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How to Pass L1a with Electric Heating

24 Aug 2017 at 12:10 PM

We are using more power than at any other point in history. And using electricity in a building has much higher associated emissions than using gas.

Much of this is due to energy wasted in generating electricity in a power station, and the energy lost during its transmission and distribution through the grid.

Even though the grid is being decarbonised by renewables and more efficient power generation, the assessment methods we use today still assume a much higher carbon intensity compared to gas.

Previously we have looked at Using Different Fuels in SAP and the Fuel Factor which highlights how using electric heating will increase the dwelling’s emissions, and also that SAP increases the target emissions to give some compensation towards meeting this.

A sample house had been modelled to determine what kind of measures  would be needed to meet compliance with electric heating. The sample house is a 3-bedroom detached house with 3 bathrooms.

Using electric heating makes it significantly harder to meet L1a compliance. Here’s what is needed for a sample detached house to pass:

When a gas system was specified, the dwelling passed with the following specification:

  • Insulation (thermal conductivity 0.022 W/mK)
    Wall 100mm
    Floor 140mm
    Roof 100mm between rafters and 90 below
  • Windows 1.4 W/m2K
  • 100% low energy lighting
  • Air test result of 5 m3/hm2
  • Accredited Construction Details (ACDs) for thermal bridging

When the heating changed to electric panel heaters for space heating and an electric immersion boiler for water heating, significant improvements were required. Different options for passing are detailed below.

Option 1 – Fabric Improvement with Waste Water Heat Recovery

  • Upgrade insulation to following thicknesses
    Wall 150mm
    Floor 150mm
    Roof 100mm between rafters and 125 below
  • Air test result of 3.5 m3/hm2 (very highly air tight)
  • Waste Water Heat Recovery for all bathrooms
  • Enhanced Construction Details (ECDs) and ACDs for thermal bridging.

These are significant improvements with demanding insulation thickness.

Option 2 – Solar PV

1.8kWp of solar PV (likely 7 or 8 panels) would be needed to meet compliance. This is assuming the panels are south facing at a 30° pitch.

Option 3 – Heat Pump

A heat pump can be used to replace either just the water heating, or both the water heating and space heating (which has a better effect). Heat pumps have the advantage of increasing the TER (because SAP applies the 1.55 fuel factor for electricity) but are much more efficient than electric panel heaters.

In our experience of assessing dwellings without a gas connection, using solar PV or a heat pump has been much more feasible than the required fabric upgrade.

It should be noted than the above measures are only based on a sample house. They are an indication of what would be required to meet L1a compliance, but actual measures required to meet compliance will vary for each house.

We fully expect regulations to become ever more stringent over the coming years. The move away from combustion engines by 2040 means that the national grid is going to be under more strain than ever.

Future proofing your projects against what will be a greater demand for efficiency makes good commercial and environmental sense.

Would you like to discuss your project?

Call us for a chat on 0330 055 34 05 or email

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