Linda Warner “As a SAP assessor, I’m asked countless client questions. Most of the time I have the answers ready but recently I was asked one that deserved serious brainstorming: can a domestic build pass Part L using only electric heating?”
Watt’s Wrong with Electric Heating?
Electric panel heaters are cheap and easy to install. But electricity as a fuel is carbon intensive and expensive. Whilst it’s true that electricity panel heaters are 100% efficient, electricity emits more than twice the carbon emissions of mains gas, and according to SAP is, can be nearly 4 times more expensive. But some developments can’t use gas, whether due to geography or infrastructure restrictions in. Can carbon hungry but easy to install electric heaters shine in these cases?
To solve this conundrum, I assessed a sample of domestic buildings, which utilised panel heaters and previously complied with Part L 2010, under Part L 2013. Of course, it was failure across the board. Under Part L 2013, Target Emission Rates have dropped by approximately 6% and the electric heaters were already stretching the limits back then. It was up to me to pull these designs up to current standards by any means necessary. With all of the design information input into SAP, I was free to tweak any variable I pleased while keeping the electric heating constant to see if it was possible to find the right balance.
The first approach was simple brute force offsetting with photovoltaic panels. This would allow for minimal changes to the designs but the amount of panels required clearly wasn’t feasible – in some cases exceeding the available roof space. They also wouldn’t be practical for every design, due to limitations from local aesthetics and over shading issues.
Next up were the thermal elements, and the designs couldn’t get away without changes here. Part L now includes what’s called an elemental recipe: set u-values for each thermal element which must be met or bettered in order for a build to pass. Had the designs used gas heating, improving the thermal performance of the floors, walls and roof would have been enough but with electric the designs still fell far short. I had limited the use of photovoltaic panels to be realistic, so I had to find other add-ons to bear the rest of the burden. Bumping glazing up from double glazing to triple saw a decent drop in emissions but the star performers were heat recovery systems.
The premise of these systems is to recover the heat that would otherwise be wasted from extracted air or shower water. Mechanical ventilation with heat recovery and waste water heat recovery, reduce the space heating and hot water demand of a property, reducing the output of the panel heaters and subsequent carbon emissions. While these scenarios showed that developers still have some flexibility even with electric heating, my advice is still to avoid it whenever possible. The designs may have been able to pass the regulations but PV panels, WWHR and MVHR systems don’t come cheap.
So what other options does a developer have if there’s no gas on site?
You could consider air source heat pumps, which take the energy from the outside air, compressing and releasing it into the home for space and/or water heating. They’re harder to install into an existing build than electric heaters but their higher efficiency makes them more energy efficient and less carbon intensive. Ultimately, you never know what’s best until you test.
At Energist, we love spending time in our virtual testing playgrounds, changing variables and tweaking designs to keep up with new technologies, new regulations and new ideas.
By Linda Warner, Senior Energy Assessor at Energist UK