A Luxembourg-based simulation study compares exhaust-air and air-source heat pumps in a residential building, highlighting differences in electricity consumption and the importance of technology selection and system sizing.
Derya Yilmaz, Luxembourg Institute of Science and Technology (LIST)
Maria Letizia Fasci, Bengt Dahlgren
Antonino Marvuglia, Luxembourg Institute of Science and Technology (LIST)
Sylvain Kubicki, Luxembourg Institute of Science and Technology (LIST)
(Note: Opinions in the articles are of the authors only and do not necessarily reflect the opinion of the European Union)
In response to the increasingly evident impacts of global warming, societies are confronted with significant climate-related challenges in advancing towards achieving climate neutrality by 2050 [4]. Within this context, the European Green Deal provides a comprehensive policy framework aimed at achieving this objective, identifying the transition from fossil fuel–based heating systems to HP technologies as a key strategic lever [6]. In parallel, leading international organisations, including the Intergovernmental Panel on Climate Change (IPCC) and the International Energy Agency (IEA), as well as consulting firms such as McKinsey & Company, emphasise the critical role of HPs in the decarbonisation of the heating sector, which remains predominantly dependent on fossil fuels [9].
Several advantages are associated with HPs. They support the decarbonisation of heating and cooling systems [11], reduce demand for non-renewable energy sources, and enhance energy security [9]. Moreover, they lower carbon and particulate matter emissions (thus contributing to improving air quality) and require low maintenance [3]. In addition, they can contribute to enhancing energy flexibility when connected to the electrical grid [6].
This study was conducted within the Horizon Europe–funded POSIFIT project, which aims to develop an advanced and dynamic integrative approach for achieving positive energy homes through smart and innovative solutions. These include the integration of active and passive strategies, such as improvements to the building envelope and HVAC systems, combined with smart management technologies, such as Building Management Systems (BMS) and Building Automation Systems (BAS).
In Luxembourg, about 10.5% of total energy consumption is attributed to residential buildings, and nearly 80% of the sector’s energy demand is met by natural gas and fuel oil products [2]. In this context, the deployment of HP technologies represents a key strategy for reducing fossil fuel use and advancing long-term decarbonization pathways in the building sector. In this study, the performance of an exhaust-air HP, which represents the current deployment in the studied pilot building, was compared to a conventional outdoor air-source HP using IESVE energy simulation software for a single-family dwelling in Luxembourg.
While electric boilers are primarily used in Eastern Europe, HPs are widely adopted in Central and Northern Europe, whereas direct heat, such as geothermal heat, district heating, and solar heat remains common in Northern Europe [12]. Although the market penetration of HPs is highest in new single-family buildings, they are becoming increasingly common in non-residential buildings, apartment buildings, and older buildings [10]. Heat produced by HPs accounts for only about 0.5% of total heat production in Luxembourg. However, the annual growth rate in the total number of installed HPs has been observed to exceed the EU 28 average [2]. Gagliano et al. (2025) state that about two-thirds of the heat demand in buildings is met by combustion-based heating systems that could be replaced by HPs. These could supply more than four-fifths of global space heating and domestic hot water (DHW) demand, with CO₂ emissions only 0.2–0.3 times those of the most efficient condensing gas boilers.
Significant innovations are driving major changes in the HP sector, including developments in working fluids, compressors, heat exchangers, novel thermodynamic cycles, smart control systems, and integration with renewable energy sources and multi-vector energy storage. In addition, advances in monitoring, diagnostics, and demand-side response mechanisms are further transforming the sector [11]. However, a survey examining Luxembourg residents’ perceptions of climate change and mitigation measures found that respondents tend to underestimate the impact of installing HPs for heating and cooling [5]. This low rating may be related to lower familiarity with the technology compared with more widely adopted solutions such as solar power (OPC, 2024). According to Decuypere et al. (2022), a lack of knowledge and information, as well as financial barriers, are among the factors that make it difficult to encourage individual homeowners to invest in energy-efficient technologies such as HPs or solar panels. Cost, housing readiness, and homeowner behaviour all influence the perceived suitability of HPs for residential heating [8].
HPs use similar components to refrigerators and air conditioners and are effective at transferring heat from external sources to where it is needed, such as for space heating in buildings or for hot water production [9]. They are generally considered high-performance because they operate by transferring heat rather than generating or converting it. Unlike boilers, which produce thermal energy by converting the chemical energy of fuel through combustion, HPs move existing heat from one location to another, allowing them to achieve higher efficiencies than conventional boilers [1]. HP technology utilises thermal energy present in the air, ground, or water to transfer heat to homes [8]. An HP is a power-to-heat (P2H) technology that moves heat from one or more low-temperature sources (Tsource) to one or more high-temperature sinks (Tsink) with the help of an external energy source [1]. Figure 1 illustrates the operating principle of an HP.
