Find out more about hybrid heating here

Hybrid heaters combine renewable energies with fossil fuels. Because of its positive ecological balance, the state supports both solar thermal systems and heat pumps. What are the advantages of hybrid heating?

Table of contents Table of contents hybrid heating: advantages, costs and funding opportunities

• Hybrid heating: An overview of the most important things
• What is hybrid heating?
• Examples of hybrid heaters
• History and distribution in Germany
• Requirements for the installation of a hybrid heater
• Efficiency and advantages of a hybrid heater
• Hybrid heating costs
• Legal provisions and grants

Table of contents Table of contents hybrid heating: advantages, costs and funding opportunities

• Hybrid heating: An overview of the most important things
• What is hybrid heating?
• Examples of hybrid heaters
• History and distribution in Germany
• Requirements for the installation of a hybrid heater
• Efficiency and advantages of a hybrid heater
• Hybrid heating costs
• Legal provisions and grants

Hybrid heating: An overview of the most important things

• A hybrid heater combines a heating system from renewable energies with a fuel-operated heater. If necessary, a heat pump or a solar thermal system works together with a second, supporting heating system so that there is enough heat even in winter.
• Gas, oil or wood heating, combined heat and power or district heating are possible as auxiliary heaters.
• Both the heat from the solar thermal system / heat pump and that from the fuel-operated heater use a heat exchanger to heat the heating water in a buffer storage tank. This means that the energy is not lost if it is not needed immediately.
• For a solar thermal system you need a sufficiently large and best south-facing roof area. On the other hand, a low heating supply temperature is crucial for the economy of a heat pump.
• Funding: As part of the 2020 climate package, the state subsidizes the purchase of hybrid heating.

What is hybrid heating?

Hybrid heating combines conventional, fuel-operated heating technology with photovoltaics or a heat pump. The fuel-operated hybrid heating subsystem ensures that sufficient power is available, especially in the late winter months. Oil heaters, gas heaters, stoves or pellet heaters are used.

The hybrid heater can therefore use several heating systems and takes advantage of both systems. The most sustainable system, i.e. the solar-powered system or the heat pump, always has priority. If the regenerative heating system no longer works optimally, the fuel-operated heating system jumps in. Depending on the design, the systems feed heat into the buffer storage in parallel or only work separately.

Buffer tanks can be connected to solar thermal systems, heat pumps and fuel-operated heating systems.

Photo: Vaillant

In addition to the options presented here, other combinations are possible, which even include up to three types of heating. For example, you can operate a heat pump with the electricity from your photovoltaic system and also have a gas heater available as support.

So-called power-to-heat systems that convert excess electricity from a photovoltaic system into heat are not yet very widespread in Germany. Due to the current climate debate, they may play a bigger role in the future.

Examples of hybrid heaters

The following table shows you the available options for hybrid heating. In the following sections you will find a description of the respective technologies.

Regenerative heating system	Supporting heating system
Solar thermal	Gas heating
Heat pump	Oil heating
	Stove
	Pellet heating
	CHP plant
	District heating

Regenerative heating system: solar thermal

With solar thermal, you use solar energy to heat water. A solar thermal system contains the following components:

• Solar panels

  Solar collectors consist of flat or tube collectors with an absorber layer that absorbs solar energy particularly efficiently. Flat-plate collectors are square panels with a protective glass layer. In the case of tube collectors, however, there is a vacuum between the absorber and the glass. In both cases there are pipes under the absorber layer. They contain a mixture of water and an anti-freeze. This liquid heats up to 95 degrees Celsius through the heat of the sun.

• Heat exchanger

  These give off the heat of the solar fluid to the heating water or the process water. Depending on the system, they are either spiral pipes integrated in the storage tank (internal heat exchangers) or plate heat exchangers located outside (external heat exchangers).

• Storage

  The buffer storage is an integral part of every solar thermal system. The medium water stores the heat provided for a few days and makes it available when required. In a single-family house, the storage tank usually contains 400 to 1, 000 liters of water.

• Circulation pump

  The electric circulation pump moves the solar fluid along the circuit. If the storage tank is full or there is no heat available, it will not work.

This is how solar thermal works.

Photo: THE HOUSE

Regenerative heating system: heat pumps

A heat pump is a thermodynamic machine. It uses the property of some gases to evaporate even at low temperatures. The so-called refrigerant absorbs the heat from the environment (reservoir) and releases it at a higher level inside the house. The process consists of four steps:

1. A compressor compresses the refrigerant to a high pressure. The refrigerant heats up, but remains gaseous.
2. The refrigerant transfers its heat to a heat exchanger and liquefies. The heat exchanger in turn heats a medium. Depending on the design of the heat pump, this is either air or water.
3. Using an expansion valve, the refrigerant returns to the original pressure and evaporates again.
4. The vaporous, cold refrigerant absorbs the heat from the environment using a second heat exchanger.

