PhaseOut – Heat pump technologies in the renovation of existing buildings

The aim is the conception, optimization, implementation, demonstration and evaluation of innovative, minimally invasive renovation solutions (thermal renovation and heating system replacement) with heat pumps and PV in multi-storey buildings on seven identical buildings. The comprehensive comparison includes various technical solution variants based on modular and scalable building technology systems as well as multifunctional building components for the exchange of decentralized gas heating by central, semi-central and decentralized heat pump solutions.

Short Description

Starting point / motivation

Austria set the target for deep renovation and for phasing out gas or oil-based heating systems to reach the ambitious climate goals. Although technical solutions for the renovation of multi-story residential buildings (i.e. window replacement, thermal insulation) and boiler replacement in central energy supply concepts already exist since several years, the renovation rate has been stagnating at a low level for years (well below 1% of the building stock).

This means that the ambitious climate protection goals in the building sector cannot be nearly achieved. The reasons for the low renovation activities are manifold. Conventional renovation processes and techniques are very expensive and require months of work on-site, including the use of scaffolding or even temporary relocation of the residents. Furthermore, there is no satisfactory technical solution available on the market for decentral heat pumps and renewable energy supply of flats.

Due to the overall limited availability of renewable electricity and district heating on national level and the increasing demand, the thermal renovation of buildings is indispensable for achieving the climate goals. Switching from fossil heating systems to heat pumps is (often) only technically feasible in combination with an appropriate thermal renovation.

Contents and goals

To achieve a significantly higher renovation rate and phase-out of fossil-based heating systems, new minimal invasive and adaptive renovation processes are required, which merge construction and building systems with energy conversion based on renewable energy sources. Standardization and industrial prefabrication enable a reduction in costs and expenditure of time for the construction while the quality of the renovation is increased.

This innovative and multidisciplinary demonstration project has the objective to further develop and demonstrate a renovation process that can contribute significantly to increase the renovation rate in the huge but challenging market of decarbonization of multi-storey buildings, by highlighting the potential of serial renovations.


The project combines three key aspects: 

  1. innovative heat pump technologies, 
  2. modular and prefabricated façades and 
  3. façade integration. 

The goal of energy efficient and minimal disruptive renovation will be reached by a holistic approach considering the whole process from conception, planning, optimization, micro and macro techno-economic evaluation to user involvement and monitoring by dedicating each an expert project partner.

Three different serial, adaptive and industrialized renovations with heat pump solutions will be compared and evaluated in seven identical multi-storey buildings. These solutions are 

  1. a central heat pump solution, 
  2. a so-called semi-central one with a central low-temperature heat pump and flatwise booster heat pumps, and
  3. flat-wise i.e., decentral heat pump solutions.

In addition to the development of two innovative compact heat pump types dedicated for renovation - a booster heat pump and a mini-split domestic hot water heat pump - a modular and prefabricated multifunctional façade that integrates either the distribution pipes, the booster heat pump or the outdoor units of mini-split heat pump and optionally decentral mechanical ventilation with heat recovery and renewables ((BI)PV) will be developed and applied in the demonstration buildings.

Expected results

To tailor a perfect solution and thus increase the acceptance for the renovation, special focus is on user-tenant involvement during all levels of the project. The resulting renovation guidelines aim to extend the knowledge on how to make systematic building renovation easier and serve as an initiator or accelerator for other projects in Austria as well as on an international level.

Project Partners

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

Contact Address

Ass. Prof. Dr.-Ing. Fabian Ochs
Technikerstr. 13
A-6020 Innsbruck
Tel.: +43 (660) 735 72 75