Solar thermal system – efficient and affordable heating

With a solar thermal system, you can use free solar energy and reduce your monthly energy costs. In addition, by installing a solar thermal system, you are demonstrating your commitment to protecting the environment, by sustainably lowering CO₂ emissions. Investing in such a solar thermal system also helps to increase the value of your property.

Whether for residential, commercial or local authority buildings, with Viessmann technology, you can futureproof your system and rely on optimal interaction of all system components. With more than 40 years' experience in the development and manufacture of solar thermal systems, you can count on our high quality and our technical expertise. Solar thermal is also the ideal way to supplement to a heating system. All Viessmann systems are designed to work in combination with a solar system, so it makes no difference whether you opt for a new condensing boiler for oil or gas, a heating system for wood, or a heat pump.

What is a solar thermal system?

In essence, a solar thermal system is a system that can be used for DHW heating and central heating backup. Solar energy is free, so you not only save on fossil energy. You will also find that your investment in a solar thermal system pays for itself within just a few years. After all, there are no costs for the energy source.

Design of a solar thermal system

In order to be able to use the sun's energy, collectors are needed to capture the rays. Viessmann Vitosol has developed tube and flat-plate collectors to do just that. These collectors absorb the sun's rays and convert them into usable heat. In addition to the collectors, a solar thermal system consists of an absorber, a solar circuit and a solar fluid or medium. When the rays hit the collectors, this energy is converted into heat in the absorber. It is first pumped into the buffer cylinder or DHW cylinder and from there, it is distributed via the circuit in the household – either as a backup for the heating circuit or to heat the domestic hot water. The heat is first transported to the corresponding storage unit by means of the solar medium. From the DHW cylinder, the heat then reaches the draw-off points, such as the taps or shower, in the form of hot water as required.


Does solar thermal always have to be combined with underfloor heating?

A widespread misconception is that solar thermal for central heating backup is only possible in combination with underfloor heating systems. Traditional radiators can also be used with a solar thermal system. The yields are only slightly lower on an annual average and are primarily determined by the somewhat higher return temperatures from the radiators.

Differentiation according to application options

You can use solar energy for your own needs in two ways. Details on domestic hot water heating and central heating backup can be found in the following sections.

Diagram: domestic hot water heating with solar thermal

At the heart of this solution is the dual mode DHW cylinder, such as the Vitocell  300-B. When there is sufficient solar radiation, the solar medium in the solar thermal system heats up the water in the DHW cylinder via the lower indirect coil.

When the temperature drops through hot water being drawn off, such as for a bath or shower, the boiler starts if necessary to provide additional heating via the second circuit. The installation of Viessmann Vitosol solar collectors is straightforward thanks to integrated flow and return pipes, a benefit that also ensures a comparatively low investment.

Further combination options of a solar thermal system with products in the Vitocell range also ensure high levels of domestic hot water heating convenience. You can also use the Vitocell 100-U/W or Vitocell 100-W dual mode DHW cylinders, combi cylinders or heating water buffer cylinders such as Vitocell 360-M. We have the right DHW cylinder to meet every need. All appliances can be seamlessly connected to the solar thermal system.

What should you consider when designing a solar thermal system?

The following rule of thumb applies when designing a solar thermal system for a detached or two-family house: if the collector area is oriented between south-east and south-west, 1.5  square metres of flat-plate collector or 1.0  square metres of tube collector area should be assumed for 100  litres of cylinder volume. Yield losses due to unfavourable orientation or inclination can be compensated for by a slightly increased collector area. Our qualified partners will assist you with your design.  

Diagram: DHW heating and central heating backup with solar thermal

The solar medium heated in the solar collectors can also be used to bring heating water up to temperature. For this, the heating circuit, via a heat exchanger, uses the water in the solar cylinder that is continuously heated by the solar collectors. The control unit checks whether the required room temperature can be achieved. If the temperature is below the set value, the boiler will also start. Solar systems for domestic hot water heating and central heating backup have an area of around one square metre for every ten square metres of living space.