Did you know? Refrigerators also use the same principle as heat pumps. The refrigerant evaporates and extracts heat from the inside of the refrigerator. The second heat exchanger releases the heat into the ambient air.

The smaller the temperature difference between the reservoir and the interior, the more efficiently the heat pump works.

Photo: THE HOUSE

Depending on the heat source and medium, heat pumps are divided into the following categories:

• Air Water heatpump

  This system uses the ambient heat to heat the domestic water of the heating system. It can therefore be connected to the existing heating system and is cheaper than the other variants. However, the efficiency drops considerably at low outside temperatures.

• Brine-water heat pump

  The geothermal heat pump is the most efficient because it uses the geothermal energy that is always available. Either 100 meter deep geothermal probes or geothermal collectors are required for this. The good efficiency is countered by extensive planning, necessary approvals for the probes and high costs.

• Water-water heat pump

  Here the heat pump takes the heat from a water reservoir. A separate well pump drives the water from depths of up to 20 meters to the surface.

• Air-to-air heat pump

  Such a heating system requires a ventilation system. The heat pump uses a plate heat exchanger to transfer the heat from the incoming air (supply air) from the outgoing air (exhaust air). The air-to-air heat pump is therefore not a thermodynamic machine in the strict sense, since it works without refrigerant. It is particularly suitable for passive houses.

A reversible heat pump with inverter technology can also be used for cooling in summer.

Photo: Panasonic

History and distribution in Germany

Solar collectors are not a new invention. Even the ancient Greeks tried to concentrate the sun's rays and use them to heat objects. In the 18th century, the natural scientist Horace-Benedict de Saussure built the first solar collector. With the Industrial Revolution and the advent of fossil fuels, the use of solar energy was initially of no interest. It was only after the oil crisis in the 1970s that researchers returned to the concept of solar thermal energy.

The invention of the heat pump, on the other hand, was only about 100 years ago. In the 1920s, the first cooling systems came on the market in the USA, which also served as heat pumps in winter. Switzerland was a pioneer in Europe. As early as the late 1930s, heat pumps or hybrid systems were heating public spaces there.

Since the saving of carbon dioxide has become the focus of politics, hybrid heating systems have also been spreading here in Germany thanks to state funding. In Germany, for example, less than one percent of new buildings had a heat pump in 2000. In 2018, it was 41 percent. Together with Sweden, Denmark and Austria, Germany maintains statistics on the performance of the installed heat pumps.

Requirements for the installation of a hybrid heater

Depending on the technology and configuration of your hybrid heating, you should consider the necessary requirements.

Requirements for a heat pump

To combine the hybrid heating with a water-water heat pump, you need good quality groundwater at a maximum depth of 20 meters. If the water is too heavily contaminated with solids or iron, the life of your heat pump will suffer. If the water reservoir is too deep, drilling is too expensive. You must also have approval from the Lower Water Authority prior to drilling.

There are similar requirements for a brine-water heat pump. For the variant with geothermal probes, you need approval from the local mining authority and the lower water authority if you could encounter groundwater during the drilling. Permits are excluded in drinking water protection areas. If you choose ground collectors, the installation is less bureaucratic, but you need a lot of space. As a rule of thumb, you should take the living space by two.

An air-to-air heat pump is a kind of air conditioning system that provides freshness in summer and warmth in winter. However, the principle only works for airtight buildings such as low-energy houses. In old buildings, the heat loss through the building envelope is too high, so that operation is not worthwhile.

Regardless of the technology, a heat pump is only economical if the heating system's supply temperature is below 45 degrees Celsius. The greater the temperature difference between the outside air or the reservoir and the heating water, the worse their efficiency will be. Underfloor heating and or other surface heating systems are ideal for heat pump operation.

Requirements for solar thermal

For a hybrid heating system with a solar thermal system, you primarily need a suitable area on the roof in a south, southeast or southwest direction. If you use the heat of the sun for heating, you should expect around 16 square meters of space for a family home with four people. However, the exact space requirement also depends on other factors such as heat loss in the house and the heating system. Mini photovoltaic systems are also suitable for smaller roofs or for installation on balconies and terraces.

Generating electricity with the power of the sun from a solar system - for yourself and for others - is 100 percent ecological.

Photo: iStock / AndreasWeber

Supporting heating system

The choice of the optimal fuel-operated heating system in combination with a heat pump or a solar thermal system for hybrid heating also depends on several criteria. For example, for pellet heating you need a room for storing the pellets. A gas condensing heating system saves space, however, just as with district heating, it must be possible to connect to the local network. A tank is required for oil heating.

