Study confirms heat pump efficiency in existing homes
2nd December 2025
GERMANY: A recent German study has concluded that heat pumps provide efficient and climate-friendly heating even in older existing buildings.
Over four years, researchers from the Fraunhofer Institute for Solar Energy Systems (ISE) conducted detailed measurements on 77 heat pumps in single-family to three-family homes. The results showed that the heat pumps achieved seasonal performance factors ranging from 2.6 to 5.4.
The CO2 emissions of the heat pumps, calculated for the first time considering time-variable factors, were found to be 64% lower in 2024 compared to those of natural gas heating systems.
The research team also conducted long-term sound measurements and investigated how photovoltaic systems can be integrated into heat pump operation. In addition to Fraunhofer ISE, two energy suppliers and nine heat pump manufacturers participated in the project.
In the first half of 2025, heat pumps outsold all other heating systems for the first time in the German market. In new buildings, heat pumps have dominated for years, with nearly 70% of new German constructions completed in 2024 using a heat pump for heating. However, homeowners of older existing buildings are still wondering whether these heat generators can also operate efficiently and climate-friendly in their homes.
The researchers maintain that these doubts are unfounded, but recognise that there is also optimisation potential.
“The results clearly show that heat pumps can be operated efficiently even in older buildings and that they provide climate-friendly heating without the need for the buildings to be renovated to new construction standards,” said Danny Günther, team leader of the Heat Pumps and Transformation of Existing Buildings project. “However, we have also uncovered optimisation potential. Based on the detailed analysis of measurement data, it can be understood which planning or installation errors occur most frequently and where inefficient operational behavior is evident.”
The examined systems included 61 air-source units and 16 ground-source systems using a brine/water heat pump.
For the efficiency evaluation, heat pumps in buildings constructed between 1826 and 2001 were considered. The heated areas ranged from 90m2 to 370m2, with an average of 170m2.
Air-to-water heat pumps were found to achieve an average seasonal performance factor (SPF) of 3.4. The lowest efficiency air-to-water heat pump had an APF of 2.6, while the highest reached 4.9. The more efficient ground-coupled systems exhibit an average APF of 4.3.
No correlation between the year of construction of the buildings and the efficiency of the heat pump was found.
The study also showed that adequately sized radiators can be operated at similarly low temperatures as underfloor heating. The new report maintains that the energy consumption of the supplementary electric heating back-up plays a minor role in the measured systems, which is also linked to the comparatively mild weather conditions during the measurement period. They accounted for only 1.3% of the electrical work in air/water heat pumps, while the share was close to 0% for ground-source heat pumps.
Partners of Fraunhofer ISE included heat pump manufacturers Bosch Thermotechnik, Glen Dimplex Deutschland, Max Weishaupt, NIBE, Panasonic Heating & Ventilation Air-Conditioning Europe, Daikin Germany, Stiebel Eltron, Viessmann, and Vaillant, as well as energy suppliers Lechwerke and Stuttgart municipal utilities.
