20 Years of Field Studies Prove: Heat Pumps Efficient in Existing Buildings

The largest studies were conducted in Germany, France, Switzerland, the United Kingdom, Austria, and the Netherlands. The longest study series, which has been running for 20 years as part of various projects, was conducted at the Fraunhofer ISE in Germany¹². The study with the most systems³ – 1,023 air-source and ground-source heat pumps – was conducted across multiple countries.

Although all studies pursued partially different objectives and measurement methods, some general statements can be formulated as results and findings.

Overall, one central finding emerges from all studies: Heat pumps operate well under real field conditions.

Efficiency in Real-World Operation

The efficiency of a heat pump is expressed as the ratio of heat delivered to electrical energy consumed. Unlike standardized laboratory values (Coefficient Of Performance – COP) and calculated SCOP values (e.g., according to EN 14825: Seasonal Coefficient Of Performance), the calculation of the actual Seasonal Performance Factor (SPF) or annual performance factor is always based on field-measured values. The evaluation period is typically one year in most cases.

The Efficiency of Air-Source Heat Pumps is Increasing

The graph shows an overview of efficiency results from air-source heat pumps from six studies conducted over the past 20 years that provided comparable values. Except for the “St. Gallen / ETH Zurich” study, the efficiency values were determined by measuring the heat actually provided and the electrical energy required for it, and then relating them to each other. In the “St. Gallen / ETH Zurich” investigation, the real internal parameters of heat pumps were measured and the efficiency of the systems was determined using validated models.

The graph shows the mean efficiency values, the range of results (outliers in both directions were not considered), as well as the number of systems evaluated and the evaluation period. Except for the “St. Gallen / ETH Zurich” study, in which heat pumps were installed in both new and existing buildings, all other studies focused on existing buildings with varying energy standards.

The mean efficiency values range from 2.6 in the oldest study to 3.7 in the study with the newest heat pumps, which also captured the most installations. The mean value reached 3.2 across all 840 investigated heat pumps. Generally, it can be stated that the average field efficiency increases over time – with the exception of the ADEME study⁴. This indicates an overall improvement in technology and a rising quality of planning and installation processes.

Ground Source More Efficient Than Air Source Heat Pumps

In addition to heat pumps with outdoor air as the heat source, all studies also investigated heat pumps with ground as the heat source. However, significantly fewer systems were measured here, which also corresponds to reality in the various markets. The trend is evident in all investigations, which underscores the overall result: ground-source heat pumps achieve on average a medium efficiency of 4.2, which is one point higher than air-source systems. The reason for the better efficiency lies in the characteristics of the heat source: thanks to connection to the ground (usually through borehole heat exchangers or ground collectors), the heat pump has access to a heat source with higher temperatures during cold seasons.

Room for Optimization and Potential

In all studies, the authors emphasize the large ranges of results, which point to malfunctions and room for optimization, but also to the associated potential. While some systems showed dysfunctions that offer improvement opportunities, others achieved impressive performance. Efficiency values of more than 4.5 for air-source and even more than 6.5 for ground-source systems demonstrate the promising potential of the technology when optimally installed and configured. The wide distribution of results is also related to the quality of the heat pumps themselves. The investigated systems come from production facilities with already varying performance figures. The subsequent spread of field efficiencies is a logical consequence.

Correct Settings are Crucial

“The greatest potential for efficiency improvement lies in the careful adjustment and readjustment of the heating controller.⁵⁶” This sentence, in a slightly modified form, is found in all investigated studies.The setting of the heating curve is particularly mentioned as a central control parameter that can be quantified. This curve defines the relationship between outdoor temperature and the supply temperature provided by the heating system. A rule of thumb, confirmed by both empirical investigations⁶ and simulations, states: A parallel reduction of the heating curve (without changing the slope of the curve) by 1 °C improves the system efficiency by 0.1. A rough estimate of cost savings for an existing building in Germany amounts to approximately €60 per year.

Conversely, incorrect settings lead to efficiency losses and unnecessary electricity consumption. For example, when the heat pump switches on and off too frequently, consumes too much energy in standby mode, or even runs outside the heating period.

Heat Pumps Also Work with Radiators

Another common finding from the studies is that heat pumps can work well not only with underfloor heating but also with radiators. When considering both systems, supply temperatures⁷ are decisive. Naturally, heat distribution systems with low temperatures achieve the highest efficiencies, but many systems with radiators also showed good efficiency values of 3.0 and higher. The often-repeated prejudice that heat pumps require radiant heating for efficient operation or even only work with radiant heating is clearly incorrect.

The Building May Be Old

“Age hardly matters.” That would be the shortest summary from the studies investigating the relationship between construction year and efficiency. While the construction year indicates the original energy standard, the current renovation status of the building and the current supply temperature of the heating system are the decisive factors for actual efficiency. The studies confirmed that heat pumps can operate efficiently even in older buildings. Reducing temperature requirements, which can be achieved, for example, by replacing the original radiators with so-called low-temperature radiators, as well as planning and hydraulic optimization, are always advantageous for efficiency improvement.

Prior renovation is beneficial but not a prerequisite. In the vast majority of cases, heat pumps function efficiently even in unrefurbished or only partially refurbished buildings. It is important that sufficient heat transfer surface area is available for the building’s heat loss to provide user comfort.

Cold Wave Test Passed

The question of how heat pumps operate at significantly sub-zero temperatures is important not only for end users (Will the house be warm enough? Won’t it be too expensive?) but also for the overall energy system (What is the electrical power demand? Is the distribution grid adequately dimensioned?). The studies also provide clear findings on this⁸.

During the coldest period of the Fraunhofer ISE project series⁹ (February 2021), the mean outdoor air temperature for the evaluated heat pumps was -3.6 °C (in the past 50 years, there have been only five months with mean temperatures below -3.5 °C in Germany). The efficiency of 17 systems (the three best in fully renovated houses were not considered) during this time was 2.3, with the range between 1.6 and 2.8. Even in cold weather, more than twice as much heat could be obtained from the ambient air for each kilowatt-hour of electricity. The system with the lowest efficiency had to operate at the lowest mean outdoor air temperature of -10.2 °C. Only five systems used backup electric heaters, which were included in the efficiency determination.

During the investigation period of the ADEME study, there was a “cold wave” in January 2024. At an average temperature of -4 °C, a mean efficiency of 2.0 was measured for the entire sample.

Conclusion

In public discourse, it is still frequently claimed that heat pumps do not function efficiently at sub-zero temperatures, in old buildings, or in buildings without underfloor heating.

These theses are clearly refuted by the evaluated, scientifically monitored field measurements. Heat pumps can fundamentally achieve high real-world efficiencies in existing buildings despite challenges from higher operating temperatures due to existing radiators or lower building efficiency standards.

The actual efficiency depends less on the type of heating or the building’s construction year than on the quality of planning and installation, correct controller settings, and low supply temperatures. Trained installers will therefore remain important. Their work can be meaningfully supported by planning tools and service programs from manufacturers and other market participants.

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The next part of the series will describe the most important technological changes in heat pumps over the last 20 years.

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