Heat Pump Cascade System Supplies the Parish Center and Rectory
71 percent heat self-sufficiency in the first year
Since January 2025, a cascade of two OVUM AirCube Heat Pumps has been supplying the parish center and the adjacent rectory—which contains two residential units—in the Protestant parish of Meersburg on Lake Constance. The previously used gas heating system, which consumed approximately 65,000 kWh of natural gas per year, was replaced by the new plant, which features three separate heating circuits and a separate domestic hot water Heat Pump. According to monitoring data, after one year of operation, the plant has achieved an annual coefficient of performance (COP) of 4.3; in December 2025, heat self-sufficiency stood at 71 percent—an additional heating element has not been necessary so far.
The project was carried out by Selg Haustechnik GmbH.
Two Buildings, One Heating Network
The project began with an energy assessment of the property belonging to the Protestant parish of Meersburg. Over time, the original parish center was expanded to include a parish office, an additional hall, and a parsonage with two residential units. While the 558 m² parish center hosts daily events, office activities, and seminars, the approximately 220 m² parsonage serves as a residence for the family of the current pastor. Both buildings are connected via a ground-source heat exchanger to form a shared heating system.
Based on the energy-efficient renovation plan, the existing gas boiler was replaced with two OVUM AirCube air-to-water Heat Pumps, which operate in a cascade configuration and also supply the parsonage via the existing connecting line. In addition, heat distribution was optimized by performing a hydraulic balancing of the heating circuits.
- 2× OVUM AirCube 520P R290 in a cascade configuration
- 3 heating circuits with separate flow temperatures
- Separate domestic hot water Heat Pump in the rectory
- Solar Power Plant and Battery Storage
Initial Situation: Central gas heating system with high distribution losses
Before the renovation, a central gas heating system in the parish center supplied both buildings via a main line, with a flow temperature of 42 °C and an annual natural gas consumption of approximately 65,000 kWh. Domestic hot water was also supplied centrally from the parish center: The hot water was produced there and distributed to both buildings via a pipe approximately 60 meters long—even though about 98% of the actual hot water demand was in the rectory. This resulted in losses along the long pipeline, which, according to the report, were roughly four times the actual hot water demand in the rectory.
Heat Pump cascade connected to a central buffer tank
The two OVUM AirCube Heat Pumps (each with 19 kW thermal output, max. 10 kW electrical output, outdoor air as the heat source, R290 refrigerant) operate hydraulically in a cascade configuration connected to a central buffer tank, which both balances the loads and provides energy for defrost cycles. From there, three separate heating circuits with different flow temperatures are supplied—tailored to the different uses of the parsonage (residential) and the parish center (office, work, and seminar activities).
To ensure optimal performance, the heating curve is continuously adjusted by the trained building superintendent; an online remote maintenance tool provided by the manufacturer also enables ongoing monitoring and quick support when needed.
- Electrical power: 2× max. 10 kW
- Heat output: 2 × 19 kW
- SCOP (manufacturer’s specification): 4.4 · JAZ (as planned): 5.5
- Flow temperatures of the three heating circuits: 42 °C, 36 °C, and 35 °C
- Launch: January 23, 2025 | Implementation period: November 6, 2024, through January 23, 2025
Generating hot water right where it’s needed
As a solution to the previous centralized hot water distribution system, a separate domestic hot water Heat Pump was installed in the rectory, while a compact 3-kW instantaneous water heater in the parish center meets the modest needs of the tea kitchen. According to the report, this made it possible to completely eliminate the transmission losses associated with the previous approximately 60-meter-long pipeline.
Installation of Outdoor Units
Selg Haustechnik GmbH installed the two outdoor units of the Heat Pump cascade on a pedestal next to the boiler room and then covered them with slatted siding to screen them from view.
Using Excess Solar Power Specifically for Domestic Water Heating
At the same time as the heating system was being modernized, another company installed a photovoltaic system with battery storage. The joint interface document published by the trade associations for the electrical and HVAC trades was used to define the interfaces between the various trades. A conscious decision was made not to raise the temperature of the heating heat pumps when there was excess PV output, as this would have led only to additional cycling given the building’s current usage. Instead, the domestic hot water Heat Pump is operated specifically during periods of excess PV power via the SG-Ready interface. According to the report, this means that approximately 45 to 55% of the Heat Pump’s electricity is generated by the PV plant itself.
Live Monitoring via MIRA
MIRA allows you to track the Plant’s operation at any time—from current power consumption to the monthly energy balance.
Operating Results After One Year
The heating load was calculated in accordance with DIN 12831; the plant is designed so that the Heat Pump cascade covers the heating demand without the need for an additional heat generator. According to monitoring data, the heating element has not yet had to be used. After one year of operation, the measured annual coefficient of performance was 4.3; in December 2025, it was 3.5. In 6,072 operating hours, the plant has recorded 879 switching cycles to date. According to the report, further efficiency gains are possible primarily through renovation measures to the building envelope of the old structure, which are already included in the renovation plan.
According to the report, the total cost of approximately €106,000 includes not only the Heat Pump cascade but also the accompanying measures—a PV plant, battery storage, hydraulic balancing, heating circuit pumps, and electric actuators for individual room control. Compared to the previous gas price, the report states that energy costs are reduced by about 50%; however, the report notes that the exact cost savings cannot be quantified, as part of the electricity used by the heat pumps is generated by the new PV plant itself, and hot water is now heated using general household electricity without a separate meter.
Implemented by Selg Haustechnik GmbH
Selg Haustechnik GmbH, an OVUM partner company, handled the planning and installation of the Heat Pump cascade in Meersburg.