Efficiency knows no age
Analyzing heat pumps in existing buildings is a promising task. Although average values indicate a trend, only a detailed examination of individual systems reveals the true potential and pitfalls in existing buildings. We take a look at six very special houses – from a historic building dating from 1826 to a property built in 1995. These detailed case studies show that it is not the age of the buildings that is decisive for high efficiency, but rather competent planning and the correct adjustment of the entire heating system, especially the hydraulics.
First, the six case studies are presented individually and in detail to highlight their specific challenges and success factors. A concluding summary summarizes the key, cross-system findings from this series.
As part of the WP-QS research project, 77 heat pump systems in existing buildings were examined using measurement technology. This analysis focuses on six selected properties that represent a representative spectrum of different conditions and operating characteristics. A detailed analysis of all the properties examined can be found in the final report of the project.1 The efficiency of the selected systems is both below and above the average efficiency values of the heat pump systems from the „WP-QS in Existing Buildings“ project.
The selection includes four outdoor air heat pumps and two ground source heat pumps with geothermal probes. The buildings represent different age classes from 1826 to 1995 and different levels of energy-efficient renovation. Particular attention is paid to the Seasonal Performance Factors (SPF) achieved, the heating circuit temperatures, and the hydraulic and control-related features of the individual systems.
Poor Energy Standard, Good Efficiency
System ID 541 is located in a building constructed in 1950 with 120 m² of heated space. The energy efficiency rating is poor – only the windows have been renovated, while other components remain in their original condition.
Despite the unfavorable building conditions, the outdoor air heat pump with radiator system achieves a remarkable SPF of 3.5. The COP value (coefficient of performance) at A2/W35 is 4.3, which indicates an efficient heat pump. There is a hybrid system, but the additional heat generator has no significant influence on operation.
The average heating circuit temperature is 42°C, the maximum 59°C. This makes this system one of those with the highest system temperatures in the comparison field. The relatively high temperatures are due to the poor energy efficiency of the building.
At 145 kWh/(m²a), the building has a high specific heat consumption, which reflects the inadequate insulation. Hot water accounts for 14% of the total energy demand, which is typical for single-family homes.
Conclusion:
Despite the high flow temperatures, the system achieves good efficiency. The system demonstrates that economical Seasonal Performance Factors can be achieved even in unrenovated old buildings with high system temperatures.
Good Energy Standard, Average Efficiency
The ID 546 system, built in 1987, has a heated area of 165 m². Unfortunately, no detailed information is available on the renovation measures carried out, but the low energy consumption suggests a good energy standard.
The outside air heat pump works with a radiator system and achieves an SPF of 3.3. The COP at A2/W35 is 3.7, which is rather low compared to other systems (for comparison: in Episode 3, the COP values of heat pumps available on the market were analyzed).
With an average heating circuit temperature of 33°C and a maximum of 47°C, this system has rather low system temperatures compared to all other outdoor air heat pumps examined. This enables, on average, efficient operation.
At only 53 kWh/(m²a), the system achieves the lowest specific heat consumption of all six properties examined. The hot water share is 22%, which can be explained by the low heating demand.
Conclusion
Despite the building’s good energy efficiency and low operating temperatures, the system’s efficiency is only 3.3. The most likely explanation for this is that the heat pump itself has a low basic performance (COP values) compared to more modern models.
High Heating Circuit Temperatures
The ID 514 system was built in 1995 and is in good original energy condition, which has been further improved by minor renovation measures. With a heated area of 170 m², it represents a typical single-family home from the 1990s.
Heat is supplied by an outside air heat pump with a radiator system and a combination storage tank. The system achieves an SPF of 3.0, which is a solid result given the comparatively high system temperatures.
It is striking that, despite being the most recent of the six properties and of all properties in the Fraunhofer ISE project, this system has the highest average (47°C) and maximum heating circuit temperatures (64°C). The heating curve is very flat, which means that almost identical flow temperatures are achieved at outside temperatures of 5°C and 15°C.
The specific heat consumption before the heat pump was installed was 144 kWh/(m²a). However, after the heat pump was installed, a significantly reduced heating energy consumption of 70 kWh/(m²a) was measured. This even includes hot water production. The heating element was not used during the measurement period.
Conclusion
The high heating circuit temperatures are not consistent with the low heating energy requirement. After optimizing the heating curve and potentially installing low-temperature radiators, a significantly higher efficiency should be achieved.
Very Good Efficiency in an Old Building
Built in 1875, the System ID 571 is one of the oldest buildings in the study. With 360 m² of heated space, it is a spacious existing building. No precise details are available on the specific renovation measures, but its visual condition indicates that only minor modernization work has been carried out.
The outdoor air heat pump with radiator system achieves a very high seasonal performance factor of 4.0. This value is significantly above the average of 3.4 for all air-to-water heat pumps examined in the project. The COP value at A2/W35 is 4.4, which indicates a highly efficient heat pump in combination with favorable operating conditions.
With an average heating circuit temperature of 36°C and a maximum of 57°C, the system operates in the low to medium range. This is remarkable for an unrenovated 19th-century building and indicates generously dimensioned radiators. The specific heat consumption is 110 kWh/(m²a), which is acceptable for a building of this age with minor renovation measures. The complex piping around the heating storage tank along with some hydraulic-related flow errors is striking.
Conclusion
Despite the hydraulic shortcomings, the system achieves excellent efficiency values thanks to the high-quality heat pump and favorable system temperatures. The system impressively demonstrates that very good SPFs can be achieved even in historic, largely unrenovated buildings.