Enjoying the savings potential of solar thermal energy

One important area of potential savings is offered by DHW heating. In our latitudes, solar collectors combined with a DHW cylinder represent the most interesting alternative to boiler operation, especially during the summer months. In a detached house, solar energy covers up to 60 percent of the energy required for DHW heating. If you combine the heating of DHW and heating water, you will save around 35 percent of the total energy required – every year. Even during spring and autumn, you may often be able to turn off your boiler when using solar energy to back up your central heating.  

The following table illustrates the savings potential:

TechnologyGas savingsCO₂ savings/yearEnergy cost savings/year
Gas condensing boiler + combi DHW cylinder with solar heating support*29 %20 %€1030

* The values given were determined by simulations assuming standard household profiles for the Potsdam site. Single-family house with 4-person household (existing with gas boiler), 150 square metres of living space and a gas consumption of 2500 cubic metres. Rounded consumption costs using standard values (EID). The energy prices of the Federal Statistical Office were used for the economic efficiency calculation. Investment costs: Viessmann gross list price plus average installation costs nationwide (may vary individually).

Note: Our calculations are theoretical examples based on various assumptions and dependent on certain general conditions. We assume no liability for the correctness and transferability to actual systems. The actual costs and possible savings may differ and must be calculated in each individual case based on the specifics of the respective location and the general conditions there.

Check the requirements before purchasing a solar thermal system

Viessmann tube collectors

The yield of a solar thermal system largely depends on the conditions on site. Particularly good results can be achieved with south facing roofs with a pitch of 30  to 40  degrees. If the roof surface faces east or west, homeowners can expect up to 20  percent less yield at a similar pitch. The somewhat less favourable orientation can be compensated for with a larger collector area. In addition to the orientation and inclination of the roof, it is also important that it is large enough and as unshaded as possible. If neighbouring houses, trees or mountains cast a shadow on the roof surface for lengthy periods during the course of a year, the solar yield will decrease. The savings in energy costs will be lower and economic efficiency will suffer as a result.

If you would like to purchase a solar thermal system, you should seek advice from our qualified partners. These experts will check the local conditions and quickly determine the best way to install the technology and integrate it into your heating system. They can also take care of professional planning and install the solar system so that it runs reliably and economically for a long time.

Quality features for solar thermal systems

To ensure that your purchase of a solar thermal system pays off, the technology needs to function reliably in the long run. An important quality feature to consider is the Solar Keymark certificate. This is a quality label for solar thermal products based on European standards. It ensures that solar thermal energy functions efficiently, and is also a prerequisite for current government subsidy programmes. 

Viessmann solar thermal systems score highly on performance and durability. Vitosol solar collectors are made of corrosion and UV-resistant materials. This is most impressively verified by quality tests according to the EN 12975 test standard. This at the same time confirm the consistently high thermal output. The patented ThermProtect temperature-dependent shutdown function also provides reliable protection against overheating. The absorbers in new Viessmann flat-plate collectors, such as the Vitosol 100-FM, change their properties (their surface structure) to radiate more energy. Viessmann vacuum tube collectors such as the Vitosol 200-TM will stop transferring solar energy to the solar medium above a certain collector temperature. The benefit: Planning of solar thermal systems is more straightforward and installation effort is lower. Components like pre-cooling vessels and stagnation coolers are not required. This increases safety and reduces investment costs. 

Solar thermal for commercial processes

Solar thermal systems are of particular interest to commercial processes that require heat at a relatively low level. The collector technology that is currently available on the market (with the exception of concentrating systems) can use solar thermal to supply heat of up to around 80  degrees Celsius across Central Europe.

Alternative solutions for high demand

In some areas of industry, process heat at very high temperatures is indispensable. These include manufacturing industries for food, fertilisers and building materials, as well as in pharmaceutical companies and hospitals. For higher heating requirements, Viessmann has a number of solutions for generating heat, steam, electricity or cooling. Fossil energy sources such as oil and gas, as well as renewable energies such as wood and biogas, can be used as energy suppliers. Large heat pumps are best suited for heat generation with environmental energy. For the simultaneous production of electricity and heat, gas-operated combined heat and power units are the system of choice.

Product range

With a wide range of flat-plate and vacuum tube collectors, Viessmann provides flexible and individual solutions for every application.