Tip: Compare the efficiency, requirements and costs of hybrid heating with other types of heating at a glance.

Efficiency and advantages of a hybrid heater

Hybrid heating works extremely efficiently and economically. The fuel-operated heating system is only used when the solar thermal system or the heat pump does not supply enough heat. With a well-planned hybrid heating system, you can save up to 40 percent of your heating costs.

Another reason for hybrid heating is the greater independence from fossil fuels. It is true that it is not possible to completely do without fuel-operated heating in our climate zone. However, you can cover part of your heating needs with renewable energies and are less affected by price fluctuations in the energy market. In the end, hybrid heating saves CO ₂ emissions. For this reason, the state rewards the change.

Efficiency of a heat pump

Manufacturers of heat pumps state the efficiency of the systems in annual performance figures (JAZ). Put simply, the annual labor factor expresses the relationship between the energy used to operate the compressor (electricity) and the useful heat provided. An annual performance factor of 4 means that 400 kilowatt hours of heat are generated from 100 kilowatt hours of electricity.

The annual performance factor is therefore a measure of the efficiency of the heat pump. Water-water heat pumps with an average annual coefficient of performance of 5 perform best here. Brine-to-water heat pumps with geothermal probes are nearby with 4 to 4.5. Air-to-water heat pumps, on the other hand, usually have an annual performance factor of 3.5.

Efficiency of wood heating

Modern wood heating systems represent an ecological possibility to supplement the solar thermal system or the heat pump. In addition to classic wood-burning stoves, wood gasifiers, pellet or wood chip heating systems are also available.

This pellet stove in a classic design is only 80 cm high and already has a smoke outlet.

Efficiency of a solar heater

On average, a solar thermal system provides between 30 and 35 percent of the heating energy required. It works most efficiently in spring and autumn when there are still many hours of sunshine.

Parameters such as the roof pitch and orientation as well as the geographical location influence the performance of the solar heating. The technology of the system is also crucial. If you use the solar thermal system for heating purposes, it is worthwhile to invest in the more expensive but more efficient vacuum tube collectors.

Efficiency of oil and gas heating

Thanks to condensing technology, modern oil and gas heating systems are particularly energy-saving and achieve efficiencies of 90 percent and above. In contrast to conventional boilers, condensing boilers use the heat of the condensing flue gases, which otherwise escapes through the chimney. In particular, natural gas in combination with condensing heating is considered a fuel with low CO ₂ emissions.

Oil heaters with condensing technology may still be installed until the end of 2025. However, all government funding ended on December 31, 2019. Modern systems can still be worthwhile as part of a hybrid heating system.

Hybrid heating costs

The purchase price and the installation costs for a hybrid heater depend on the selected variant and your heating requirements. The following table provides an overview of the average purchase, development and maintenance costs for a system in a single-family home. In addition to these expenses, you must also take into account the expenses for fuel (oil, gas or wood) or the purchase of district heating.

Type of heating	acquisition cost	Development costs	Average Maintenance costs / year
Solar thermal system	around 6, 000 euros	1, 500 to 3, 000 euros	150 to 250 euros
Air to air Heat pump	5, 000 to 10, 000 euros	2, 000 to 6, 000 euros	50 to 100 euros
Air Water- Heat pump	4, 000 to 10, 000 euros	500 to 2, 000 euros	50 to 100 euros
Water water- Heat pump	6, 000 to 12, 000 euros	From 4, 000 euros	50 to 100 euros
Brine water Heat pump with Geothermal probes	8, 000 to 12, 000 euros	From 7, 000 euros	50 to 100 euros
Brine water Heat pump with Earth collectors	8, 000 to 12, 000 euros	From 3, 500 euros	50 to 100 euros
Oil condensing heating	6, 000 to 9, 000 euros	1, 000 to 2, 500 euros	150 to 250 euros
Gas condensing heating	3, 000 to 6, 000 euros	1, 000 to 2, 500 euros	100 to 250 euros
Pellet heating	10, 000 to 15, 000 euros	1, 000 to 2, 500 euros	150 to 250 euros
CHP plant	From 15, 000 euros	From 5, 000 euros	350 to 500 euros

Legal provisions and grants

Since 01.01.2020, the Federal Office of Economics and Export Control (BAFA) has been funding solar thermal systems in both new and existing buildings with up to 30 percent of the acquisition costs as part of the 2020 climate package. The prerequisites are a calculated yield of at least 525 kilowatt hours per square meter per year, a sufficient collector area and a buffer storage with enough capacity.

If you install a hybrid heater with the combination of heat pump and gas, you will also receive a subsidy of up to 30 percent of the purchase price, including installation and commissioning costs. If you replace an oil heater with a hybrid heater, the BAFA bonus increases to up to 40 percent of the costs incurred.

Timo Jochmann