Figure 1. Operating principle of a heat pump. Source: Adapted by the authors from Adamo et al. (2025).
Air source HPs, which resemble conventional air conditioning units, extract heat by using fans to blow air across a large surface area. Ground source HPs capture heat from the earth through networks of horizontal pipes or deep boreholes. Less common water source HPs draw heat from water bodies such as lakes, rivers, and seas [9].
The pilot building (see Figure 2) was constructed by a public developer that plans, builds, and sells affordable housing units, mainly for middle-income households.
Figure 2. Luxembourg pilot building.
The building is a three-storey semi-detached single-family house located in Betzdorf, Luxembourg. The project location is classified as a Cfb climate zone according to the Köppen-Geiger classification system. The building is designed as a well-insulated dwelling, featuring clay block walls, mineral wool insulation in the roof, triple glazing, wooden window frames, and 15 monocrystalline solar panels. Table 1 illustrates the building envelope, PV panels, and HP characteristics.
Table 1. Building characteristics.
In this study, we considered two HPs. The first (HP1) is an exhaust-air HP. Specifically, it corresponds to the current installation in the house, which integrates two HPs operating in parallel, one serving domestic hot water production and the other space heating and cooling. Both HPs use building exhaust air as the heat source. Before HP operation, heat is recovered from the exhaust air via a ventilation heat recovery unit to preheat the incoming outdoor air. The remaining thermal energy in the exhaust air is then extracted by the HPs. The main advantage of this type of HP is its higher efficiency compared with systems using lower-temperature heat sources, such as outdoor air–source HPs. However, the capacity of exhaust-air HPs is inherently limited by the available ventilation airflow. As a result, they are often unable to meet the full heating demand of a building, particularly when domestic hot water demand is significant, as in well-insulated single-family houses. This limitation increases reliance on an auxiliary heating system, which is typically less efficient.
In contrast, a conventional outdoor air–source HP can be sized to match the total building load and can therefore supply both space heating and domestic hot water with little or no need for auxiliary heating. For this reason, we also considered a second HP (HP2), an air source heat pump, which can cover the entire heating demand of the building with minimal or no auxiliary heating. It is a modern air-to-water heat pump that uses R290 (propane) as a refrigerant—a climate-friendly refrigerant with a low global warming potential—and is controlled by an inverter to maximise part-load operation.
Figures 3 and 4 illustrate the comparative performance of HP1 and HP2. For each system, the reported values correspond to the mean of the results obtained across all scenarios utilising that HP configuration.
HP2 performs better than HP1. In fact, HP1 requires 7.1 MWh of electricity during the year, while HP2 requires 2.7 MWh, i.e., 2.6 times less electricity. The difference in performance is primarily driven by space-heating demand. HP1 consumes approximately 5.5 MWh of electricity to provide space heating over the year, compared to about 1.4 MWh for HP2 (Figure 3). This reflects the higher efficiency of HP2 under the considered operating conditions. In contrast, the difference in electricity consumption for domestic hot water production is smaller, as shown in Figure 4, indicating that the performance gap between the two systems is less pronounced for this end-use.
The underlying reason for these results is the difference in installed capacity of the two systems. Due to its limited capacity, HP1 is frequently unable to fully meet the space-heating demand and therefore relies more heavily on the auxiliary heating system, which leads to increased electricity consumption. HP2, by contrast, can cover the full building heating demand with little or no auxiliary operation, resulting in significantly lower annual electricity use. Since domestic hot water production is prioritised by both HPs, HP1 is often able to meet the full DHW demand, resulting in a smaller difference in electricity consumption between HP1 and HP2 for this end-use compared to space heating, for which HP1 heavily relies on the auxiliary system.
Figure 3: Energy consumption of the heat pumps to satisfy the space heating demand, average for HP1 and HP2.
Figure 4: Energy consumption of the heat pumps to satisfy the domestic hot water demand, average for HP1 and HP2.
The study compares the performance of an exhaust-air heat pump (HP1) and a conventional outdoor air-source heat pump (HP2) for a residential building in Luxembourg using dynamic energy simulation. The results indicate that the air-source heat pump provides significantly better overall energy performance, requiring about 2.6 times less electricity annually than the exhaust-air HP, mainly due to its ability to fully meet the building’s space-heating demand without relying heavily on auxiliary heating. In contrast, the limited capacity of the exhaust-air HP increases dependence on auxiliary systems and raises electricity consumption. These findings highlight the importance of appropriate HP sizing and technology selection when designing efficient low-carbon heating systems for residential buildings in Luxembourg and similar climates.
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