Why energy statements moved away from gas CHP

CHP energy statement strategies used to write themselves. A gas fired combined heat and power engine sized to the domestic hot water load, a communal heat network fed from it, and a carbon saving at the second stage of the energy hierarchy that got the scheme most of the way to its target. That approach is now close to unarguable on a new residential development in England, and the reason is arithmetic rather than fashion.

Gas CHP saves carbon only when the electricity it generates displaces grid electricity with a much higher carbon intensity than the gas burned to make it. The grid stopped being that dirty.

How the carbon case for gas engines changed

A CHP engine converts gas into electricity and useful heat. The carbon accounting gives the scheme credit for the displaced grid electricity, and charges it for the gas consumed. When grid electricity was assumed to carry roughly two and a half times the carbon of gas per unit delivered, the credit outweighed the charge and the engine looked good.

The Standard Assessment Procedure carried those assumptions into every residential energy statement. Under the earlier SAP 2012 factors, grid electricity was taken at about 0.519 kgCO2 per kWh against about 0.216 for mains gas. The credit was large and CHP dominated the middle of the energy hierarchy.

Then the generating mix changed. Coal came off the system, offshore wind came on, and the published factors were revised to match.

The factor that did it

SAP 10 reduced the grid electricity factor to about 0.233 kgCO2 per kWh, with gas broadly unchanged at about 0.210. The Greater London Authority adopted the revised factors for London Plan energy assessments ahead of their formal use in building regulations, which is why London schemes felt the change first.

Run the same engine through those numbers and the result inverts. The electricity displaced now carries a carbon content close to that of the gas burned, so the exported credit no longer covers the fuel, and once heat network losses are included many CHP schemes show a carbon increase against a high efficiency boiler and grid electricity.

Check the version of the calculation methodology that applies to your scheme, because the factors continue to be revised and the assessment methodology underpinning Part L is itself being replaced as the Future Homes Standard is implemented. An energy statement prepared on superseded factors is the single most common reason these documents are sent back.

Two practical points follow. The direction of travel is one way, because the grid continues to decarbonise while gas does not. And the same arithmetic that removed the case for CHP is what makes heat pumps look progressively better each time the factors are updated.

Air quality objections to gas fired plant

Carbon was only half of the problem. A gas engine running in a residential building emits oxides of nitrogen at ground level in an area where people live, and in air quality management areas that is a planning objection in its own right.

London guidance sets emission standards for gas engine plant and requires an assessment of the effect on local concentrations, with flue heights, dispersion modelling and in some cases abatement. Outside London, environmental health officers ask similar questions where the site sits in or near a management area, supported by an air quality assessment.

The awkward part is that the carbon case and the air quality case pulled in opposite directions for years. Applicants were proposing plant that harmed local air quality to hit a carbon target that the plant no longer delivered.

What replaced CHP in heat network design

Communal and district heating did not go away. What changed is the heat source.

Ambient loop networks, running at low temperature with a water source heat pump in each dwelling or each building, have become the standard proposal on larger residential schemes. Large centralised air source or water source heat pumps serving a conventional network are used where the plant space and the acoustic constraints allow. Waste heat recovery from data centres, electricity substations, sewers and industrial processes is the most attractive option where a source exists within economic distance.

Where a ground source scheme is proposed, the borehole consents and abstraction issues need to be programmed early, as set out in ground source heat pump borehole licences and permits.

Heat network zoning changes the question again in designated zones, where connection to an identified network can be required rather than chosen, which we cover in heat network zoning and the connection requirement.

Consented schemes built around an old strategy

Plenty of permissions granted between 2015 and 2021 carry conditions or obligations requiring a CHP installation, sometimes with a specified electrical output. Building that plant now would cost money, harm air quality and increase emissions.

Councils generally understand this. The energy officer who wrote the condition has watched the same factors change. What they need is evidence rather than an assertion: a comparison of the consented strategy and the proposed replacement using the current factors, a demonstration that the revised scheme meets or exceeds the consented carbon saving, and confirmation that the network can still be connected to a future low carbon source.

Do not simply build something different from the approved details. A condition requiring a specific plant type is enforceable, and a building control sign off does not cure a planning breach.

Amending a permission

The mechanism depends on what the permission says. Where the requirement sits in a condition, an application under section 73 to vary that condition is the normal route, and it produces a new permission with the condition reworded. Where the requirement sits in a section 106 obligation, a deed of variation is needed and the council will want the replacement strategy documented in the same way.

Where the change is small, for example a change of plant model within the same technology, section 96A may be available. It is not the right route for a change of heat source, because that alters the substance of what was approved.

Build the evidence pack once and use it for both the planning application and the discharge of the related conditions. It should contain the revised energy strategy, the carbon comparison on current factors, the air quality position, the plant space and acoustic implications, and the effect on the whole life carbon assessment where one was submitted.

What to propose instead

Start with demand reduction, because a lower peak heat demand shrinks the plant and the network losses that follow it. Fabric performance, form factor, glazing ratios and airtightness do more for the final answer than any plant selection.

Then choose the heat source on the basis of the site rather than the policy wording: available electrical capacity from the distribution network operator, plant space, external space for air source units and their acoustic effect on neighbours, and the presence of any recoverable waste heat nearby.

Finally, design the network for the temperature the source can deliver. A network designed around 80 degree flow temperatures cannot be retrofitted to a heat pump without replacing emitters in every dwelling, and that mistake is being repeated on schemes being consented now.

An illustrative example. A 180 unit scheme with a consented gas CHP strategy is reassessed on current factors. The engine now shows a carbon increase against the baseline, the air quality officer objects to the flue, and the replacement strategy is an ambient loop with individual water source heat pumps. The plant room shrinks, the flue disappears, and the section 73 application turns on one comparison table.

Enter your postcode on PlanScope to see which energy and sustainability reports your scheme is likely to need, and draft the strategy before the plant room is drawn.

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.