For the study, researchers monitored 77 heat pump systems in single- and multi-family buildings, recording seasonal performance factors (SPF) ranging from 2.6 to 5.4. For 2024, heat pump CO₂ emissions, calculated using time-variable factors, were 64 percent lower than those of gas boilers. The project also examined long-term noise levels and the integration of photovoltaic systems. Two energy suppliers and nine heat pump manufacturers partnered with Fraunhofer ISE on the research.
Heat pumps are gaining traction in Germany, topping sales of heating systems for the first time in early 2025. In new builds, nearly 70 percent of homes completed in 2024 use a heat pump. Despite this progress, many owners of older buildings remain sceptical about whether heat pumps are suitable for retrofit. The study shows these concerns are largely unfounded. “Our results make it clear: heat pumps work efficiently in older homes, delivering climate-friendly heating without the need for major renovations,” says Danny Günther, team lead at Fraunhofer ISE. “Still, we identified room for optimisation, such as planning and installation errors and inefficient operation.”
Seasonal performance factors ranging from 2.6 to 5.4
The monitored heat pumps outperformed those in a previous project, with air-to-water units averaging an SPF of 3.4, up from 3.1. Ground source units averaged 4.3, compared to 4.1 previously. No correlation was found between building age and efficiency. Electric heating rods, used as backup during cold spells, accounted for just 1.3 percent of electricity consumption in air-to-water systems and close to zero in ground source systems.
Heat pumps could cut UK household heating bills by half
Compared to gas boilers, heat pumps reduced CO₂ emissions by 68 percent, based on the 2023 German electricity mix. When calculated using quarterly updated emission values, savings were slightly lower at 64 percent. Fraunhofer ISE developed a method for long-term noise measurement, testing five air-to-water heat pumps. In two cases, background noise was the dominant factor, while in three cases nighttime noise limits were exceeded—though levels always remained below ambient thresholds. These issues could have been avoided through better device selection, positioning or soundproofing.
Combination with PV and recommendations
The study also assessed PV and heat pump combinations. Without battery storage, PV-equipped homes achieved 25 to 40 percent autonomy and 22 to 37 percent self-consumption. With a battery, autonomy rose to 32 to 62 percent and self-consumption to 40 to 83 percent.
A symphony of energy assets – how orchestration cuts costs
Despite good overall efficiency, many systems were oversized and some exhibited high switching frequencies. In several cases, combined storage tanks did not reliably separate temperatures for heating and hot water, resulting in unnecessary heating at elevated temperatures. To address these issues, the team developed a process matrix outlining common quality gaps and solutions across planning, installation and commissioning. The recommendations provide guidance for planners, installers and operators.
Of the 77 systems studied, 61 were air-to-water heat pumps and 16 were ground source. The buildings dated from 1826 to 2001, with floor areas ranging from 90 to 370 m² (average 170 m²). Older homes had undergone above-average insulation upgrades. Partners included leading manufacturers and utilities, with funding from the Federal Ministry for Economic Affairs and Energy. (hcn)