High Efficiency Potential of Geothermal Heat Pumps
System ID 569 is located in the oldest of the buildings shown, dating from 1826. The heated area is 240 m². Following extensive renovation work, the building now meets average energy standards.
The ground source heat pump with geothermal probes works with a mixed heat transfer system consisting of radiators and underfloor heating. With an SPF of 5.1, this system achieves the highest value of all six properties examined. In addition, a pellet boiler is installed, but this has no significant impact on operation – the heat pump covered almost 100 percent of the energy requirements.
The system operates at 32°C, the lowest average heating circuit temperature of the six systems. The maximum temperature is 60°C. These favorable temperature levels result from the combination of underfloor heating in some areas and the energy-efficient renovation that has been carried out. At 95 kWh/(m²a), the specific heat consumption is moderate.
The system has the highest performance factor (the performance factor of a heat pump describes the ratio of the actual COP achieved to the theoretically maximum possible COP according to the Carnot process and is therefore a measure of efficiency compared to the physical ideal) of all ground source systems at 0.52, which certifies the very good efficiency quality of the heat pump. The COP value at B0/W35 is also very good at 4.8.
Conclusion
This system exemplifies the high efficiency potential of geothermal heat pumps in renovated old buildings with low system temperatures. The theoretical potential of both the high-quality heat pump and the excellent heat source has been fully confirmed in practice. The system was additionally
equipped with a pellet boiler. However, operation has shown that the heat pump can cover almost the entire heating load on its own, making the pellet boiler largely redundant as a backup.
Flaws During Installation Lower System Efficiency
System ID 573 is installed in a building dating from 1850 with 205 m² of heated space. Following renovation work, the building now meets average energy standards. A solar thermal system has also been installed.
The ground source heat pump with geothermal probes supplies a radiator system and achieves an SPF of 3.8. This is a rather moderate value for geothermal heat pumps, which is due to identified hydraulic obstacles.
The average heating circuit temperature is 42°C, the maximum is 51°C. Despite the geothermal source, the temperature level ranges from medium to high, which limits efficiency. The specific heat consumption is 104 kWh/(m²a), which is an average value.
The system has complex piping around the heating storage tank with a hydraulic error: the pipes were connected to the storage tank in the wrong order. This leads to a high temperature drop of more than 4 Kelvin after the storage tank in parallel connection.
Conclusion
Hydraulic errors significantly reduce system efficiency, as shown by the comparison with System ID 569. This system illustrates the importance of careful hydraulic planning and competent installation for the overall efficiency of the system.
Summary and Conclusion
The analysis of the six selected heat pump systems provides important insights for heat pump integration in existing buildings:
Efficiency potential regardless of building age: The systems examined impressively demonstrate that heat pumps can also achieve high seasonal performance factors in historic buildings. The spectrum ranges from 1826 to 1995, with no direct correlation between building age and efficiency. Rather, the decisive factors are the system temperatures and the hydraulic design.
Significance of low system temperatures: The systems with the lowest heating circuit temperatures (ID 569: 32°C, ID 546: 33°C, ID 571: 36°C) achieve the best efficiency values. This can be attributed to several factors: oversizing of the original heat distribution system, replacement with generously dimensioned radiators, energy-efficient renovation, or a combination of these measures. The analysis shows that even unrenovated buildings can achieve low system temperatures with the appropriate design.
Advantage of geothermal heat pumps: With Seasonal Performance Factors (SPF) of 5.1 (ID 569) and 3.8 (ID 573) on average, the two geothermal heat pumps achieve significantly higher efficiency values than the outdoor air heat pumps (3.0 to 4.0). This confirms the fundamental efficiency superiority of geothermal sources, although system 573 shows that hydraulic faults can significantly reduce these
advantages.
Critical role of hydraulics: Systems ID 571 and ID 573 in particular demonstrate that poor hydraulic design and installation can significantly impair system efficiency. Complex piping, incorrect flow patterns, or mixed-up connections significantly reduce the achievable SPF. Careful hydraulic design and quality assurance during installation are therefore essential. Rare heating element usage: In four of the six systems, the heating element was either not used at all or only minimally (less than 2%). This shows that even in unrenovated old buildings with higher system temperatures, largely monoenergetic operation is possible. However, the measurement period was comparatively mild in terms of climate.
Assessment of renovation requirements: The results put into perspective the frequently expressed assumption that heat pumps in existing buildings necessarily require comprehensive renovation measures. System ID 541 (unrenovated building from 1950) and System ID 571 (barely renovated building from 1875) achieve good SPFs of 3.5 and 4.0, respectively. The decisive factor is the design of the overall
system for adapted, but not necessarily very low, system temperatures.
The field measurement results underscore the great potential of heat pump technology in existing buildings. At the same time, they show that proper planning, design, and installation are crucial for achieving high efficiency levels.
Previous Episodes in the Series
Part 1: Beyond the Noise, What the Heat Pump Truly Means for Our Society
Part 2: 20 Years of Field Studies Prove: Heat Pumps Efficient in Existing Buildings
Part 3: From Niche to Norm, 20 Years of Progress in Heat Pump Technology
Part 4: The Heat Pumps Fact-Check, 10 Myths Scientifically Debunked
- D. Günther et al., „WP-QS im Bestand: Entwicklung optimierter Versorgungskonzepte und nachhaltiger Qualitätssicherungsmaßnahmen für Wärmepumpen im EFH-Bestand,“ Fraunhofer-Institut für Solare Energiesysteme ISE, Freiburg, Abschlussbericht, Okt. 2025. ↩︎