How Ductwork Routing Affects Kitchen Exhaust Performance

The route your exhaust duct takes from the hood to the discharge point is one of the most consequential decisions in any commercial kitchen build or renovation. In our experience, it is also the decision most often made purely for convenience — shortest path, least disruption to the ceiling void — rather than for airflow performance. That trade-off usually shows up later as a system that struggles to clear smoke, a fan that runs harder than it should, and grease deposits accumulating faster than your cleaning schedule can handle.

Why Does Duct Routing Matter So Much for Exhaust Performance?

Air moving through a duct experiences resistance — what engineers call static pressure loss. Every metre of straight duct adds a small amount. Every bend, every offset, every transition from one size to another adds more. When you add enough of these together, the fan at the end of the system simply cannot generate sufficient airflow to keep up with the heat and grease load your cooking line produces.

The practical result is a kitchen that feels stuffy, hoods that don't fully capture smoke and steam, and grease-laden air spilling out into the dining area or back-of-house corridor. We've walked into kitchens where the chef is complaining about heat and odour, and within five minutes of tracing the duct run we can see exactly why — two 90-degree elbows back to back right off the fan outlet, or a duct that drops vertically and then turns horizontal three times before it exits the building.

Good duct routing keeps resistance low, maintains capture velocity at the hood face, and gives grease particles the best chance of making it all the way to the discharge filter — not settling inside the duct.

What Are the Most Common Routing Mistakes We See?

Too Many Tight Bends

A sharp 90-degree elbow can be equivalent in resistance to several metres of straight duct. When a run has three or four of these — often unavoidable in a constrained ceiling void — the cumulative effect is significant. Where we cannot eliminate bends, we use swept elbows with a larger centreline radius, or we split the run differently to reduce the number of direction changes. On one kitchen we refurbished in a shophouse, re-routing the duct to exit through the rear wall instead of the shared ceiling void cut two bends from the run and measurably improved hood capture — the owner had never had the system perform that well.

Undersized Duct Sections

Sometimes a duct is sized correctly at the main run but then reduced — to squeeze past a beam or a pipe rack — without being restored to the correct size afterwards. That constriction acts like a pinch point. Velocity spikes, pressure drops, and grease is thrown against the duct wall at the narrowest point and accumulates there. We always try to maintain consistent cross-sectional area across the full run, or increase the duct area to compensate if a shape change is unavoidable.

Long Horizontal Runs Without Adequate Slope

Grease is not just carried in vapour form — it also condenses on duct walls and flows as liquid. A horizontal duct should slope back towards a collection point so that liquid grease drains to where you can clean it, rather than pooling in a low spot and hardening over months. When we design or assess a duct run, we factor in drainage gradient as a basic requirement, not an afterthought.

Discharge Point Location

Where the duct terminates matters too. A discharge point that faces into a prevailing wind, or that exhausts into a recessed shaft with poor cross-ventilation, can create back-pressure that effectively throttles the fan. We assess the external discharge environment — not just the internal duct path — before finalising any layout.

How Does Poor Routing Affect Grease Build-Up and Fire Risk?

Grease accumulation inside a duct is a direct function of how long grease-laden air spends in contact with duct walls, and how often it slows down or changes direction. A poorly routed duct with excessive bends and low air velocity is essentially a grease trap running the full length of your ceiling. SCDF guidelines exist around grease duct construction and access for exactly this reason — regular cleaning is not optional, and cleaning a badly routed duct is harder, more expensive, and less thorough than cleaning a well-designed one.

We've inspected ducts that have never been properly cleaned because the cleaners couldn't physically reach certain sections — access panels were in the wrong places, or sections were boxed in with no provision for entry. That situation is both a compliance failure and a serious fire hazard. When we design a duct run, we specify access panel positions at every bend and at intervals along straight runs so that our own cleaning crews — or any approved contractor — can reach every surface. We fabricate those panels ourselves, so we're not waiting on a supplier.

Can Variable Speed Drives Help Compensate for a Difficult Duct Layout?

To a degree, yes. We supply and install variable speed drives (VSDs) on our fan systems, and they give us the ability to dial up fan speed during peak cooking periods and back it off during quiet times. This saves energy and reduces noise. However, a VSD is not a substitute for a well-designed duct route. Running a fan at maximum speed to overcome a poorly routed system causes early bearing wear, increases noise significantly, and still may not deliver adequate capture velocity at the hood if the static pressure loss is high enough. We always say: get the routing right first, then use a VSD to optimise — not the other way around.

What Should You Do if Your Existing Duct Layout Is Already Fixed?

Not every kitchen has the luxury of a redesign. When we take on a maintenance contract or are called in to improve an underperforming system, we start with a proper assessment: we map the duct run, measure actual airflow at the hood face, inspect internally for grease build-up and any obstructions, and check the fan duty point against what the system actually needs.

From there, the options range from a thorough clean to restore flow, to upgrading the fan motor or impeller for greater pressure capability, to modifying specific sections of duct where a small re-route is feasible without major disruption. We stock our own MV fans, motors, and control panels, so we can often carry out modifications quickly without long lead times. We always confirm any structural or compliance implications with the relevant authority before we proceed.

Frequently Asked Questions

How do we know if our duct routing is causing our exhaust problems?

The clearest sign is a hood that visibly fails to capture smoke or steam during busy cooking periods, even though the fan appears to be running. Other indicators include unusually fast grease build-up in the duct, persistent kitchen odour in adjacent areas, and a fan motor that runs hot. When a client calls us with any of these complaints, duct routing is always one of the first things we assess — we check the physical layout, measure airflow, and compare it against what the system was designed to achieve.

Does duct length alone affect performance significantly?

Length matters, but bends matter more. A long, straight duct run in good condition is far less problematic than a shorter run with multiple tight elbows and transitions. That said, very long runs do accumulate resistance, and for those situations we may recommend an inline booster fan at an intermediate point rather than oversizing the main fan — it's more energy efficient and easier to maintain.

Are there NEA or SCDF requirements specifically about duct routing?

Yes — both NEA and SCDF have requirements covering grease duct construction, materials, clearances from combustibles, access panel provision, and discharge termination. The exact clauses depend on the type of premises and the cooking process involved. We always confirm the specific requirements with the relevant authority before we finalise a design or quote, rather than assuming a one-size-fits-all rule applies.

Can we reroute an existing duct without a full kitchen shutdown?

In many cases, yes. We've carried out partial duct modifications during off-peak hours — late night or early morning — to minimise disruption to kitchen operations. The feasibility depends on the extent of the work and the access available. When we assess a site, we give a realistic picture of what can be done with minimal downtime and what would genuinely require a planned shutdown period.

We fabricate our own ductwork — does that make a difference?

It makes a significant difference to speed and fit. Because we fabricate components in-house, we can produce bespoke duct sections, transitions, and access panels to the exact dimensions a specific route requires, rather than adapting standard off-the-shelf pieces to fit awkward spaces. That means tighter joints, better sealing, and a finished installation that performs the way we designed it to.

If your kitchen exhaust system isn't performing the way it should — or you're planning a new build and want the duct layout done right from the start — get in touch with us for a site assessment and quotation. Our team runs 24/7 standby service, so whether it's a planned review or an urgent issue, we're available when you need us.

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