result.
Flat rectangular ducting can help in low ceiling voids, but it must be correctly sized to maintain the required free area and airflow characteristics. It should be fabricated and supported to a standard appropriate for the application, especially where grease, heat or industrial contaminants are involved.
Plan bends, branches and transitions carefully
The duct route needs to reflect how air will actually travel through the system. Sudden transitions, poorly formed branches and sharp take-offs cause turbulence and uneven extraction. This is particularly problematic on systems serving multiple capture points, where one branch may pull strongly while another underperforms.
Use properly designed transitions to move between circular and rectangular ductwork or between different duct sizes. Keep transitions gradual rather than abrupt. For branched systems, position balancing dampers where they can be accessed during commissioning and future adjustment.
A common mistake is allowing building constraints to dictate every detail of the route. Structural steel, pipework and cable trays matter, but extraction ducting has its own operational requirements. Coordinate services early so the duct run has a defined corridor rather than being forced through whatever gaps remain.
Choose a discharge point that protects people and premises
The outlet is not merely the end of the duct. Its location affects odour control, re-entrainment, neighbouring properties, staff comfort and compliance. Exhaust air should discharge where it cannot be readily drawn back into air intake louvres, openable windows, doors or nearby ventilation systems.
For commercial kitchen extraction, roof-level discharge is frequently preferred because it provides better dispersion away from people and adjacent buildings. However, the right solution depends on the building height, surrounding properties, planning considerations, local authority requirements and the type of cooking or process being extracted.
Wall discharge can be suitable in some applications, but it needs careful assessment. A discharge point beside a delivery area, public pavement, staff entrance or neighbouring boundary is likely to create complaints and operational problems. If odours, smoke or grease vapour are present, additional treatment such as electrostatic precipitation, carbon filtration or odour-control equipment may be required as part of the wider design.
Do not position the discharge where wind effects are likely to push extracted air back towards the building. The local roofline, parapets, taller neighbouring structures and prevailing conditions can all affect dispersion. Site-specific judgement is essential.
Build in access for cleaning and inspection
An extraction route that cannot be cleaned cannot be managed properly. This is a major consideration for kitchen grease extract and for industrial applications that generate dust, oil mist, fumes or process residue. Access doors should be included at sensible intervals and close to changes in direction, riser bases and other areas where deposits are likely to collect.
Access panels must be large enough for the intended cleaning method and positioned where a contractor can reach them safely. Hiding them above a fixed ceiling or behind permanently installed equipment may make the system look tidy initially, but it creates a costly maintenance issue later.
For grease extract systems, duct construction, access provisions and cleaning arrangements should be designed with recognised UK industry guidance in mind. Fire safety requirements, insurance expectations and the nature of the cooking operation all influence the specification. A low-volume café operation and a high-output restaurant line should not be treated as identical projects.
Consider fire separation, materials and supports
Ductwork passing through walls, floors or fire compartments requires careful coordination. Penetrations may need suitable fire-stopping, and the duct arrangement must not compromise the building's fire strategy. Grease extract has additional risks because combustible deposits can accumulate inside the ductwork if cleaning is neglected.
Material selection also matters. Galvanised steel may suit many general ventilation applications, while stainless steel is often the stronger choice where hygiene, moisture resistance, corrosive environments or grease handling are priorities. The correct gauge, jointing method and finish depend on the duty of the system.
Support ductwork properly throughout the route. Large duct sections, external risers and long horizontal runs need secure, appropriately spaced supports that can carry their weight without stressing joints. Allow for thermal movement where hot extract is involved, and protect external ductwork against weather exposure and corrosion.
Allow for fan location, noise and energy use
Fan position changes the way a duct system performs. A roof-mounted fan can keep much of the ductwork under negative pressure, which can help limit leakage of odours or contaminants into the building. An internal fan arrangement may suit other layouts, provided there is adequate access for servicing and noise is controlled.
Do not leave attenuation until the end of the project. Fans, high velocities, bends and discharge points can all contribute to noise. If the site is near offices, flats, hospitality venues or other sensitive receptors, acoustic control needs to be built into the design rather than added as a last-minute compromise.
Energy efficiency follows good routing. A shorter, smoother duct route with correctly sized components reduces system resistance. That means the fan can achieve the required airflow without unnecessary power consumption. Variable-speed control can offer further savings where extraction demand changes through the working day, but it must not reduce airflow below the level needed for safe, effective capture.
Check the route before fabrication begins
Before ductwork is manufactured, walk the route on site. Confirm ceiling heights, beam positions, existing services, access routes for installation, roof penetration locations and the final discharge point. Drawings are essential, but they do not always reveal the pipework, structural details or access restrictions found on an operating site.
A proper pre-installation check also prevents costly alterations to stainless steel ductwork after fabrication. At CanopyMan, this practical approach helps ensure custom-built extraction systems are designed around the premises rather than forced into an unsuitable standard layout.
The best exhaust duct route is the one that delivers the required airflow with the least resistance, reaches a safe discharge point and remains accessible for the life of the system. Treat ducting as a core part of the extraction design, and it will support cleaner air, quieter operation and dependable long-term performance.