The demonstrated heat pump system is developed within «HEAVEN» project of Fa. Viessmann and Fraunhofer ISE: the innovative heat source hydraulics utilize the synergies of air (availability, costs) and ground sources (performance) and allow regeneration and defrosting by intelligent control.

The building was originally built in 1963 and renovated in 1995. The building features a heated floor area of 2112 m² on 5 floors with a total of 30 apartments. The building was renovated exchanging the windows with insulating glazing (Uwin = 1.7 W/m²K), and adding layers of thermal insulation (styrofoam, d = 60 mm, l = 0.035 W/mK) to the façade, cellar and to ceiling, thus reducing the overall U-value of the opaque envelope from Uenvelope = 1.71 W/m²K to  0.323 W/m²K.

Key facts

Building Heat pump and source
Location Karlsruhe, Germany
Number of HP 1
Construction 1965 Installed power 55.4 kW
Heat distribution radiator heating Operation mode bivalent
Heated area 2112 m² living Heat source  PVT collectors
(solar+ambient)
Level of insulation good
(renovation in 1995) 
Heating system Domestic hot water
Heat demand 60 kW Type of system central
Heating temperature 55 / 45 °C
Max. temperature 62 °C
Circulation system yes

Description of the technical concept

Heat Pump Technology

  • Cascade of two heat pumps with
    27 kWth + 43 kWth
  • Viessmann 351.B27 and 351.B43

Heat Source

  • Outdoor air unit: 40 kWth nominal power
  • Ground source heat exchanger:
    18 probes à 18 m length, double U tube, 30 kWth
  • Hydraulic source system developed within research project «HEAVEN» by Fraunhofer ISE and Fa. Viessmann.

→ Bivalent system with condensing gas boiler as back up (bivalent parallel operation)

→ Two drinking water storages (2x 850 l)

→ One heating buffer storage (850 l)

→ Heat pump integrated into district energy systems via SG ready to allow optimization of self-consumption of PV and CHP electricity

LowEx Measures

Heat Distribution System

  • Original radiators are mostly sufficient to allow a reduction of nominal heating temperatures
  • 13 of 150 radiators were identified as “bottleneck” radiators. These radiators are exchanged with radiators of larger heating power
  • Reduction of heating temperatures to 55/45°C
  • Hydronic balance

Drinking water distribution

  • Two external freshwater stations
  • Circulation

Karlsruhe-Durlach HEAVEN data sheet