Part L 2021 carbon factors and the heat pump sums

The part l 2021 carbon factors heat pump question has one short answer. Grid electricity is now treated as far less carbon intensive than it was under the previous methodology, roughly 0.136 kgCO2 per kWh in SAP 10.2 against 0.519 under SAP 2012, while mains gas barely moved at around 0.210. That single change reversed the compliance arithmetic. A heat pump scheme that failed the old Target Emission Rate can clear the new one comfortably, and a gas boiler scheme that passed easily now needs help to get over the line.

Part L 2021 came into force in England in June 2022 and delivers roughly a 31 per cent reduction in carbon emissions for new dwellings against the 2013 standards.

What changed in the calculation, not just the factors

Two things moved at once, and conflating them causes most of the confusion.

The carbon factors changed, because grid generation has decarbonised. And primary energy became the principal metric for dwellings, sitting alongside the emission rate rather than beneath it. A compliant dwelling has to pass the Target Primary Energy Rate and the Target Emission Rate, and meet the limiting fabric and services standards, with the separate fabric energy efficiency target of the previous edition removed.

The primary energy metric is the one that keeps electric heating honest. Electricity has a primary energy factor well above 1, because generation and distribution losses are counted, while mains gas sits close to 1. So the emission rate flatters electric heating and the primary energy rate pulls it back. A design that passes one and fails the other is a design with a sizing or an efficiency problem, usually both.

The notional building and what it assumes

Compliance targets are derived from a notional dwelling of the same size and shape as yours, specified in Approved Document L Volume 1. Read the specification table before you assume anything, because the notional building is where the real standard lives.

What matters about it for this argument is that the notional dwelling is specified with improved fabric values, a gas boiler where mains gas is available, waste water heat recovery, and photovoltaics on houses. The targets are then calculated from that specification.

The consequence is counter-intuitive. Because the notional building includes photovoltaics, a real gas boiler scheme without them starts behind. And because the notional fabric is already good, there is less headroom to trade fabric improvements against a worse heating system than designers are used to.

How a heat pump scheme compares

An air source heat pump running at a seasonal coefficient of performance around 2.8 to 3.2 delivers heat at well under half the carbon intensity of a gas boiler once the SAP 10.2 electricity factor is applied. On the emission rate, that is a straightforward pass on most house types.

The primary energy rate is where the design work is. Three inputs decide it:

• the design flow temperature, because a heat pump sized for 55 degrees performs materially worse than one sized for 45 or lower
• the emitter sizing, since low flow temperatures need larger radiators or underfloor distribution, and a scheme that keeps the old radiator schedule will not achieve the assumed efficiency
• the hot water strategy, including cylinder size, coil area, standing losses and whether an immersion heater is doing more work than anyone admits

Get those right and primary energy follows. Get them wrong and the SAP output will say so, usually about 3 weeks before the planning submission is due.

Fabric still earns its keep. A low flow temperature only works in a dwelling with a low heat loss, so the fabric and the heating system have to be designed as one thing. Junction detailing is part of that, and the issues in thermal bridging detail at planning stage feed straight into the calculated heat loss.

Where gas boilers now struggle

Gas is not prohibited by Part L 2021. It is simply harder.

A house with a gas boiler can comply, generally by adding photovoltaics, improving fabric beyond the limiting values, and adding waste water heat recovery. The combination is workable on a detached house with a decent roof. It becomes difficult on a flatted scheme, where roof area per dwelling is small and the array has to compete with plant, access and any blue or green roof.

There is also the policy direction to think about. The Future Homes Standard has been consulted on and announced, with start dates that should be checked on gov.uk rather than assumed, and the clear trajectory is away from new fossil fuel heating. Specifying gas on a scheme that will complete several years after permission invites a question at committee that is awkward to answer.

Refrigerant choice is the other side of the heat pump argument, and planning officers now ask about it, as set out in refrigerant choice and global warming potential.

The fabric trade-off, and its limits

Part L sets limiting values for fabric and services that cannot be traded away, whatever the overall calculation says. That floor is the thing people forget when they try to buy compliance with a larger array.

The air permeability line causes the most trouble in practice. A design assuming 3 m³/h.m² at 50 Pa and an as-built test at 5 produces a dwelling that does not comply with the submitted calculation, and the fix at that stage is expensive.

Overheating is the related risk. A well sealed, well insulated dwelling with large south-facing glazing will fail Approved Document O, and the top-floor units described in overheating in roof level and mansard flats are where it shows up first. An Overheating Risk Assessment using dynamic modelling is often needed where the simplified method cannot be met.

What the energy statement has to show

Planning officers are not building control, and they do not need the full compliance pack. They need enough to see that the submitted strategy is the one that will be built.

Include the heating and hot water strategy in a sentence, the assumed seasonal performance and design flow temperature, the fabric values proposed against the limiting values, the installed generation capacity with the roof plan, and a table of Target and Dwelling Emission Rates and Target and Dwelling Primary Energy Rates. Where the local plan carries its own carbon policy, show the development against both the national target and the local one, because they are measured differently.

That last point catches schemes in London in particular. A percentage improvement policy measured against a Part L 2021 baseline is a much harder test than the same percentage measured against the 2013 baseline, since the easy savings have already been taken into the baseline. Check which baseline the adopted policy and its guidance refer to before you promise a number.

Illustrative example. A pair of semi-detached houses with an air source heat pump at a 45 degree design flow temperature, underfloor heating at ground floor, oversized radiators above, 3.5 kWp of photovoltaics and a design air permeability of 4. That passes both metrics with room to spare. Swap the heat pump for a gas boiler and the same houses need the array roughly doubled and the fabric improved to hold the primary energy rate.

PlanScope drafts an Energy Statement from your project answers and inserts a flag wherever a calculated figure is missing, so the SAP outputs still come from your assessor. Reports should be reviewed before submission, and compliance modelling needs a qualified energy assessor. This is general guidance, not legal advice; check the current Approved Document L and your local plan policy 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.