Part L 2021 – EPC Implications

The long-awaited uplift to Part L (2021) was released in December and is in force from June 2022.  It makes for interesting reading when considering the impact for those buildings that already are or have plans to heat electrically via heat pumps and those that continue to be heated via gas boilers.   

Currently, when conducting an EPC assessment, a building is assessed under Part L 2013 which assumes a carbon factor for grid electricity of 0.519kgCO2/kWh, when taking into account the seasonal Coefficient of Performance (sCoP) of a heat pump (typically 3-4), there is not much of a ‘win’ with respect to improving the EPC rating from installing a heat pump compared to a gas boiler.  In light of the expected strengthening of the Minimum Energy Efficiency Standards (MEES) rating of ‘E’ to ‘B’ by 2030, the case for replacing a gas boiler with a heat pump has not been as strong as it should be.

Carbon Factors

The new Part L moves away from a fixed carbon emissions factor to monthly factors, this change mimics reality where the carbon emissions factor is typically higher over the winter months. 

CO2 Monthly Emissions Factors for Grid Electricity































These carbon factors should result in approximately an 81% reduction in carbon emissions compared to a gas boiler situation assuming a sCoP of the heat pump of 3.5. 

Heat Pumps

Our modelling of the new regulations is showing that for those buildings that are heated electrically, there will be a likely betterment of the EPC rating due to the change in the carbon factors.  The reason for this is that the ‘Reference Building’, the building that the Actual Building is compared against will remain specified with a gas fired boiler.  On our test building, the EPC rating improved by 10 Asset Rating points just by assessing under Part L 2021.

Gas Boilers

The unintended consequence of the change in the electrical carbon factors is for buildings heated via gas boilers to worsen their ratings.   The answer as to why this is the case is due to how EPCs are compared to a Reference Building.  Within this building the lighting figures are poor and under the new Part L the contribution to the carbon emissions of the lighting lowers.

In the building we’ve completed an EPC assessment for, the lighting carbon contribution has dropped from 65% of the carbon emissions for the Part L 2013 EPC to only 28% in a Part L 2021 EPC.   This affect is mitigated in a heat pump heated building due to a) the carbon factor being lower for electricity than gas and b) the primary energy usage from a heat pump is significantly lower than a gas boiler.

The effect of this change is that those buildings heated by gas that do have an EPC ‘B’ now may not still have an EPC ‘B’ when 2021 Part L comes into force.  The risk would be assessing now and lodging an EPC ‘B’ and then not carrying out any upgrade works in the interim years to 2032.  If this were to occur, the building would likely be worse than a ‘B’ and would no longer comply with MEES legislation. 

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KJ Tait Engineers can assist with any aspect of MEES and EPCs from general guidance to assisting in completing calculations.