Pilot project by NHW and Viessmann Climate Solutions: Heat pump heats a building with twelve apartments

Kassel, April 9, 2025 – The Nassauische Heimstätte | Wohnstadt (NHW) Group and Viessmann Climate Solutions, a North Hesse-based company with a global reach and part of the Carrier Group, are jointly testing sustainable and efficient heating systems for the housing industry. Together with the Kassel-based installer Roth & Liebermann, the use of a heat pump is being tested in a pilot project in a typical residential building with a total of twelve apartments in Heinrich-Pierson-Straße in Oberzwehren. The existing (fossil-fuel) heating system in the residential building of Hesse's largest housing company is being upgraded with a Viessmann heat pump, and has been producing highly efficient heat for the residents in the meantime.

During a press tour, NHW Managing Director Monika Fontaine-Kretschmer, Oliver Happel (Project Manager at Viessmann Climate Solutions) and Simone Fedderke (Head of the Department for Urban Development, Construction, Climate Protection, Environment and Transport), together with Roth & Liebermann Managing Director Patrick van de Meent, visited the current status of the project.

Efficient modernization with the Vitocal 250-A PRO heat pump from Viessmann

Heat pumps are often used in new construction projects or for modernizations. The aim of the pilot project by NHW and Viessmann Climate Solutions is to efficiently integrate heat pumps into existing buildings with as little effort as possible and to gather practical experience without subjecting the buildings to a cost-intensive full modernization. “We are continuously working on cost- and energy-efficient solutions that benefit our tenants and our environment,” said NHW Managing Director Monika Fontaine-Kretschmer. “This pilot project is another important building block in our sustainability strategy, with which we want to reduce energy consumption and CO2 emissions in our neighborhoods. It also contributes to the 'Practical CO2 reduction in the building sector' initiative, which we joined a few months ago.” The aim of the initiative is to organize a broad public discourse and persuade decision-makers in politics to fundamentally rethink climate policy in the building sector and align it with realistic, achievable CO2 reduction targets. It was launched by renowned scientists from the fields of architecture and engineering in mid-November 2024 and now has almost 500 supporters.

“For this joint pilot project, we worked closely with NHW to select an existing building in which the Vitocal 250-A PRO heat pump can be operated in a hybrid system with the existing heat generator with little modernization effort. The Vitocal 250-A PRO impresses with its high flow temperatures. This means that a large proportion of the annual heat requirement can also be effectively covered by the existing heating network. On particularly cold days, the additional heat generator then provides support and absorbs the peak loads. This pilot project is a perfect example for the future, showing that heat pumps can also be used efficiently in existing buildings and with the existing heating network,” says Oliver Happel, Project Manager at Viessmann Climate Solutions.   

“On our way to climate neutrality, we want to move away from fossil fuels and towards renewable energies from the region,” emphasized Kassel's municipal climate advisor Simone Fedderke. “A crucial step is municipal heat planning, with which we want to create the basis for a secure, independent and sustainable heat supply. However, we are also constantly exchanging ideas with other players such as housing companies and are delighted when - as in this exemplary cooperation project - further contributions are made along the way.”

How the Vitocal 250-A PRO heat pump works

The compact Vitocal 250-A PRO air/water heat pump heats, cools and produces domestic hot water. To do this, the sound insulated outdoor unit quietly and efficiently uses the outside air as a heat source. A scroll compressor with infinitely variable output adjustment and electronic expansion valves ensure a flow temperature of up to 70 °C with very high efficiency and therefore low consumption costs. With infinitely variable output adjustment, the Vitocal 250-A PRO is a heating and cooling solution that can be tailored to individual project requirements in larger residential and commercial buildings and businesses. One appliance generates up to 39.5 kW heating output. For higher outputs, up to four units can be cascaded, whereby only one indoor unit is required.  

The Vitocal 250-A PRO indoor unit offers numerous functions. Up to three mixed heating/cooling circuits and a seasonal buffer cylinder are already included. Hot water can be generated using a cylinder charging system or a cylinder charging pump. Instead of a DHW cylinder, the Vitotrans 353 fresh water station can also be used, which conveniently generates DHW using the instantaneous DHW principle. If required, an external heat generator can be integrated to cover the peak load. The integration of the Vitocal 250-A PRO into the building management system (BMS) is a further plus point: this enables the heat pump and the hydraulic system to be monitored and controlled from a central control room. External setpoint specification for the combined heating and cold water buffer cylinder is possible via the standard BACnet/IP or ModbusTCP/IP connection.

Corporate Group Nassauische Heimstätte | Wohnstadt

The Nassauische Heimstätte | Wohnstadt (NHW) Group, based in Frankfurt am Main and Kassel, has been offering comprehensive services in the areas of housing, construction and development for over 100 years. It employs around 890 people. With 60,000 rental apartments at 112 locations in Hesse, it is one of Germany's leading housing companies. The Kassel Regional Center manages around 17,500 apartments, including around 5,000 in the city of Kassel. It carries out sustainable urban development projects under the NHW brand ProjektStadt. It is a founding member of the Wohnen.2050 initiative to increase the impact of climate protection in the housing industry. With hubitation, NHW also has a start-up and ideas network for innovative housing. www.nhw.de/

About Viessmann Climate Solutions

Founded in 1917 as a heating technology manufacturer, today Viessmann Climate Solutions is a leading global provider of sustainable climate (heating, cooling, water and air quality) and renewable energy solutions. The Integrated Viessmann Climate Solutions portfolio seamlessly connects products and systems via digital platforms and services, creating an individualized feel-good climate for users. Viessmann Climate Solutions is part of Carrier Global Corporation, global leader in intelligent climate and energy solutions that matter for people and our planet for generations to come. For more information, visit https://www.viessmann-climatesolutions.com/en.html.

Press contact:

Nassauische Heimstätte Wohnungs- und Entwicklungsgesellschaft mbH 
Schaumainkai 47 ⋅ 60596 Frankfurt am Main 
Jens Duffner (Press spokesman)
Phone: +49 69 678674-1321
E-Mail: jens.duffner@naheimst.de
Internet: www.nhw.de  

Viessmann Climate Solutions
Viessmannstraße 1 ⋅ 35108, Allendorf (Eder) 
Jörg Schmidt (Deputy Head of Communications)
Phone: +49 151 15168389 
E-Mail: smdj@viessmann.com
Internet: www.viessmann-climatesolutions.com/de