Biomass and CHP flue height: dispersion and design
Calculate the flue before you draw it. Here is how dispersion, building wake and the Clean Air Act 1993 thresholds set chimney height for biomass and combined heat and power plant.
Flue height for a biomass boiler or a combined heat and power engine is an air quality calculation before it is an elevation decision. The height has to disperse the plume clear of the building's own wake and away from nearby windows, intakes and balconies, and on plant above the thresholds in section 14 of the Clean Air Act 1993 the chimney height needs the local authority's separate approval. So the architect who draws a flue to suit the roofline and then asks the air quality consultant to justify it has the sequence backwards, and the redesign usually costs more than the original study would have.
Why the height is set by dispersion
A flue discharges hot gas containing nitrogen oxides, carbon monoxide and, for biomass, particulate matter. How much of that reaches a person at a window depends on the efflux velocity, the temperature, the wind and the geometry of the surrounding buildings.
Buildings generate a recirculating wake downwind that extends to roughly two and a half times their height. A flue terminating inside that wake will bring plume down to roof level and to facades, which is why a short flue in the lee of a taller neighbour performs worse than a flue on an exposed roof of the same height.
Biomass makes this sharper than gas. Particulate emissions are higher, and the nitrogen oxide emission per unit of useful heat is typically higher than a modern gas boiler's, so the same heat output needs more dilution.
Emission limits applied to combustion plant
Three regimes can apply to the same unit, and they answer different questions.
Section 14 of the Clean Air Act 1993 requires the local authority to approve chimney height for a furnace burning solid fuel at or above 45.4 kg per hour, or liquid or gaseous fuel at or above 366.4 kW. Approval is refused if the height is insufficient to prevent smoke, grit, dust, gases or fumes from becoming prejudicial to health or a nuisance. The traditional calculation method behind this was written around sulphur dioxide, which makes it a poor fit for a low-sulphur biomass or gas installation, so authorities increasingly expect a modern dispersion assessment alongside the statutory application.
Environmental permitting applies to medium combustion plant, broadly units of 1 MW to 50 MW thermal input, with emission limit values for nitrogen oxides and, for solid fuel, dust. Specified generators, including standby diesel sets operating above set hours, have their own permitting route.
Local policy applies to everything else. In London the Mayor's guidance sets maximum nitrogen oxide emission rates for boilers and combined heat and power expressed per unit of energy output, and a scheme exceeding them is expected to change the plant rather than raise the flue. The mechanics of that system are covered in air quality neutral and the emissions benchmarks.
Screening methods for chimney height
Work through three levels and stop when the answer is defensible.
- Geometric screening. Establish the height of the host building and everything within a few building heights, identify the wake envelope, and set a trial height clear of it.
- Simple dispersion screening using a point source model with the actual emission rate, efflux velocity, exit temperature and flue diameter, testing concentrations at the nearest facades and intakes against the air quality objectives.
- Detailed modelling with building effects included, where the trial height is marginal, where the site sits in an Air Quality Management Area, or where the environmental health officer asks for it.
The input that most often gets fudged is the emission rate. Take it from the manufacturer's data at the operating load the scheme actually needs, not the best figure on the datasheet at optimal load with a clean filter. Biomass plant spends a lot of its life at part load, and part load emissions are worse.
| Input | Where it comes from | Why it matters |
|---|---|---|
| Emission rate for nitrogen oxides and dust | Manufacturer data at the design load | Drives concentrations at receptors |
| Efflux velocity and exit temperature | Flue sizing, not assumed | Controls plume rise above the terminal |
| Flue diameter and terminal type | Mechanical design | A cowl or rain cap suppresses plume rise |
| Surrounding building heights | Survey and context model | Sets the wake the plume has to clear |
| Receptor locations | Windows, balconies, intakes, play space | Where the objective has to be met |
Watch the terminal detail. A decorative cowl added late to tidy the appearance can reduce plume rise enough to change the result, and it is the sort of change that never gets back to the air quality consultant.
Building downwash and adjacent structures
Downwash is the mechanism behind most complaints about plant emissions in courtyards and light wells. The plume leaves the flue, is caught in the recirculation zone behind a parapet, plant screen or adjoining block, and is brought back to facade level within a few metres.
