HTWP – Bosch

Development of heat pump solutions for multi-family houses

Funding code: 03ET1377 A/B/C

Funding period: 01.01.2016 until 31.12.2021

Project partner: Fraunhofer ISE, Bosch Thermotechnik, Robert Bosch GmbH



Development of two heat pump solutions for multi-family houses:

  • A bivalent system, consisting of an electric heat pump and a fossil generator, for space heating and DHW heating with optimized operation for different optimization goals (primary energy, CO2 emissions, investment cost, …)
  • An electric heat pump with a refrigerant circuit with reduced charge based on a natural refrigerant



Bivalent System

The main challenges posed by the installation of heat pumps in existing multi-family houses are the required high flow temperatures – specially to comply with health regulations with regard to domestic hot water –  and the limited availability of adapted heat sources in the surroundings.

A promising solution for the renovation consists in installing so-called bivalent (hybrid) systems in which an electric heat pump and a fossil power heat generator operate in parallel through a superior control system.

With this solution the heat pump (incl. source) must not be dimensioned for maximum performance and the heat generators can run more efficiently (for example with regard to the operating costs or primary energy consumption).


Bivalent temperatures and shares of the heat generators gas boiler and heat pump in covering the heat load under variation of use (renovated or non-renovated existing buildings) as well as the optimisation objectives (primary energy consumption, CO2 emissions and consumption costs); results of simplified, static calculations.

Heat pump with natural refrigerant

In order to meet the reduction in emissions of fluorinated greenhouse gases (F-gases) envisaged by the F-Gas Regulation (Regulation (EU) No. 517/2014), a heat pump based on natural, flammable refrigerants that is suitable for existing MFHs is being developed in parallel in a second part of the project. To minimize its hazard potential, the refrigeration circuit is among other things designed for a minimal charge using new evaporator concepts. A corresponding security concept is also being developed. In order to meet the high requirements for sound emissions in densely built-up urban areas, current developments in psychoacoustics are incorporated into the entire development.

Test stand for measuring refrigerant heat exchangers (evaporators and condensers) at Fraunhofer ISE

Work plan

Bivalent (hybrid) heat pump heating systems in the field of multi-family building renovation

  • Basics and state of the art
  • Simulation-based evaluation of different dimensioning and operating strategies for different system concepts
  • Controller development and implementation
  • Demonstration projects

Heat pump with natural refrigerant for use in multi-family buildings

  • Development of refrigerant-reduced evaporators and optimization of the freezing and defrosting strategy
  • Design of the refrigeration circuit
  • Technical evidence
  • Simulation-based assessment of device performance in the application context

Contact persons


Björn Nienborg, project manager   
Thermal systems and buildings
Fraunhofer ISE
Heidenhofstraße 2, 79110 Freiburg
+49 761 4588-5883

Dr. Bernd Bierling
Bosch Thermotechnik GmbH
Postfach 13 09, 73243 Wernau
Tel.: +49 7153 306-1673

Dr. Christian Glueck
Bosch Thermotechnik GmbH
Postfach 11 61, 35453 Lollar
+49 (0)6441 418-2340

Dr. Fabian Schmid
Robert Bosch GmbH
Renningen, 70465 Stuttgart
Tel.: +49 711 811-24215