Heat pumps beat gas heating systems even in old buildings

05.11.2025

Source: E & M powernews

A multi-year Fraunhofer project shows: Heat pumps can also work efficiently in unrenovated existing buildings - but it also revealed planning and installation errors.

Heat pumps also provide efficient and climate-friendly heating in existing buildings. This is the conclusion of a research project by the Fraunhofer Institute for Solar Energy Systems ISE, the institute announced on November 4. In the project, the scientists spent four years taking measurements on 77 heat pumps in one to three-family homes.

The systems examined in the study achieved annual coefficients of performance of between 2.6 and 5.4, an improvement in efficiency compared to previous studies. Taking into account time-variable electricity mix data, the CO2 emissions of the heat pumps were 64 percent lower than those of natural gas heating systems.

The study shows that efficiency is independent of the year of construction of the building. Both panel heating and sufficiently large radiators enabled low flow temperatures. Heating rods were hardly ever used, but according to the researchers, this was also due to the comparatively mild weather conditions during the period measured.

On average, air-to-water heat pumps achieved an annual coefficient of performance (COP) of 3.4, i.e. they generate 3.4 units of heat from one unit of electricity. The air/water heat pump with the lowest efficiency achieved an APR of 2.6, the one with the highest 4.9. In the previous project, the average was 3.1.

Combination with photovoltaics increases self-sufficiency

The researchers at Fraunhofer ISE also analyzed the combination of heat pumps with photovoltaic systems. A classic approach to increasing self-consumption of locally generated PV electricity is to raise set temperatures when there is surplus PV electricity.

According to these study results, operating the heat pump increasingly with solar power can have advantages: Solar electricity is cheaper than electricity from the grid, even with heat pump tariffs, heat pumps can be operated in a more climate-friendly way and the distribution grid can be relieved at certain times.

The results of the study of six heat pump/PV combinations: Without a battery, buildings with a PV system achieve 25 to 40 percent self-sufficiency and 22 to 37 percent self-consumption. With a battery, these ranges shift significantly upwards with values for building self-sufficiency of 32 to 62 percent and for building self-consumption of 40 to 83 percent.

Project also highlights problems

Despite the good efficiency values measured, the research project also revealed potential for optimization. For example, many heat pumps were oversized in relation to consumption, and the switching frequencies for some systems were also very high. In some of the systems with combination storage tanks, the temperature levels for space heating and DHW heating were not reliably separated, which in some cases led to unnecessary heat being supplied at DHW temperature level.

Based on the analysis of the measurement data and the feedback from the stakeholders, the Fraunhofer ISE research team therefore created a process matrix in the final report. This documents typical sources of error during planning, installation and commissioning and shows how these can be avoided.

The project examined 61 air/water and 16 ground source heat pumps. Around half of the buildings examined were retrofitted with insulation. The results support the assumption that heat pumps are an efficient and climate-friendly heating option even in partially renovated or unrenovated existing buildings.

The project partners were nine manufacturers, two energy suppliers and the Federal Ministry for Economic Affairs and Energy, which funded the project.

The detailed results of the research project "Heat pump quality assurance in existing buildings", or "WP-QS in existing buildings" for short, can be found on the ISE project page.

Author: Heidi Roider