Combustion plant in new development: policy resistance

Propose a gas boiler, a combined heat and power unit or a standby generator in a new scheme and you are now proposing the part of the building that attracts the most hostile consultee response. Environmental health officers ask about emissions, air quality officers ask about dispersion, and the planning officer asks why the scheme is not all electric. The policy direction in England has moved decisively away from combustion for space heating in new buildings, and the practical consequence is that any combustion plant has to be argued for rather than assumed.

The route through is evidence: emissions rates for the specific plant, a dispersion assessment where the context demands one, and a flue design that survives a sense check against the nearest window.

Why combustion plant attracts objection

Three separate policy pressures converge on the same piece of equipment.

Air quality is the first. Many local authorities in England have declared air quality management areas under the local air quality management framework, and nitrogen dioxide is the pollutant most often behind the declaration. New combustion sources inside or near an air quality management area are therefore scrutinised against the annual mean and hourly objectives in the Air Quality Standards Regulations 2010, which set nitrogen dioxide at an annual mean of 40 micrograms per cubic metre and an hourly mean of 200 micrograms per cubic metre not to be exceeded more than 18 times a year.

Carbon is the second. Part L 2021 of the Building Regulations shifted the primary energy and carbon targets far enough that a gas boiler scheme struggles to show a surplus, and the Future Homes Standard is expected to tighten that further. Check the commencement position before relying on it in a submission, because the transitional arrangements matter more than the headline.

Heat network and local policy expectations are the third. Many local plans now carry a heating hierarchy that puts connection to an existing network first, a site network second, and individual heat pumps third, with combustion last and only where justified.

Air quality and emissions standards

For plant below the environmental permitting thresholds, planning is the only regime that controls emissions, which is why officers lean on it hard.

The evidence an Air Quality Assessment should carry for combustion plant:

• the plant schedule, with rated thermal input, fuel, expected annual running hours and the manufacturer's emissions data
• emissions expressed in the units the local policy uses, which for boilers is normally milligrammes of nitrogen oxides per kilowatt hour and for engines and turbines normally milligrammes per normal cubic metre at a stated oxygen reference
• background concentrations taken from the national background maps and any local monitoring, with the year stated
• receptor locations, including the scheme's own windows and balconies, not only existing neighbours
• the assessment method, following the Institute of Air Quality Management guidance on planning and air quality

The Greater London Authority publishes emissions benchmarks for boilers, engines and turbines through its sustainable design and construction guidance, and a London scheme has to evidence compliance with the version current at submission. Outside London, local plan policies vary, and the honest position is that many authorities have no numeric standard and will simply ask whether the best available plant has been specified.

Medium combustion plant between 1 and 50 megawatts thermal input falls within the environmental permitting regime, and generators above certain capacities fall within the specified generator rules. Where that applies, say so in the planning submission and explain that emissions limits will be controlled through the permit, because an officer who does not know that will try to control them by condition instead.

Where a gas connection is still proposed

There are schemes where a gas connection remains the defensible answer. The argument has to be made on the evidence rather than on habit.

The cases that tend to succeed:

1. A constrained electrical supply, evidenced by a formal point of connection response from the distribution network operator showing the cost or the lead time for the capacity an all electric scheme would need.
2. A process load that cannot practically be electrified, such as certain commercial kitchen or industrial applications.
3. A phased development where a site heat network is planned but the first phase needs interim provision, with a condition securing later connection.
4. Retrofit and conversion schemes where the existing distribution system and emitter temperatures make heat pumps unworkable without intervention that would harm a listed building.

In each case the Energy Statement has to show the alternative was assessed and why it fails, with figures. A statement that asserts heat pumps are unsuitable without a capacity letter or a temperature analysis will be treated as unevidenced, and the problem of electrical capacity is worth understanding properly before the argument is made, as the issues around peak electrical demand and all-electric heating show.

Standby generators and their treatment

Standby generators get waved through at design stage and then cause trouble at discharge of conditions, because they are diesel, they sit on a roof or in a yard, and they are tested regularly.

The points that matter in a planning submission are the testing regime, the stack height, the location relative to air intakes and openable windows, and the noise. A monthly test of a few minutes at a sensible hour reads very differently from a weekly load bank test for an hour. State the regime and accept a condition on it. Life safety generators serving fire pumps and smoke control have a stronger case than resilience generators serving data or commercial continuity.

Noise and air quality interact here in an awkward way. Acoustic attenuation on a generator exhaust adds back pressure and length, which pushes the designer towards a lower discharge point, and a lower discharge point worsens dispersion. Resolve that conflict on the drawing rather than leaving two consultants to specify incompatible solutions, and have the Noise Impact Assessment and the air quality work use the same plant location and the same stack height.

Flue design and discharge height

Flue height is the single most effective mitigation available, and the least costly to change on a drawing.

The general principles are straightforward. Discharge above the roof line of the building it serves and above any local parapet, clear of the recirculation zone created by the building itself, away from air intakes, openable windows and balconies, and high enough that the plume is not drawn down into a street canyon. Where taller buildings sit nearby, the effective height is governed by their influence, not by the building the flue is attached to.

Dispersion modelling is the way to test it on anything other than a small domestic scale boiler. The modelling needs the building geometry, surrounding heights, meteorological data for a representative year, the emission rate and the exit velocity and temperature. The output that matters is the process contribution at each receptor compared with the objective and the background, and the way flue height and dispersion modelling interact is the whole subject of energy centre flue height and dispersion modelling.

One design note that saves arguments: decide the flue route early. A flue added after the facade design is fixed ends up boxed out in a position nobody likes, and a plant room moved to suit a flue late in the process takes floor area out of the scheme.

Evidence that satisfies environmental health

Environmental health officers read a small number of things and form a view quickly.

Give them a one page summary at the front of the assessment with the plant, the emission rate, the stack height and the modelled contribution. Officers deal with many applications and a buried conclusion gets missed.

Conditions and testing

Expect three conditions, and draft them yourself rather than accepting a standard form that cannot be complied with.

The first fixes the emissions performance: plant shall not be installed unless its nitrogen oxides emission rate is no greater than the stated figure, with manufacturer certification submitted before installation. The second fixes the physical arrangement: flue height and position as shown on the approved drawing, with no reduction without consent. The third requires commissioning evidence: a test report confirming emission rates and, where appropriate, periodic retesting.

Resist conditions that require an absolute air quality outcome on site, because you cannot control the background. Resist open ended monitoring obligations with no end date. And ask for the emissions figure to be expressed as the policy expresses it, so that an acceptable plant does not fail a condition on a unit conversion.

Enter your postcode on PlanScope to see which reports a scheme with on site combustion plant is likely to need.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Air quality and acoustic assessments are specialist disciplines and councils usually expect a suitably qualified author, so professional review is strongly advised. This article is general guidance rather than legal advice.