Combined heat and power after the Part L 2021 uplift
Gas combined heat and power no longer displaces high-carbon grid electricity, so it tends to raise regulated emissions on new residential schemes. Here is where it still has a case and how to evidence it.
Gas combined heat and power used to be the easy win in an energy statement. It generated electricity on site that displaced grid electricity with a high carbon factor, and the saving flattered the whole scheme. That arithmetic has gone. The carbon factors behind Part L 2021 put grid electricity well below mains gas per kilowatt hour, so a gas CHP unit now tends to raise regulated emissions on a residential scheme rather than lower them. If you are proposing one, the energy statement has to explain why, in terms an officer and a consultee can test.
Why grid decarbonisation changed the arithmetic
The case for CHP always rested on a comparison. Burn gas on site, get heat and electricity, and count the carbon avoided by not importing that electricity from a grid running on coal and gas.
As wind and solar displaced coal, the carbon content of a grid kilowatt hour fell steeply and kept falling. The comparison inverted. Electricity imported from the grid is now less carbon intensive per unit than gas burned in an engine, and an on-site engine no longer displaces anything dirty.
Part L 2021 brought that reality into the compliance calculation. The uplift set a 31 per cent reduction in carbon emissions for new dwellings against the 2013 standard, and 27 per cent for other buildings, with primary energy added as the lead metric. The carbon factors used in the calculation moved at the same time.
Carbon factors and which set applies
The single most common error in an energy statement is a factor mismatch. The SAP and SBEM calculations behind Part L compliance use the factors built into the approved methodology for that version. Government greenhouse gas conversion factors, published annually for corporate reporting, are a different set with a different purpose.
Use the compliance factors for the compliance case. Use the reporting factors, clearly labelled, only where you are presenting an operational or whole life figure that sits outside Part L. Mixing them produces a document where two tables disagree and neither can be relied on.
Check which version of the methodology applies to your submission before you model anything. Building Regulations have transitional arrangements, and a scheme with a commencement notice under an earlier edition is assessed under that edition. The article on choosing between SAP and SBEM on a mixed-use scheme covers how this plays out where residential and commercial floorspace share a building.
Air quality, and the objection you did not expect
A gas engine is a combustion plant with a flue. It emits oxides of nitrogen, and it does so at ground level in the middle of a residential development rather than at a power station somewhere else.
Authorities in London apply published emission limits to boilers and CHP plant through the Greater London Authority's sustainable design guidance, and several authorities outside London have adopted similar wording. Inside an Air Quality Management Area the environmental health team will look at the stack height, the dispersion modelling and the operating regime before anyone looks at the carbon case.
That combination catches schemes out. A CHP unit can fail on air quality grounds even where a carbon argument could be made for it, and there is no mitigation that makes a gas engine emit nothing.
Where combined heat and power still earns its place
There is a real case for CHP, and it is narrower than it used to be.
The strongest one is a site with a steady, year-round demand for both heat and power. Leisure centres with pools, hospitals, hotels and certain industrial processes run their engines at high load factors for most of the year, which is where the efficiency case holds. A residential block with a summer demand consisting of hot water alone does not.
Two other situations come up. Sites where the electrical grid connection is constrained and importing the full load is slow or expensive. And existing heat networks with an installed engine, where the question is not whether to fit CHP but how to plan its replacement.
| Situation | Typical load factor | Carbon case under current factors |
|---|---|---|
| New residential block | Low outside winter | Weak, usually worse than a heat pump |
| Leisure centre with pool | High, year round | Arguable, needs modelling |
| Industrial process heat | High, steady | Often the strongest case |
| Constrained grid connection | Varies | A resilience case, not a carbon one |
Alternatives inside the energy hierarchy
The hierarchy has not changed: reduce demand first, then supply heat and power efficiently, then add renewables, then monitor what the building actually does.
For most new schemes the be clean layer is now served by heat pumps rather than engines. Air source units where space allows, water source or ambient loop arrangements on larger sites, and waste heat recovery where a nearby source exists. Connection to an existing or planned heat network is worth testing early, and heat network zoning is changing the answer in designated areas.
Fabric remains the least costly carbon you will ever buy. A scheme that argues about plant while leaving the be lean layer thin usually gets asked about it.
Designing a network that outlives the plant
Plant has a life of fifteen to twenty years. A district heating network in the ground has a life measured in decades. Design the network for what will replace the engine, not for the engine.
In practice that means low flow and return temperatures, so a heat pump can be connected later without replacing the pipework or the emitters. It means physical space and electrical capacity in the energy centre for that future plant. It means avoiding the oversizing that locks a network into high temperatures to keep an engine at acceptable load.
An illustrative example. A 180 unit scheme is designed around a gas CHP unit sized for the winter peak, with a network running at 80 degrees flow. Five years on, the operator wants to swap to heat pumps. The emitters cannot deliver at 55 degrees flow, so every flat needs new radiators before the plant can change. Designing the network at the lower temperature from the start would have cost more in emitter size and nothing in pipework. Figures here are illustrative.
Explaining the choice in the submission
The Energy Statement has to do three things if CHP is in the scheme.
- Set out the counterfactual. Model the same building with heat pumps and present both results using the same factors and the same version of the methodology.
- Justify the load profile. Show the heat and power demand across the year, not a peak figure, because the case for an engine lives or dies on how many hours it runs at sensible load.
- Deal with air quality separately and properly, with emission limits, stack design and any dispersion modelling the authority expects.
Where the scheme is large enough to need a Whole Life Carbon Assessment, the plant and the network appear in it as embodied carbon too, and a replacement cycle within the assessment period has to be counted.
Say plainly if the CHP is there for resilience or grid constraint rather than carbon. Officers respond better to an honest operational case than to a carbon argument that falls apart under the current factors.
Energy and carbon documents are ones councils expect a suitably qualified author to prepare, and professional review is strongly advised before submission. An AI-assisted draft can build the structure, pull the policy framework together and mark where your modelling outputs go, then flag the inputs it does not hold.
Enter your postcode on PlanScope to see which reports your scheme 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.
Frequently asked questions
Does gas CHP still reduce carbon emissions on a new residential scheme?
Usually not under the carbon factors behind Part L 2021. Grid electricity now carries a lower factor per kilowatt hour than mains gas, so an on-site engine no longer displaces high-carbon imports. Model the same building with heat pumps using the same methodology version and present both results before proposing an engine.
Which carbon factors should an energy statement use?
Use the factors built into the version of SAP or SBEM that applies to the Building Regulations submission. Government greenhouse gas conversion factors published for corporate reporting are a different set for a different purpose. Mixing the two produces a document with tables that contradict each other, which reviewers notice quickly.
What did the Part L 2021 uplift actually change?
It set a 31 per cent reduction in carbon emissions for new dwellings against the 2013 standard and 27 per cent for other buildings, and introduced primary energy as the lead compliance metric. The carbon factors used in the calculation were updated at the same time, which is what reversed the case for gas engines.
Can a CHP unit be refused on air quality grounds?
The plant itself is not refused, but the application can be. A gas engine emits oxides of nitrogen at ground level within the development. Authorities in London apply published emission limits to boilers and CHP, and inside an Air Quality Management Area the environmental health team will scrutinise stack design, dispersion and the operating regime.
How do I future proof a heat network designed around CHP?
Design for the plant that will replace the engine. Use low flow and return temperatures so heat pumps can be connected without replacing emitters, leave physical space and electrical capacity in the energy centre, and avoid oversizing that forces high network temperatures to keep an engine loaded.
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.