24 Multi-Family Buildings in Rhein-Erft Switch to Heat Pumps in 10 Weeks

Within 10 weeks in Rhein-Erft-Kreis (North Rhine-Westphalia), 33 ZEWOTHERM ZEWO LAMBDA air-source heat pumps replaced gas and oil heating in 24 multi-family buildings serving approximately 300 apartments. The transition to the new system was carried out under an inter-building contract heating model without any interruption to residential occupancy.

Project Parameters

The 24 buildings are owned by housing company Erftland and are located in five towns: Bergheim, Kerpen, Bedburg, Elsdorf, and Frechen. The buildings were constructed between 1986 and 2005, with a total residential floor area of 17,359 m². Prior to the retrofit, heating was supplied predominantly by natural gas; three buildings operated oil-fired boilers. The annual heat demand of the entire complex was estimated at approximately 1.5 million kWh, with associated CO₂ emissions of around 310 t per year. The total thermal load of the 24 buildings is approximately 600 kW.

Technical Solution

An individual system design was developed for each building based on its thermal load and structural characteristics. Modulating air-source heat pumps from the ZEWO LAMBDA EU13L, EU15L, and EU20L series, manufactured at ZEWOTHERM's plant in Remagen, were installed. Depending on the required capacity, units were installed individually or in cascade. The units operate on the natural refrigerant propane (R290), deliver a supply water temperature of up to 70 °C, and feature a monobloc design: the closed refrigeration circuit is entirely housed within the outdoor unit, so no refrigerant piping is routed into the building.

In addition to the heat pumps, ZEWOTHERM supplied buffer tanks ranging from 300 to 800 L, hydraulic stations, and ZEWO Powerbox units.

Financial Model

The key condition of the project was inter-building contract heating: an energy service company assumes responsibility for heat supply and recovers its investment through heat sales over the contract period. In this project, vaestro and ZEWOTHERM covered the majority of capital costs; Erftland contributed a relatively modest construction share, which allowed it to limit its own capital outlay and retain funds for other measures on the housing stock.

Budgeting the 24 buildings as a single package also delivered an organizational benefit: planning, procurement, and installation work were coordinated across five sites rather than managed one building at a time.

Progress and Results

During peak periods, installation crews converted up to four buildings per week. Within 10 weeks, all 33 heat pumps were installed and the heating of all 24 buildings was switched to the new system. The speed was enabled by a standardized methodology, parallel work on multiple sites, and a phased commissioning process with ZEWOTHERM's service team.

Following the heating replacement, the energy efficiency of the buildings improved by up to three classes in some cases. The retrofit also brought electrical networks and metering panels into compliance, creating a technical foundation for potential future Mieterstrom projects (electricity from local generators for residents).