Transforming a Derbyshire Property into an Energy-Efficient Haven

Carpenterstation · 2025-03-12T14:53:17Z

When we moved into our Derbyshire home in April 2014, it was already a work in progress. The ground floor was insulated and heated with a wet underfloor system, while the upstairs had two heating circuits: one for the two bathrooms and another for the three bedrooms and hallways. Although the house felt a little small for our needs at the time, it came with planning permission for an extension. We began the extension in 2015 and completed it in early 2018, increasing the house size to 450m². The ground floor continued to use underfloor heating, while the five bedrooms and five bathrooms were heated by radiators on separate circuits. The house is well-insulated, with walls built to modern standards (using thermalite blocks and 100mm insulation) or dry-lined plasterboard inside thick, rendered old walls. The lofts are also well-insulated. However, the 11 exterior doors remain the weakest point in terms of insulation, despite all windows and doors being double-glazed. We also have four solid fuel fires, which we use sparingly during the coldest months of December through February. Energy Efficiency Challenges Despite our efforts to keep heating costs manageable (never heating the house above 20C and setting the water temperature to 55C) the running costs were significant. My usual approach was to encourage everyone to “put on a jumper,” tape up the immersion switch, lock down the nine thermostats, and turn the TRVs (thermostatic radiator valves) down to 2 or 3. While our system wasn’t inefficient, it was powered by an LPG boiler (first a Glowworm, then a Vaillant, replaced in 2020). LPG proved expensive and difficult to source, with prices rising steadily. By 2022, the cost per kWh had reached 5p, and it was clear that prices would only continue to climb. The Turning Point: Embracing Renewable Energy Our journey toward renewable energy began in 2016 when Octopus Energy took over our failing electricity supplier. Their innovative tariffs made solar panels, batteries, and two-way inverters increasingly attractive. Although I had installed a 4kW solar system in a previous home, the economic case for solar at this property only became viable in early 2023, as panel and battery prices dropped. Key milestones in our journey include: Completing the extension in March 2018. Replacing the gas hob with an induction hob in November 2019 for better air quality and ease of use. Installing a smart meter in March 2022, enabling 30-minute interval monitoring. Transitioning to a full electric vehicle (EV) in September 2022. Installing 29 solar panels (10kW peak) and two 5kWh batteries with a two-way inverter in March 2023. Moving to the Agile Octopus tariff with export capabilities, despite a challenging process. Replacing the front door in August 2023 to improve insulation. Flooding during Storm Babet in 2023, which led to replacing 70m² of carpeted floors with stone. Replacing the LPG system in January 2025 with two Mitsubishi Ecodan cascade air source heat pumps (17kW total), a new plant room and a hot water tank. Adding a third 5kWh battery to optimise low-cost electricity imports and mitigate peak prices. Switching to the Cosy Octopus tariff in February 2025. Energy Usage and Cost Analysis Our investments in solar panels and air source heat pumps totalled around £30k, with a long payback period anticipated. However, these changes have allowed us to maintain energy costs close to 2022 levels, despite rising prices. 2022:Cost: £3,758 | Usage: 39.7MWh | Average cost: 9.5p/kWhHeating was heavily used from November to April, with minimal usage from May to September. 2023:Cost: £4,253 | Usage: 36.5MWh | Average cost: 11.7p/kWhThe installation of solar panels and batteries reduced net usage from 10.7MWh to 7.5MWh. Time-of-use tariffs were exploited in Q4. 2024:Cost: £3,020 | Usage: 30.6MWh | Average cost: 9.9p/kWhBatteries and solar panels helped absorb price inflation, with net electricity imports dropping to 1.5MWh. 2025 (so far):Cost: £880 | Usage: 5.3MWh | Average cost: 16.7p/kWhTarget: 12MWh (COP 3) at 12p/kWh. Energy usage in January and February has significantly reduced compared to previous years. Key Takeaways and Lessons Learned Our journey has been a steep learning curve, with several unexpected challenges and valuable lessons: ASHP Performance: I knew that running an air source heat pump would differ from a gas boiler’s on-off cycling, but I underestimated the complexity of a cascade system. The ASHP struggles to heat the house from zero to 20°C, and defrost cycles every 40 minutes reduce efficiency during freezing conditions. Installing an ASHP in frosty weather is not ideal. Frost Damage: I didn’t anticipate frost damage to the concrete plinth under the ASHP, which now needs to be redone. Air Quality and Circulation: Constant temperatures led to mustiness, as the gas system’s heating and cooling cycles had previously forced more thermodynamic circulation. We now leave internal doors open to encourage airflow and of...