To achieve its climate protection targets, Austria has committed to transitioning from gas- and oil-based heating systems to renewable systems, as well as to high-quality energy-efficient building renovations. Although technical solutions for refurbishing multi-storey residential buildings (such as window replacement and insulation) and for replacing boilers (in centrally supplied heating systems) have existed for many years, the renovation rate has stagnated at well below 1%. If this trend continues, the climate targets will not even come close to being met. The reasons for the low level of activity are diverse. Conventional approaches are expensive, labor-intensive on-site, require scaffolding, and sometimes necessitate temporary relocation of residents. In addition, there is no satisfactory solution for decentralized heat pumps and renewable heat supply for apartments. Given the limited availability of renewable electricity and district heating, and rising demand, thermal renovation of buildings is indispensable for achieving overall climate goals. The transition from gas- or oil-based heating systems to heat pumps is often only technically feasible in combination with adequate thermal renovation and is generally recommended only in conjunction with it.

The low rate of renovation is due to a variety of barriers. Historically low prices for fossil fuels and electricity meant that energy-efficient renovations—at conventional standards—often did not offer attractive returns for investors. Moreover, the accumulated reserves are usually insufficient to cover comprehensive renovations, meaning these can often only be implemented with significant rent increases. Another disadvantage is that costs and benefits are rarely assessed over the entire lifecycle of buildings.

To increase the renovation rate, new minimally invasive and scalable renovation methods are required, combining innovative systems for the thermal envelope and renewable heat supply. Standardization and industrial prefabrication enable cost reductions, lower on-site effort, and higher quality. The innovative and multidisciplinary demonstration project "PhaseOut" aimed to further develop such solutions and to demonstrate optimized renovation processes—particularly with regard to serial/industrialized approaches—in order to unlock the large but challenging market potential in the existing building stock.

Three key aspects were combined with the goal of minimally invasive renovation: innovative heat pump technology, modular and prefabricated façade elements, and façade-integrated building services. All stages of the renovation process were considered in a holistic approach—from concept, planning, and optimization to operational and economic evaluation and user involvement—by competent project partners. Four different renovation variants were compared and evaluated in seven identical buildings:

1. central heat pump per building with decentralized fresh water stations in each apartment
2. central low-temperature heat pump with apartment-level booster heat pumps (semi-central system)
3. central heating heat pump and individual decentralized domestic hot water heat pumps in each apartment
4. central heating heat pump and individual decentralized electric boilers in each apartment

In addition to developing two innovative, compact heat pump types specifically for renovation—a booster heat pump and a mini-split domestic hot water heat pump—a modular and prefabricated façade was developed and implemented in the demonstration buildings. All apartments were also equipped with a decentralized, façade-integrated ventilation system, developed as part of the project. Heat pump-based renovation solutions in multi-family buildings are technically feasible, highly energy-efficient, and a key lever for decarbonizing the building stock—provided they are planned integrally and supported by simulation ("digital twin"). Simulation results and findings from the demonstration project show that compact, apartment-based or semi-central heat pump concepts (e.g., booster heat pumps for domestic hot water), in combination with adequate thermal renovation and thus low supply temperatures, enable significant reductions in CO₂ emissions and energy costs.

Façade-integrated heat pumps and prefabricated façade systems allow for minimally invasive, serial renovations with short construction times and manageable building physics conditions, as confirmed by hygrothermal analyses. High-quality thermal renovation also enables the use of compact monoblock R290 heat pumps with reduced noise emissions, resulting in high user acceptance. To achieve high acceptance of renovation measures—and thus high implementation and connection rates—maximum transparency and the involvement of residents in all process steps are essential. Through intensive and systematic tenant involvement, a very high implementation rate of 35 out of 42 apartments was achieved. This exceeded both the original ambitious plan and typical experience by a wide margin.

Recording the existing condition of each apartment is a complex but essential task. Despite 42 structurally identical apartments, the conditions and configurations of the existing heating systems (e.g., location of gas boilers, position and size of heat emitters) varied greatly, resulting in 42 different situations. To provide suitable and cost-effective solutions for each individual case, a range of modular and scalable heat pump solutions is necessary. Overall, it is clear that only modular, flexibly combinable heat pump concepts—coupled with renewable electricity generation and high planning quality—can deliver economically viable and widely scalable renovation solutions for multi-storey residential buildings.

Based on the experiences from the FFG projects PhaseOut and SüdSan, and incorporating new developments, a renovation guideline was developed to serve as an initiator and accelerator for further projects in Austria and internationally, thereby contributing to achieving climate targets and a climate-neutral building stock. The results of monitoring over the period 2026–2028 will be published subsequently. The technologies developed within the project (the booster heat pump and the decentralized façade-integrated ventilation system) are being commercialized by industry partners and are therefore available on the market for future renovation projects.

Project management

Universität Innsbruck, Institut für Konstruktion und Materialwissenschaften, AB Energieeffizientes Bauen

Project or cooperation partners

• AEE - Institut für Nachhaltige Technologien
• drexel und weiss energieeffiziente Haustechniksysteme GmbH
• Energieinstitut Vorarlberg
• IDM Energiesysteme GmbH
• Klimatherm GmbH
• Kulmer Bau GesmbH & Co KG
• Nussmüller Architekten ZT GmbH
• Ingenieurbüro Rothbacher GmbH
• SOZIALBAU gemeinnützige Wohnungsaktiengesellschaft

Ass. Prof. Dr.-Ing. Fabian Ochs
Technikerstr. 13
A-6020 Innsbruck
Tel.: +43 (660) 735 72 75
E-mail: fabian.ochs@uibk.ac.at
Web: www.uibk.ac.at