Three design responses work. Raise the terminal clear of the wake, which is the honest but visually awkward answer. Move the flue to the highest part of the roof so less of the building is upwind of it. Or reduce the source, which is the only response that also solves the problem for future neighbouring development.
Plant screens deserve particular care. A louvred enclosure added for visual reasons around a flue terminal can create exactly the recirculation the height was calculated to avoid.
Visual effect of a compliant flue
Once the height is calculated, it becomes a design problem, and pretending otherwise produces bad elevations. A flue that has to terminate three or four metres above a parapet will be seen, so draw it properly, with the terminal, any supports and the bracketry, on every relevant elevation and on the roof plan.
In a conservation area or on a listed building this is where schemes stall. The heritage objection to a tall flue is legitimate and the air quality requirement is not negotiable downwards, so the reconciliation usually has to come from the plant selection rather than from the flue. Grouping several small flues into one shared discharge often reads better than a row of individual terminals.
Alternatives that avoid the problem
The most effective air quality mitigation for combustion plant is not having it. Heat pumps remove the flue and the emission entirely, at the cost of plant space, electrical capacity and a different noise question, which brings its own assessment through the Noise Impact Assessment.
Connecting to an existing heat network moves the combustion off site to a source already permitted and modelled. Low nitrogen oxide burners, flue gas recirculation and selective catalytic reduction reduce emissions from plant you are keeping. On biomass specifically, check the fuel specification the emission figures assume, because a boiler run on wet woodchip rather than dry pellet performs differently from its type test.
For kitchen extraction the parallel question of odour and grease is dealt with under separate guidance, set out in commercial kitchen extraction and the EMAQ guidance.
Evidencing the design for environmental health
Environmental health officers read these submissions looking for four things: the emission rate and its source, the model and its inputs, the receptors assessed, and the conclusion expressed against the objectives at those receptors. Give them a table of each rather than a narrative.
Then state what the approval depends on. If the result holds only with a particular terminal type, or at a particular flue height above the parapet, say so, because that is what a condition needs to secure. Offer the condition wording. And keep the flue height in the Air Quality Assessment consistent with the height on the elevations, because the discrepancy between the two is the single most common reason these submissions come back.
PlanScope drafts an Air Quality Assessment structured to the sections an environmental health officer expects and flags the site-specific figures it cannot verify rather than filling them in, which is the right behaviour for emission rates that have to come from manufacturer data. For specialist technical reports, qualified professional input may be required.
Enter the site postcode on PlanScope to see which environmental documents a scheme with combustion plant is likely to need.
This is general guidance, not legal advice, so check the position with your local authority's environmental health team 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.
Frequently asked questions
When does a chimney need Clean Air Act approval?
Section 14 of the Clean Air Act 1993 requires local authority approval of chimney height for a furnace burning solid fuel at or above 45.4 kg per hour, or liquid or gaseous fuel at or above 366.4 kW. Approval is refused where the height would not prevent emissions becoming prejudicial to health or a nuisance, and it is separate from planning permission.
Why does a flue have to clear the building wake?
A building creates a recirculating zone downwind that extends to roughly two and a half times its height. A flue terminating inside that zone brings the plume back down to roof and facade level, so concentrations at windows, balconies and ventilation intakes rise sharply. Parapets, plant screens and taller neighbours all extend the problem.
Does biomass need a taller flue than gas?
Usually. Particulate emissions are higher and nitrogen oxide emissions per unit of useful heat are typically higher than a modern gas boiler's, so the same heat output needs more dilution. Part load operation makes it worse, which is why emission rates should be taken at the load the scheme actually runs at rather than the datasheet optimum.
Can a cowl or rain cap be added after the assessment?
Not safely. A cowl or decorative terminal suppresses plume rise and can change the modelled result, and it is the sort of late detail that never reaches the air quality consultant. Fix the terminal type before the assessment is finalised and reflect it in the condition wording offered with the application.
What is the alternative to a tall flue on a heritage site?
Change the plant rather than the height. Heat pumps remove the flue and the emission, a connection to an existing heat network moves combustion off site, and low nitrogen oxide burners or selective catalytic reduction cut emissions from plant you keep. Grouping several flues into one shared discharge also reads better than a row of terminals.
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.