A high-performance extraction hood can still underperform if the duct route is poorly planned. Knowing how to plan hood duct routing before fabrication or installation prevents weak capture, excessive fan noise, grease build-up, expensive alterations and avoidable downtime. The route between the hood and discharge point is not simply a way of getting air outside. It is a working part of the extraction system and must be designed around airflow, fire safety, maintenance access and the realities of the building.
For commercial kitchens, factories and workshops, the best route is rarely the shortest line on a drawing. It is the route that delivers the required air volume with the fewest restrictions, remains practical to clean and inspect, and discharges safely away from people, openings and neighbouring properties.
Start Hood Duct Routing With the Extraction Duty
Ductwork should follow the extraction requirement, not the other way round. Before deciding whether a duct rises through the roof, runs along a rear elevation or passes through an internal service route, establish what the hood needs to remove.
In a commercial kitchen, this means considering the cooking line beneath the canopy, the appliance heat load, fuel type, grease output and expected operating hours. A solid-fuel appliance, for example, creates a very different extraction challenge from a low-duty electric cooking suite. In industrial settings, identify the contaminant at source: welding fume, oil mist, dust, steam, vapour or process heat all affect hood design, airflow rate and filtration requirements.
The fan must overcome the resistance created by the entire system. This includes the hood, filters, duct length, bends, transitions, attenuators, ESP units where specified, weather louvres and discharge components. If the duct route is drawn first and the fan selected later, there is a real risk of specifying a fan that is either too weak for the final pressure loss or unnecessarily expensive to run.
Keep the Route Direct, Smooth and Properly Sized
Every metre of duct and every change in direction adds resistance. The practical aim is to use the shortest viable run with the fewest bends, while preserving safe access and a compliant discharge location. A direct vertical rise to roof level is often effective where the building layout allows it, but it is not automatically the right answer if roof access, structural penetrations or adjacent air intakes create problems.
Avoid undersizing the duct to make it easier to conceal. A duct that is too small raises air velocity and pressure loss, increasing fan power, noise and the rate at which grease or particulate can collect in the system. An oversized duct can also cause problems if the velocity falls too low for the duty. Duct dimensions must be selected against the required airflow and application, rather than chosen from a convenient standard size.
Where a change in duct size is needed, use a gradual transition rather than an abrupt step. Likewise, bends should be planned with suitable radii wherever possible. Tight elbows create turbulence and increase resistance. They may be unavoidable in an existing building, but they should be treated as a design constraint that must be accounted for in fan selection and access planning.
Plan Bends, Branches and Vertical Risers Carefully
A route with several sharp bends may look tidy above a ceiling or behind a partition, but it can make the whole system harder to balance and maintain. Where bends are necessary, keep them spaced apart rather than placing several restrictions together. This gives the airflow a chance to stabilise and helps reduce noise.
Vertical risers deserve particular attention. They are commonly the most efficient method of reaching a high-level discharge point, but they need secure support, weather protection and suitable roof penetration detailing. On taller runs, duct sections must be supported correctly so that joints are not carrying the load of the system. The route also needs to allow for inspection doors and cleaning access at appropriate points.
Avoid unnecessary branches on grease-laden kitchen extract systems. A shared route can appear economical at first, but mixing duties can complicate airflow control, cleaning responsibilities and fire risk management. If more than one hood or process source must connect to a common system, the system should be designed as a complete installation, not assembled from separate duct runs after the fact.
Choose the Discharge Point Before Finalising the Route
The discharge point often determines the entire routing strategy. Extracted air must leave the building where it will not create nuisance, recirculate into supply air systems or affect neighbouring occupiers. A wall-mounted discharge may be easier to install than a roof terminal, but it can be unsuitable near windows, doors, fresh-air intakes, public walkways or neighbouring premises.
For grease extract, a high-level discharge is commonly preferable because it gives fumes a better opportunity to disperse. However, the building height, surrounding properties, prevailing conditions and local authority expectations all matter. A discharge point that works on an isolated industrial unit may be unsuitable for a restaurant beneath flats or within a dense high street.
Consider access at the same time. Fans, ESP units, filters and terminal components require routine inspection and servicing. Locating equipment in an inaccessible roof void or above a fragile roof surface may lower installation cost on paper while increasing maintenance cost for years afterwards.
Build Cleaning Access Into the Duct Design
Grease, dust and process residues do not disappear once they enter the duct. They settle where airflow slows, changes direction or meets rough internal surfaces. That makes duct cleanliness a design issue as much as a maintenance issue.
Grease-bearing ductwork should use durable, suitable materials and properly sealed joints. Access doors must be placed where technicians can reach the sections most likely to accumulate deposits, especially around bends, changes in direction, fan connections and long horizontal runs. Access panels hidden behind fixed ceilings, boxed-in services or permanent equipment are of little practical value.
For commercial kitchen projects in the UK, recognised guidance such as DW/172 should inform the approach to kitchen ventilation design. The final specification may also be influenced by building control, environmental health requirements, fire strategy, landlord conditions and insurer expectations. Treat these as design inputs at the start, not paperwork to resolve after the duct has been installed.
Coordinate With the Building, Not Just the Ceiling Plan
Duct routing often clashes with items that were not obvious during an early survey: structural beams, drainage stacks, sprinkler pipework, cable trays, ceiling services and refrigeration plant are common examples. A proper site survey should trace the proposed route from hood to discharge point and identify every level change, penetration and support position.
This coordination is particularly valuable in refurbishments. Existing premises rarely provide a clear route, and compromises are often required. It may be better to use a slightly longer route with good access and sensible bend radii than force a short run through a congested service zone. The correct answer depends on the building, the extract duty and what can be safely altered.
Also check whether make-up air has been considered. Extracting large volumes of air without a planned replacement air path can leave the space under negative pressure. Doors become difficult to open, the hood may capture poorly and heated or cooled air is wasted. Supply air should support the hood's capture performance without creating draughts across the cooking or work area.
Specify Materials and Components for the Actual Duty
Not all ductwork is interchangeable. Kitchen extract carrying grease-laden vapour requires a different approach from general ventilation or light-duty fume extraction. Material grade, thickness, joint type, access provisions and fire performance must suit the application.
Premium-quality stainless steel ductwork is often the practical choice where hygiene, durability and corrosion resistance are priorities. It also provides a clean, professional finish in visible installations. Galvanised duct may be suitable for certain non-grease ventilation duties, but the decision should be based on the extracted air and required system life, not simply initial material cost.
Filtration also changes routing decisions. An ESP unit can reduce grease and smoke emissions where correctly specified and maintained, but it introduces pressure loss and requires service access, electrical provision and a workable cleaning regime. The duct route, fan duty and equipment position must therefore be designed together.
Turn the Design Into an Installation Plan
Before manufacture begins, confirm the duct sizes, route elevations, penetration details, support locations, access doors, fan position and discharge arrangement. A clear drawing avoids the common site problem of arriving with fabricated duct sections that cannot pass around a beam or through the available opening.
Custom fabrication is especially valuable where standard straight sections will not fit the site. Purpose-made offsets, transitions, bends and connection pieces can reduce on-site adaptation, improve joint quality and keep the installation moving. CanopyMan designs and manufactures extraction systems around the actual conditions of commercial and industrial premises, helping customers avoid the cost of treating ductwork as an afterthought.
A well-routed system is easier to install, easier to clean and cheaper to operate. Give the duct route the same attention as the hood and fan, and the finished extraction system will deliver the dependable performance your operation needs.