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Heat pump cascade heats single-family house


The Schneider family bought a house in the Pfalz region of Germany. To minimize the utility costs of the house built in the late 1970s, they decided to renovate their home to save energy. Instead of an oil-fired boiler, two air-to-water heat pumps interconnected in a cascade heat all 267 m2 of the building. Some of the electricity to operate the heat pumps is provided by the new photovoltaic system on the roof. 



A narrow driveway leads up to the Schneiders’ house. In Maxdorf. The 257 m2 single-family home is situated on a slope at the back of a lot that was once 1500 m2. “The former owner was unable to sell such a large property, so he split it into two lots,” explains Daniel Schneider. The computer scientist bought the lot with the house, built in 1977, early last year. The undeveloped lot was sold separately. Extensive renovation and modernisation was needed before the family could move in. The roof was insulated and re-tiled, preparing it for installation of a photovoltaic system.

The age of the heater was only one reason it had to be replaced; the location of the lot was also a factor. Splitting the property into two lots created a new problem: “In the past, the oil for heater was delivered from the street,” explains Schneider. The tanker drove over the undeveloped part of the property, getting as close as possible to the house. “But if a new house is built there, that won’t work anymore,” says Schneider. The Schneiders’ house would then no longer be accessible from the street, only via the driveway. There would be room for the tanker, but the oil hose would have to be laid across the property and around the house, because the boiler room is in the back of the building. The family was hesitant to rely on this option and decided to look for an alternative. 



“Gas would have been one option. The connection cost was acceptable, but laying the supply line would have been very expensive,” says Schneider. Another factor was that the family was already considering a type of heating that works with renewable energy. The homeowners liked the idea of an environmentally friendly heating solution that is not dependent on fossil fuels and their fluctuating prices. This is why the Schneiders requested bids for air-to-water heat pumps from several specialized crafts businesses. “We didn’t want a geothermal heat pump, because the rocky ground would have made drilling difficult.”

One bid proposed integrating the home’s wood-burning stove with integrated water heat exchanger into the hot water supply, meaning that the heat pump system would operate bivalently. But the Schneiders intended to use the fireplace only for the cosiness aspect and did not want to be committed to always having a fire to keep the water warm. Olaf Mayer, proprietor of his own plumbing and heating business, ultimately suggested the ideal solution for the building: an Ecodan heat pump cascade that can heat the house monovalently.


The two heat pump outdoor units - Ecodan by Mitsubishi Electric, 8 kW each - are equipped with the globally patented Zubadan technology. The system works at 100% capacity at subfreezing temperatures down to -15°C. The manufacturer guarantees proper functioning even at temperatures as low as -28°C. The outdoor units are connected to the hydroboxes in the boiler room via refrigerant lines. The wall-mounted hydroboxes do not take up much space, and they have a built-in heat pump manager that automatically controlls the outdoor units.



Mayer consciously chose two units with a lower capacity interconnected in a cascade over a single heat pump that could supply the same total capacity. “Besides the safety aspect and the energy saving potential, the government subsidies available were another advantage,” explains the master craftsman. Ecodan cascades have a special control system that calculates the maximum achievable coefficient of performance (peak COP) as a function of the power consumption of the outdoor units, the current heat output and the outdoor temperature, and controls the system accordingly. So the units are controlled such that they support one another and together provide the output needed. This is much more efficient than running one module at full capacity while the other is in standby mode.

A side effect of this is that both units wear the same and have approximately the same operating times. The wide modulation range facilitated by cascading is another benefit. The redundancy feature increases operational reliability. if one module is off, the other devices ensures that heat continues to be supplied. When applying for a government subsidy, both systems are considered, meaning that owners can receive more funding. The German Federal Office for Economic Affairs and Export Control (BAFA) grants 1500 EUR for an electrically operated, modulating air-to-water heat pump up to 37.5 kW. If the heat pump us used as a monovalent heat generator in an existing building, the subsidy increases 30 percent to 2250 EUR.

And since it can be connected to an intelligent power grid via an integrated interface, the Ecodan system meets the requirements for the “Smart Grid Ready” certificate. If a buffer tank of at least 300 l is added to such a heat pump, the homeowner receives another 500 EUR load management bonus. The Schneiders installed an 800 l multi-function tank. Since the service life of the compressor was optimised, the buffer tank has a positive effect on system efficiency.



The effort required to install an air-to-water heat pump is comparable to that required to install a gas or oil condensing boiler. The heat distribution could remain virtually the same. There is floor heating throughout the building, with the exception of the bathroom, which has radiators. The old radiators were replaced with modern elements. Replacing the radiators in existing buildings can be beneficial, because it increases the efficiency of the heat pump system. Large radiators with high water volume can provide the heat needed even at lower flow temperatures.And heat pumps run more economically at lower flow temperatures. So the cost of investing in new radiators is recouped through the lower operating costs.

A photovoltaic system with 10 kW output was also installed as part of the renovations. To accommodate the system, the old fibre-cement roof tiles had to be replaced. “Except for the trusses, everything was replaced. And better insulation was installed as well,” explains Schneider. No further insulation was required, as the building was constructed of solid poroton bricks. “At some point we just have to replace the windows,” plans Schneider.

Photovoltaic panels were installed on both sides of the roof, since those are the east and west sides. The electricity that they generate is used for household electronics and to power the heat pump. “A module regulates the system such that it draws power from the photovoltaic system whenever possible,” explains Mayer. This saves the Schneiders a lot of money. In the future the combination of photovoltaic and heat pump system will allow even greater autonomy in supplying electricity and heat, because battery storage technology continues to evolve. Experts are predicting that in only a few years battery storage will be used more and more in private homes. Then the electricity from the photovoltaic system will easily be able to be buffered and then used by the heat pump when needed.


The Schneider family fulfilled their dream of owning a home when they bought their single-family house. Although the structure is in very good condition, the family decided to implement energy-saving measures to keep operating costs low over the long term. This is why they replaced the oil-fired boiler, which was almost 30 years old, with an air-to-water heat pump cascade. This single heat generator covers all of the heating needs. The efficiency of the Ecodan system selected not only keeps operating expenses low, it also allowed the Schneiders to benefit from multiple government subsidies. A photovoltaic system was also installed. It not only powers the household appliances, it also produces some of the electricity needed to run the heat pump system. This means even lower operating costs.