Almost half of European buildings are equipped with fossil fuel boilers installed before 1992. With a share of 46%, natural gas is the largest primary energy source for heating and cooling. In Vienna alone, around 440,000 households use gas combination boilers or low-efficiency gas heaters for space heating or domestic hot water preparation. Renovating such systems could reduce the EU's total energy consumption by 5 – 6% and cut CO₂ emissions by around 5%. Nevertheless, on average less than 1% of the national building stock is renovated annually in the EU. Although already established and market-leading in the new-build sector, heat pumps are still far less common in renovation projects — especially in large-scale residential buildings. As a result, the energy efficiency and CO₂ reduction potential of heat pumps in this segment is either not being realized or only insufficiently utilized, as individual households in multi-apartment buildings rarely can switch to a heat pump. However, replacing gas with environmentally friendly heat pumps is indispensable on the path toward climate-neutral cities. The "Gasthermenersatz" project addresses exactly this challenge.

In the project, a prototype of a decentralized, noise-optimized heat pump solution with serially and parallel-connected refrigerant circuit modules was developed, built, and tested. Depending on the configuration of a certain number of these hermetically sealed modules, efficient operation can be achieved according to heating load and temperature requirements. The resulting heat pump represents a drop-in alternative to gas boilers in large residential buildings. For practical operation, a thermal storage unit and access to a heat source are still required. Ideally, the latter is shared and derived either from ambient air or geothermal energy, both of which can be accessed through the now unused chimney. Communication between the heat pump and storage is managed by a central controller. This "smart" communication enables full utilization of synergies for maximum decarbonization.

For experimental evaluation, a rack for the modular heat pump system was introduced, allowing the thermal capacity and performance of each individual module to be measured at different temperature levels and media. Up to eight modules could be installed in total, with connections for four different heat pump modules used in the project. A key feature of the rack is its flexible combination options, which can be easily realized with pre-installed 3-way valves, for example through parallel or serial module connections.

Development proceeded in two iterative loops, each with three work packages: (1) concept development (component, module, and system level; modeling in Modelica/Dymola), (2) laboratory setup (interfaces, safety, integration at the AIT test bench), and (3) laboratory tests according to EN 14511 with automated data acquisition (LabVIEW, Matlab, Python). The system was dimensioned for apartments up to 130 m² with a maximum heating load of approx. 10 kW. The A3 refrigerant R290 was used, and by reducing the refrigerant charge to 150 g per module, safety requirements according to IEC 60335-2-40 could be simplified.

Two different module types were developed and manufactured: Module 1.2 (reciprocating compressor 10 cm³) and Modules 2.X (rotary compressor 17 cm³; variants with different plate heat exchangers and additional components). Heating capacity and COP were compared as performance indicators. Results showed that Module 1.2 outperformed the 2.X modules in all pressure ranges except under operating condition B10/W25. Among the rotary compressor modules, Module 2.3 performed worse than 2.0 and 2.2, which was attributed to the evaporator with 18 plates acting as the limiting factor in this design. The isentropic efficiencies for underfloor heating and domestic hot water preparation were above 0.45 for both reciprocating and rotary compressors. However, measurements showed that the efficiency of the reciprocating compressor was better at higher pressure ratios than that of the rotary compressor. Some of the deviation between measured values and manufacturer data can be attributed to heat losses in the uninsulated refrigeration and hydraulic lines. At high supply temperatures and low heating loads, these losses had an increased impact. In addition, optimization potential was identified in subcooling and superheating, which could further improve performance and efficiency, while the overall trend is expected to remain.
Considering the system as a whole, combined operation of the modular heat pump seems more suitable for applications requiring both heating and domestic hot water simultaneously. The efficiency of combined operation with an intermediate temperature level is slightly lower than in parallel operation, but the advantage lies in the simultaneous use of the heat source for both purposes and a high DHW output for rapid charging of a hot water storage tank.

The experimental results highlight the potential of a modular heat pump as a sustainable solution for replacing gas boilers in residential buildings. Moreover, it was found that the heat pump module could be further optimized through technical adjustments, particularly regarding refrigerant charge and refrigerant circuit component configuration. Successful implementation of the decentralized heat pump concept for gas boiler replacement is considered realistic and would strengthen Austria's role as a technology leader in innovative environmental technologies. Nevertheless, development risks remain in the areas of product safety (due to the use of a flammable refrigerant) and economic viability, since gas combination boilers are established products that have been mass-produced for decades. The insights gained from this project contribute to the further development and fine-tuning of modular heat pump solutions, which could make a significant future contribution to renewable energy deployment.

Project management

OCHSNER Wärmepumpen GmbH

Project or cooperation partners

AIT - Austrian Institute of Technology GmbH

Stephan Preisinger
Ochsner-Straße 1
A-3350 Stadt Haag
Tel.: +43 (5) 04245 - 548
E-Mail: stephan.preisinger@ochsner.com
Web: www.ochsner.com