Energy centre flue height and dispersion modelling

Energy centre flue height is decided by dispersion modelling and then argued about on visual grounds. Those are two separate conversations with two separate consultees, and the sequence matters: model first, then design the flue, then place it on the elevation. Architects who fix the roofscape before the air quality consultant has run anything usually end up doing the drawings twice.

The output of the modelling is a minimum height above a defined datum. Everything above that is a design choice.

Why flue height is a planning matter

Combustion plant serving a development, whether gas boilers, combined heat and power or a peaking generator, emits oxides of nitrogen. The planning question is whether the ground-level concentrations at nearby receptors remain within the objectives, and whether the development makes an existing exceedance worse.

The relevant objectives for nitrogen dioxide are an annual mean of 40 micrograms per cubic metre and a one-hour mean of 200 micrograms per cubic metre not to be exceeded more than 18 times a year. Both come from the Air Quality Standards Regulations 2010. Where the site falls within an Air Quality Management Area, expect the environmental health officer to scrutinise the assessment closely and to ask what the scheme contributes.

There are two parallel regimes to check at the same time. Chimney height approval under section 14 of the Clean Air Act 1993 applies to certain furnaces and is a separate consent from the local authority. And combustion plant rated between 1 and 50 megawatts thermal input falls within the medium combustion plant provisions of the Environmental Permitting (England and Wales) Regulations 2016, which bring their own emission limits and a permit from the Environment Agency.

Neither of those removes the planning requirement. They do mean the flue you design has to satisfy more than one test, and it is worth establishing which regimes apply before the modelling brief is written.

Dispersion modelling inputs and outputs

The assessment runs a dispersion model, in practice one of the established atmospheric dispersion packages, over several years of meteorological data from a representative station.

The inputs a reviewer will check:

• Plant schedule: thermal input, fuel, emission concentration at a stated reference oxygen level, flue gas volume flow, exit temperature and efflux velocity.
• Operating profile: hours of use, load pattern, whether the plant runs at night, and whether standby plant is assumed to operate.
• Stack parameters: height above local ground level, internal diameter, and the position of the termination relative to the building.
• Meteorological data: station, years used, and the justification for choosing it.
• Surface roughness and any terrain effects.
• Background concentrations, normally taken from the Defra background maps for the relevant grid square and year, with the verification applied.
• Receptor locations: existing sensitive receptors, proposed receptors within the scheme itself, and any at height on neighbouring buildings.

The output is a process contribution at each receptor, which is added to the background to give a predicted environmental concentration. The assessment then compares that against the objectives and reports the percentage of the objective the plant contributes.

Receptors within the development are the ones most often forgotten. A flue serving a block of flats can put its own top-floor balconies in the plume, and that is a point the case officer will raise once the layout is published.

Building downwash and surrounding heights

A plume caught in the recirculating cavity behind a building comes down to ground level close to the source, and the model will show it.

The rule of thumb used at feasibility stage is that a stack should terminate at around two and a half times the height of the dominant nearby building to clear its wake. That is a screening figure only. The model, with the building effects module switched on and the surrounding buildings entered with their real dimensions, is what produces the answer.

Two errors recur. Omitting buildings that are consented but not yet built, which is common on phased regeneration sites where the neighbouring plot rises after yours. And entering the flue height above the roof rather than above ground level, which produces a number that looks fine and is wrong.

On a phased site the plume from an early phase has to work against the final built form, not the interim one. That is one of several reasons to settle the energy strategy at masterplan stage, as our note on energy masterplanning across a phased site sets out.

Air quality standards applied

The assessment is judged against the objectives, but the descriptive part is judged against the professional guidance. The Institute of Air Quality Management publishes the criteria most consultants use to describe the magnitude of a change at a receptor and to reach an overall judgement.

Two things to state clearly in the report:

That last row is the one that shortens the determination. Presenting results at, say, 1 m below and 2 m above the proposed height shows the officer how much margin exists, and it lets the design team argue for the lower option with evidence rather than assertion.

Where the London Plan applies, the Greater London Authority sets emission limits for boilers and combined heat and power plant, and an air quality neutral assessment is also required. Check the current figures in the authority's published guidance rather than carrying them across from an earlier scheme.

Visual effect of a tall flue

A flue tall enough to work is often taller than the architecture wants. This is where the two conversations collide.

Three design responses are commonly accepted. Group the flue with other roof plant inside a single screened enclosure that reads as part of the building. Express it deliberately as a slender element with a considered profile, which works better on a contemporary scheme than an apologetic one. Or relocate the energy centre to the tallest block on the site, so the same absolute height is less visible.

What rarely works is a screen that stops below the termination, leaving a short length of flue poking above it. Officers notice, and it reads as an afterthought because it is one.

Where the site is near a heritage asset or within a conservation area, the flue becomes part of the harm assessment and needs to appear in the verified views. Get it into the townscape work at the right height from the first iteration.

Coordinating with the architectural design

The practical coordination items, in the order they usually bite:

1. Riser space from the plant room to the roof, sized for the flue and its clearances, including any secondary containment.
2. Structural support and wind loading on a freestanding flue, which affects the roof structure and sometimes the core.
3. Maintenance access to the termination, including a safe working platform where required.
4. Separation from openable windows, balconies, terraces and air intakes on the same and neighbouring buildings.
5. Acoustic attenuation, because the same plant that needs a flue usually needs a silencer and both compete for the same space.

Item four is the one that generates late changes. A flue that satisfies the dispersion model can still sit too close to a fresh air intake or a terrace, and moving it at that stage means rerunning the model.

The decision about whether to have a gas-fired energy centre at all is upstream of all of this, and it is covered in our note on the all-electric new homes decision. An all-electric scheme removes the flue question entirely and changes the energy statement argument.

Conditions securing the specification

Expect a condition fixing the flue height and the emission limits, and a second requiring evidence before occupation.

Word the first by reference to a named drawing and revision showing the termination height above ordnance datum, rather than a height above roof level that shifts if the parapet changes. Word the second around commissioning test results demonstrating the emission concentration assumed in the assessment, submitted to and approved by the authority.

Where standby or peaking plant is included, the condition should limit its running hours, because the modelling almost certainly assumed a limit. A scheme that models 50 hours a year of generator running and then operates it commercially is in breach.

Councils normally expect an air quality assessment of this kind from a suitably qualified specialist, and professional review is strongly advised. Enter your postcode on PlanScope to see which reports a site with an energy centre is likely to need.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.