Kitchen extraction: stack height, discharge and odour control
Terminating a metre above the ridge at the right efflux velocity does more for an odour case than any filter. What environmental health officers look for in a commercial kitchen extraction scheme.
Kitchen extraction stack height is the single design decision that most often settles whether a restaurant or takeaway gets permission. Filtration reduces the odour concentration in the airstream. Where and how you discharge it decides whether the remaining odour reaches somebody's bedroom window. Environmental health officers know this, which is why a scheme with excellent carbon filtration terminating at first floor level on a rear elevation still attracts an objection, and a simpler system terminating a metre above the ridge often does not.
Get the termination right and the rest of the design becomes a conversation about detail.
Discharge position decides more than filtration
Odour dispersion depends on how far the plume gets from the building before it comes back down to ground or to an opening window. A duct that discharges low on a rear elevation sits inside the building's own wake, where air tumbles and the plume is dragged down the face of the wall.
Terminating above the roofline puts the discharge into cleaner air moving over the building. The plume rises, dilutes and travels. Nothing in the filter train achieves the same effect, because filtration works on concentration and termination works on dilution and distance.
That is also why the nearest receptor matters more than the postcode. A first floor flat above the unit, a hotel window ten metres away across a light well, or a residential garden backing onto the yard all change the answer, and they change it before anyone specifies a filter.
Terminating at or above ridge level
Published guidance on odour and noise from commercial kitchen exhaust systems, which most environmental health teams work from, expects discharge at roof level, and in practice officers look for a termination at least a metre above the ridge of the building and above nearby buildings that could be affected.
In a terrace of three storey buildings with flats above, that means a duct running the full height of the rear elevation. It is the part of the scheme applicants resist, because it is visually prominent and expensive, and it is the part that is hardest to argue away.
Where a ridge termination cannot be achieved at all, on a listed building or where the only route is through a neighbour's demise, the fallback is a case built on distance, enhanced odour control and a discharge point positioned away from openings, supported by assessment rather than assertion. Expect it to be harder.
Design the duct route at the same time as the internal layout. A riser that has to be retrofitted through a residential flat above is a lease problem as well as a planning one.
Efflux velocity and the termination detail
Velocity at the discharge point does the work of pushing the plume clear of the building. Guidance for these systems points to a minimum around 10 m/s, with 15 m/s preferred for reliable dispersion, measured at the terminal rather than in the duct.
Three details undo it.
- A conical rain cap over the outlet deflects the plume sideways and downwards. Use a vertical open discharge with a drain at the base of the riser instead.
- A cowl, louvre or bird mesh with a small free area throttles the outlet and drops velocity below the design figure.
- An oversized duct chosen for pressure loss reasons reduces velocity at the terminal. Where that is unavoidable, the terminal itself can be reduced in area to recover it.
State the design volume flow rate, the terminal free area and the resulting efflux velocity in the submission. An officer who can check the arithmetic in two lines is an officer who does not need to ask for more information.
Filtration for different kinds of cooking
Odour load varies enormously by cooking method. Grilling and frying of fatty foods, charcoal cooking and heavily spiced dishes produce far more odorous compounds than baking or boiling, and the published guidance works through a risk assessment that pairs the cooking type with the sensitivity and proximity of receptors to arrive at a level of odour control.
| Stage | What it does |
|---|---|
| Baffle grease filters at the canopy | removes grease aerosol, protects everything downstream |
| Electrostatic precipitator | removes fine particulate and some grease carryover |
| Ultraviolet or ozone treatment | breaks down odorous compounds in the airstream |
| Activated carbon | adsorbs residual odour, the final stage |
Carbon is the stage most often specified and most often neglected afterwards. It has a finite adsorption capacity, it becomes ineffective long before anything visibly changes, and a system with exhausted carbon performs as though it has none. If you specify carbon, specify the replacement interval in the same sentence.
Mesh grease filters of the throwaway type belong in a domestic cooker hood. On a commercial canopy they blind quickly and let grease into the ductwork, which is a fire risk as much as an odour one.
Fan noise and the attenuation that goes with it
The fan usually arrives as a second application problem. A centrifugal fan on a flat roof or in a rear yard is assessed as an industrial or commercial sound source under BS 4142:2014+A1:2019, against the background sound level at the nearest noise sensitive receptor, and late night operation is the difficult period because background levels fall while the extract keeps running.
Attenuators in the duct on both sides of the fan, anti-vibration mounts, flexible connections and an acoustic enclosure are the usual package. Specify them at design stage, because retrofitting an attenuator into a completed riser is rarely possible.
Where the extract runs past midnight, expect the council to look at the operating hours as well as the plant, and expect a Noise Impact Assessment to be asked for alongside the odour case.
Maintenance, records and the condition on your permission
Permissions for this kind of plant almost always carry a condition requiring the approved system to be installed before the use starts and maintained thereafter. Some go further and require a maintenance log to be kept and made available on request.
Build the regime into the operating routine: grease filter cleaning on a weekly or more frequent cycle depending on throughput, ductwork cleaning to the industry grease hygiene standard at an interval set by the cooking load, carbon replacement on the manufacturer's schedule, and a dated record of each.
That record is also your defence if a complaint is made later. Planning permission does not immunise a business against a statutory nuisance abatement notice under Part III of the Environmental Protection Act 1990, a point to read alongside statutory nuisance after planning permission is granted.
What to submit with the application
A proportionate submission covers the cooking type and menu, the canopy and extract layout, the duct route and termination height shown on the elevations, the filtration train with the model and specification of each stage, the design flow rate and efflux velocity, the fan specification with attenuation, the nearest receptors with distances, and the maintenance regime.
A Ventilation/Extraction Statement pulls that into one document, and where the receptor picture is difficult an Odour Assessment carries the risk assessment properly. Both are stronger when the environmental health officer has seen the proposal at pre-application stage.
This is general guidance, not legal advice; check the position with your local planning authority and its environmental health team before relying on it.
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Frequently asked questions
How high does a commercial kitchen extract duct have to be?
Published guidance on commercial kitchen exhaust systems expects discharge at roof level, and environmental health officers generally look for a termination at least a metre above the ridge of the building and above nearby buildings that could be affected. Lower terminations sit in the building's wake, where the plume is pulled back down the elevation.
What efflux velocity should the discharge have?
Guidance for these systems points to a minimum of around 10 metres per second, with 15 preferred for reliable dispersion, measured at the terminal rather than in the duct. Rain caps, restrictive cowls and oversized terminals all reduce it. State the flow rate, terminal free area and resulting velocity in the submission.
Do I need planning permission for an extraction duct?
Usually yes, where the duct is fixed to an external elevation, because it is an external alteration affecting the appearance of the building. On a listed building it will also need listed building consent, and in a conservation area the design and position will be assessed against the character of the area. Check with the council before ordering.
Does carbon filtration remove the need for a tall stack?
No. Filtration works on the concentration of odour in the airstream and termination works on dilution and distance from receptors. Officers treat them as complementary. A scheme with heavy carbon filtration discharging at first floor level near openable windows is usually a harder case than a simpler system terminating above the ridge.
How often does the system need cleaning?
Grease filters are typically cleaned weekly or more often depending on throughput, ductwork is cleaned to the industry grease hygiene standard at an interval set by the cooking load, and carbon is replaced on the manufacturer's schedule. Keep dated records, because permissions frequently require the system to be maintained and the log produced on request.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.