In almost every planning energy statement, one decision moves the carbon figures more than any other: how the building is heated. Choosing a heat pump over a gas boiler can be the single step that takes a scheme from failing its carbon target to comfortably passing it. This article explains the carbon maths behind heat pumps vs gas boilers, and why the Future Homes Standard has effectively settled the argument for new homes.
Why heating dominates the carbon calculation
Space and water heating account for the majority of a typical dwelling's regulated energy use. Because the energy statement reports carbon savings against a Part L baseline, the emissions produced by the heating system carry disproportionate weight. Lighting and ventilation matter, but swap the heat source and the whole carbon profile of the building shifts.
The carbon maths: SAP emission factors and efficiency
Two numbers drive the result: the carbon emission factor of the fuel, and the efficiency of the appliance.
- Gas has a fixed carbon emission factor and a modern condensing boiler runs at roughly 90% efficiency. Every unit of heat therefore carries close to a full unit of gas emissions.
- Electricity has been decarbonising rapidly as renewables take a larger share of the grid, so under SAP 10.2 its emission factor is now far lower than it once was — and lower, per unit of energy, than gas.
A heat pump then compounds that advantage. Its seasonal coefficient of performance (SCOP) is typically around 3, meaning it delivers about three units of heat for every unit of electricity. Combine a low-carbon fuel with an efficiency above 300% and the heating emissions collapse compared with a gas boiler burning fossil fuel at 90%. That is why, in SAP 10.2 modelling, a well-designed heat pump dwelling routinely posts a large carbon saving over the notional building.
Where heat pumps sit in the energy hierarchy
Heat pumps belong to the Be Green stage of the Be Lean, Be Clean, Be Green hierarchy — low-carbon technology that meets the demand left after the fabric has done its work. For London schemes chasing the London Plan Policy SI2 35% on-site carbon reduction target, a heat pump is often the measure that makes the target achievable without an unrealistic array of other technologies. Paired with rooftop solar PV, it usually clears the target on-site, reducing any carbon offset payment.
The Future Homes Standard settles it
From a policy standpoint the debate is closing. The Future Homes Standard, published in March 2026, is designed so that new homes are net-zero-carbon ready, with fossil-fuel heating (gas boilers) no longer permitted in new homes from 2027. In practice that means air source heat pumps or low-carbon heat networks become the default, backed by a much larger solar PV requirement. Schemes being designed now should assume a heat-pump future rather than retrofit it later.
Fabric first still comes first
A heat pump is not a substitute for good fabric. Because heat pumps deliver heat at a lower flow temperature than boilers, they work best in a well-insulated, airtight dwelling with appropriately sized emitters. The Be Lean stage — strong U-values, airtightness and efficient glazing — is what lets the heat pump run efficiently and keeps running costs down for occupiers. An energy statement that leans on technology while ignoring fabric tends to attract officer scrutiny.
When a gas boiler still appears
There are edge cases — some existing-building conversions, certain off-gas-grid situations, or phased sites — where the picture is more nuanced, and hybrid or alternative low-carbon solutions may feature. But for new-build housing the trajectory is clear, and a modern energy statement should justify any retained combustion heating rather than assume it. Heating is only one part of a scheme's environmental case; the biodiversity, water and greening side is handled separately in a sustainability statement.
How Fortress Associates can help
Fortress Associates prepares automated, site-specific energy statements for planning applications across every UK local authority, and the report is free. We build the Part L 2021 / SAP 10.2 baseline, model the heating strategy through the full Be Lean, Be Clean, Be Green hierarchy with CO₂ savings at each stage, and address the London Plan SI2 35% target and carbon offset for London schemes. Start on our energy statement page, get in touch via the contact page, or browse our other services.
Sources & further reading
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