Combined heat and power and the air quality objection
The carbon argument for gas combined heat and power has narrowed as the grid decarbonised, while the local nitrogen dioxide argument has not moved. An energy statement has to face that rather than bury it.
Gas combined heat and power used to be the easy answer on a large residential scheme. It is now the fastest route to an environmental health objection, because the carbon arithmetic that justified it has shifted while the local air quality arithmetic has not. Anyone preparing a CHP air quality planning case in 2026 is arguing for a combustion plant in a place where the authority is under a statutory duty to bring nitrogen dioxide down, and the energy statement has to face that squarely rather than treat it as a footnote.
Why the carbon case and the air quality case pull apart
The carbon case for gas CHP rested on displaced grid electricity. When the grid was coal-heavy, generating on site and using the waste heat beat taking power from the network and burning gas in a boiler. That advantage has narrowed as the electricity grid has decarbonised, and the carbon factors used in Part L calculations now reflect it.
The air quality case never changed. A reciprocating gas engine running for most of the year emits oxides of nitrogen at ground level in a built-up area, where a heat pump emits none at all. So the same plant that once scored well against a carbon policy now scores badly against an air quality policy and only moderately against the carbon one.
That is the tension a case officer sees. An energy statement that presents CHP as an environmental benefit without addressing emissions reads as out of date, and environmental health will say so in consultation.
Emission standards authorities expect
Several authorities publish emission standards for combustion plant serving new development, and London is the most developed example. The Greater London Authority sets separate nitrogen oxide standards for gas boilers and for CHP engines, expressed on different bases, and requires the manufacturer's certified data rather than a generic figure. Verify the current standard on the London Plan guidance pages before you specify anything, because these numbers are revised.
Outside London the position varies. Some authorities adopt the London standards by reference in a supplementary planning document, some rely on the air quality chapter of the Planning Practice Guidance, and some have nothing written down and rely on the environmental health officer's judgement case by case. Ask at pre-application which applies.
Plant size also brings a separate regime into play. Combustion plant rated between 1 and 50 megawatts thermal input falls within the medium combustion plant provisions of the Environmental Permitting Regulations, with its own emission limit values and permitting duties. That permit is independent of planning permission, and having one does not answer a local air quality objection.
Flue height and dispersion modelling
Flue height is where most CHP applications are won or lost. The principle is simple. Raise the discharge point far enough above the building and its neighbours, give it enough efflux velocity, and the plume disperses before it reaches anyone's window.
The problem is that flue height is a planning matter as well as an air quality one. A 3 metre stack above a parapet on a six storey block is an air quality solution and a townscape objection at the same time, particularly in a conservation area or within the setting of a listed building.
Work it in this order.
- Get the manufacturer's certified emission data for the specific engine, not an indicative figure from a brochure.
- Run a dispersion model with local meteorological data and the surrounding building heights included, so the downwash effect of your own block is captured.
- Test the minimum flue height that meets the nitrogen dioxide objective at the nearest sensitive receptors, including balconies and any school or care home nearby.
- Take that height to the architect before the elevations are drawn, and test whether it can be integrated rather than added.
- Report the modelling in the Air Quality Assessment with the input assumptions visible, because the reviewer will want to change them.
If the height that works is a height the authority will refuse on design grounds, you do not have a scheme. That is a feasibility answer, and it is cheaper to find it in month two than in month nine.
Air quality management areas and what changes inside one
Local authorities declare air quality management areas under Part IV of the Environment Act 1995 where an air quality objective is not being met, and then have to produce an action plan. The annual mean objective for nitrogen dioxide is 40 micrograms per cubic metre, and it is the one most urban declarations turn on.
Inside a declared area, three things change in practice. The baseline concentration is already at or above the objective, so any addition is being made to an exceedance. The authority has a published action plan it has to demonstrate progress against, which makes approving new combustion sources politically hard. And officers will apply their air quality policy strictly rather than in the round.
A scheme inside an air quality management area proposing gas CHP needs a very specific benefit to point at. Connection to an existing heat network with a committed decarbonisation pathway is one. A phased energy strategy where CHP is transitional and the plant room is sized for a later heat pump retrofit is another, and the energy masterplanning approach on a phased site sets out how to evidence that without promising something the site cannot deliver.
Options that avoid combustion
The honest position on most residential schemes now is that the combustion question can be designed out.
Air source heat pumps at building or dwelling scale remove the flue question entirely and bring a noise question instead, which is usually easier to condition. Ambient loop networks with water source or ground source heat pumps work well on larger sites with the space for boreholes. Direct electric with high fabric performance is viable on small schemes where the heat demand is already low. Connection to an existing heat network sidesteps on-site generation, although the emissions of the network source still belong in the assessment.
Each of these has a cost and a plant space consequence, and pretending otherwise in an energy statement does not help. The decision on whether to take a gas connection at all is usually better made at feasibility than defended at appeal.
Conditions on emissions and operating hours
Where CHP is accepted, expect conditions rather than a clean permission. Common ones restrict the plant to the certified emission performance modelled in the assessment, require commissioning test results to be submitted before occupation, fix the flue height and termination detail by reference to a drawing, limit operating hours where noise or emissions at night are the issue, and require a maintenance regime.
Read them carefully before you agree to them. A condition that ties the scheme to a particular engine model can become a section 73 application when that model is discontinued, and a condition requiring post-commissioning monitoring against a modelled figure needs a monitoring method everyone has agreed in advance.
What the energy statement should concede
The strongest energy statements name the conflict rather than hiding it. Set out the carbon performance under the current Part L methodology, set out the local air quality position with the baseline concentrations stated, and explain why the chosen option is the right balance for this site.
Concede the weaknesses. If the carbon saving from CHP is modest under current factors, say so and justify the choice on heat network connection or resilience grounds instead. If the flue height is at the limit of what the elevation can absorb, show the architectural treatment. An Energy Statement that argues one side only invites the officer to find the other side for themselves.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified air quality consultant before relying on it.
Browse the PlanScope report types to see which energy and air quality documents a scheme of your size 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.
Frequently asked questions
Is gas CHP still acceptable in a planning application?
It can be, but it needs a specific justification rather than a general carbon claim. Authorities will look at certified engine emission data, modelled ground level concentrations at the nearest receptors, and whether a non-combustion option was properly tested. Inside an air quality management area the case is much harder to make and often fails on flue height or townscape grounds.
What emission standards apply to a CHP engine serving new homes?
There is no single national planning standard. The Greater London Authority publishes nitrogen oxide standards for boilers and CHP engines that many authorities adopt or echo, and plant between 1 and 50 megawatts thermal input also falls within the medium combustion plant provisions of the Environmental Permitting Regulations. Confirm the current figures with the authority before specifying.
How is flue height decided?
By dispersion modelling using certified emission data, local meteorological data and the surrounding building heights, tested against the relevant air quality objective at the nearest sensitive receptors. The height that works technically then has to survive a design assessment, which is why the answer should reach the architect before elevations are drawn.
Does an environmental permit satisfy the planning air quality test?
No. A medium combustion plant permit controls emission limit values and operation under a separate regulatory regime. The planning assessment asks a different question, which is whether the development is acceptable in land use and amenity terms at this location. Holding a permit is evidence of compliance, not a planning answer.
What does air quality neutral mean?
It is a policy approach, developed most fully in London, that requires a development's building and transport emissions to be no worse than a defined benchmark for that use and location. Schemes that exceed the benchmark have to provide mitigation or offsetting. Check the current guidance because the benchmarks and the calculation method have been revised.
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.