Extract termination height and odour control selection

The default expectation for a commercial kitchen extract is that it discharges vertically, at high velocity, above the ridge of the building it serves and any building immediately adjoining it. Most refusals on kitchen extraction are refusals of a flue that stops short of that, or of a system where the odour control equipment does not match what is being cooked. Get those two decisions right and the rest of the application is administrative.

Risk banding the cooking process

Guidance used across England bands kitchen extract systems by risk, taking account of the type of cooking, the scale of the operation and how close sensitive receptors are. A sandwich bar with a panini press is not a tandoori restaurant, and the system should not be either.

The factors that push a scheme up the bands are chargrilling, frying, solid fuel cooking, high-odour cuisines, long operating hours, large covers, and residential windows within a short distance of the discharge point. Flats above the unit put almost any kitchen into the top band, because the receptor is at the same height as the problem.

Do the banding first and record it. Every later decision follows from it, and an environmental health officer will start there.

The banding also sets expectations with the client. A tenant who has budgeted for a basic filter box needs to know at heads of terms stage, rather than at the point of refusal, that a chargrill in a building with flats above carries a five-figure abatement cost and a flue running up a rear elevation that the freeholder has to agree to.

Matching odour control to the band

Low risk systems can often work with grease filtration and good dispersion alone. Medium risk systems usually add carbon filtration or electrostatic precipitation. High risk systems need multiple stages in series: coarse and fine grease removal, then electrostatic precipitation or ultraviolet treatment, then a carbon bed sized for the airflow.

Two details decide whether a high risk system works in practice.

Carbon beds are sized by contact time, which means volume of carbon against airflow. An undersized bed on a large extract rate gives a few weeks of performance and then nothing. The quotation should state the carbon mass or bed volume and the design airflow, not simply that carbon is fitted.

Electrostatic precipitators need cleaning on a schedule and they need someone to do it. A unit installed in a ceiling void with no access hatch will not be cleaned, whatever the maintenance schedule says.

Termination height above the roofline

The aim of the discharge point is to release the plume above the building envelope, clear of pressure zones and away from openable windows.

Terminating above the ridge of the building, by a margin rather than flush with it, is the position guidance points to and the one authorities apply. Where a taller building stands immediately adjacent, the ridge of that building matters too, because a plume released below it will be drawn down into the lee. Measure the separation to any openable window, ventilation intake or balcony and put those distances on the section drawing.

The flue itself is operational development in planning terms and needs permission in its own right on most buildings. On a listed building it needs listed building consent, and a rear external flue on a listed terrace is one of the harder consents to obtain. In a conservation area the visual effect of a 6 metre stainless duct on a rear elevation is a real planning issue, not a formality.

Discharge velocity and efflux direction

Discharge vertically, upwards, and fast. A cowl, a Chinese hat or a horizontal louvre defeats the purpose by slowing the plume and pushing it back towards the building.

Efflux velocity is what gives the plume momentum to clear the building's wake. Guidance points to a working minimum in the region of 10 to 15 m/s at the discharge point for kitchen extracts, and the calculation follows from the duct diameter and the extract rate, so it should be stated on the drawing rather than asserted in the covering letter.

Rain protection is achieved with a drained cowl-free termination or a stub stack with a drain, not with a cap. If the mechanical consultant proposes a cap, ask what the exit velocity becomes.

Fan noise travels with the air

The extract fan is a noise source and it usually runs late into the evening. Councils routinely refuse or condition on this rather than on odour.

An assessment to the relevant industrial and commercial sound method, comparing the rating level with the background sound level at the nearest residential facade at the quietest operating hour, is what an environmental health officer expects. Attenuators, anti-vibration mounts and a fan selected below its peak efficiency point all help. So does not putting the fan on a party wall.

Where deliveries and waste collection accompany the kitchen, the same officer will look at those too, and our note on night deliveries and servicing noise covers how those hours get conditioned. A Noise Impact Assessment is a report type where councils usually expect a suitably qualified author, and professional review before submission is strongly advised.

The evidence a council will want

Put all of it in one place. A Ventilation and Extraction Statement should carry the following.

1. The cooking process, covers, operating hours and the resulting risk band.
2. The extract rate in cubic m/s, with the calculation behind it.
3. The odour control specification, stage by stage, with carbon volume or precipitator rating.
4. A section drawing showing the termination height against the ridge and against any adjoining building.
5. Distances to the nearest openable windows, intakes and balconies.
6. Efflux velocity at the discharge point and confirmation that the termination is uncapped.
7. The maintenance regime, with intervals, access arrangements and record keeping.

Where the odour risk is high or the receptors are close, a full Odour Assessment will be expected alongside it.

When the flue cannot go up

Sometimes it has nowhere to go. A leasehold that stops at the ceiling, a listed upper storey, a freeholder who will not consent, a shared stairwell with no route.

Be straight about what a low-level discharge requires. Odour abatement has to be specified well above the band, usually several stages with substantial carbon, because dispersion is no longer helping. The discharge should be directed away from openings, at the highest point available, with the greatest separation achievable. The maintenance regime has to be tighter and evidenced, because the whole system now depends on it.

Expect scepticism. Many authorities have had bad experiences with low-level systems that worked for a year, and some will refuse rather than condition. A pre-application discussion with the environmental health team, with the specification in front of them, is worth more than any amount of argument after a refusal.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

Browse the PlanScope report types to see what a change of use to a hot food unit is likely to need.

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