FiTNeS - Facade integrated modular Split-heat pump for new buildings and refurbishment

The goal of FitNeS was the development of modular split heat pumps with compact and silent façade-integrated outdoor units for heating and domestic hot water preparation (and optionally cooling in combination with PV). The outstanding features of the concept are a modular design with a high degree of prefabrication and representing a visually and architectonically attractive, economic and sustainable solution for both new constructions and renovations. One of the main development goals is the minimization of sound emissions by means of optimized flow control.

Short Description


completed (February 2023)

Goals, Contents and results

For the buildings of the future - so-called nZEBs according to the EU Buildings Directive (EPBD) - both for new buildings and for renovation, efficient and cost-effective heating technologies are needed that can be flexibly operated with renewable energies (i.e. mainly PV). Expectations for a comfortable indoor climate (especially in summer) are rising, and at the same time efficiency in the building sector must be increased significantly. Heat pumps are seen by most experts as one of the key technologies in the building sector, but the access to heat sources is very limited, especially in inner-city areas and in high-density urban areas. Accordingly, there are currently no real alternatives to decentral (storey or flat-wise) gas boilers available on the market. The E-boilers frequently used for hot water in renovation projects are not an acceptable solution in terms of the energy transition and, in view of rising electricity prices, are also increasingly not an alternative from an economic point of view. Compact heat pumps for heating and domestic hot water are needed to replace gas-fired boilers in small flats. Often, there is no more space available than the gas-fired boiler used to occupy.

A promising solution would be so-called mini-split heat pumps, because these are available at low prices due to the extremely high number of units. Heat pumps with small outputs (in the range of less than 2.5 kW) could be designed compactly and cost-effectively, thus offering new possibilities. However, these systems only represent a real alternative if their acceptance can be increased through improved design, architecturally attractive integration into the building envelope and significantly reduced noise emissions.

In the framework of the project FitNeS, modular air-water split heat pumps with very compact and silent façade-integrated outdoor units for heating and/or domestic hot water supply (and optional cooling in combination with PV) were developed. The concept is characterised by the fact that it enables modular construction with a high degree of prefabrication and thus represents a visually attractive, economical and ecological solution, especially for thermally renovated multi-storey residential buildings. Based on the technical and non-technical boundary conditions, various concepts for façade-integrated outdoor units were developed and evaluated in terms of design, façade construction, accessibility (for maintenance), building physics, efficiency, etc. Various concepts for different evaporator-fan configurations and fan types were investigated. From a pre-selection, the best concepts were developed in detail and optimised with the help of simulations (CAE, CFD). One of the main development goals was to minimise sound emissions through optimised flow guidance. For this purpose, functional models of compact, silent and visually attractive outdoor units were developed and tested in the laboratory. As the best variant, a configuration with a vertical evaporator with four parallel axial fans arranged with suction was further developed. The prototype was made with an attractive design with a deep-drawn plastic cover and tested.

The specification for the refrigeration circuit was the use of an alternative refrigerant. After screening, the choice fell on R290 as a natural refrigerant with a very low GWP. It has good thermodynamic properties and is suitable for split HP applications, is cost-effective and there is no shortage in sight. Another decisive factor for the project was the high component availability. The permissible charge of 150 grams when using R290 as a refrigerant is a challenge for a split heat pump. For the 1.5 KW outdoor split heat pump, the limit can be met. Functional samples for a modular split HP with a façade-integrated outdoor unit were built and then measured in detail in the laboratory.

With the aim of enabling high-quality, minimally invasive serial renovation, concepts with prefabricated façades were investigated and further developed. Particularly in the frequently occurring small flats, there is often no space for an indoor installation and façade integration is thus a possibility to make such renovation solutions possible at all. For this purpose, a holistic and systematic approach was used to investigate and optimise the aspects of building physics, efficiency, primary energy savings and practicality (e.g. maintenance), with a focus on architectural design and an appealing design. The verification of compliance with efficiency as well as comfort criteria and indoor air quality is carried out through extensive dynamic building and system simulations with models validated by the laboratory experiments. The implementation of a test façade with a façade-integrated outdoor unit made it possible to define interfaces and gain practical experience during installation. The façade with the heat pump outdoor unit was installed in a so-called PASSYS test cell and equipped with sensors. Functional tests were carried out.

For two demonstration buildings, renovation variants and concepts for the use of the developed heat pump for space heating, domestic hot water heating and combined operation were investigated. Detailed multi-zone simulations of a flat in the buildings were used to compare different renovation variants. An essential factor for efficiency is the flow temperature, which depends on the type and size of the radiators. Simple design and dimensioning tools are necessary for the future use of heat pumps in refurbished multi-storey residential buildings.

The developed prototypes of decentralised compact heat pumps represent an important step towards replacing the many gas floor-standing boilers in existing buildings. The improvement of the design and reduction of noise emissions through façade integration of the heat pump outdoor units will greatly increase the acceptance of such solutions. For widespread use, further work is necessary in the direction of modularising the components in order to be able to implement flexible concepts for different heating delivery systems as well as domestic hot water heating. Other important tasks are simple structural solutions for coupling the new systems to existing heat delivery systems (such as radiators), and competitive installation through better and broader training of refrigeration technicians or combined training of refrigeration and heating technology.

The PhaseOut project builds on the results of the FiTNeS project. A variant optimised for DHW heating is being developed and will be used in a demonstration project and compared and evaluated with alternative heat pump system concepts. The development of business models for holistic thermal refurbishment and the replacement of gas boilers based on the developed decentralised compact heat pump requires the development of innovative maintenance concepts.

Project Partners

Project management

Universität Innsbruck, Institut für Konstruktion und Materialwissenschaften

Projekt- bzw. KooperationspartnerInnen

  • Drexel Solarlufttechnik und Lüftungsbau GmbH
  • Innsbrucker Immobilien GmbH & CoKG
  • element design - Stephan Breier e.U.
  • wInterface GmbH
  • Drexel und Weiss Energieeffiziente
  • Ingenieurbüro Rothbacher GmbH

Contact Address

Fabian Ochs
Technikerstr. 13
A-6020 Innsbruck
Tel.: +43 (512) 507 63603