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Solar thermal system calculation – size and design

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Nothing beats good planning, and this is especially true when calculating a solar thermal system. A number of aspects need to be clarified in advance. It is advisable to clear up a few basic points so that you can calculate the size and design of solar thermal system that suits your own needs.

Calculating the size and design

You can install a solar thermal system with flat-plate or tube collectors. Vacuum tube collectors tend to be used where there is less roof space available. They are more efficient than flat-plate collectors and have particularly good thermal insulation due to the vacuum.

There's another aspect you need to consider – do you only want to heat your domestic hot water or relieve the heating system as well? Whatever you decide, the rest of the system must be matched accordingly. If the system will be used solely for domestic hot water heating, you really need to consider a DHW cylinder. This in turn influences the area of the collectors as well as the question of how large the cylinder should be, which ultimately depends on how many people live in the household. In any event, the individual framework conditions must be discussed with a Viessmann trade partner and included in the calculation. Key points include:

  • Number of people in the household
  • Size of the living space
  • Type of collectors
  • Subsidy conditions
  • Required coverage
  • Domestic hot water or central heating backup (detached house)?
  • In a detached house = energy condition of the building + heat demand
  • Insulation

Size and design for DHW heating

The following sizing overview can be used as a guide for DHW heating:

OccupantsDHW demand per day (60  °C) in lCylinder volume in lCollector, number of Vitosol-FM/-FCollector, Vitosol-TM area
260250/3002 x SV / 2 x SH1 x 3  m²
390250/3002 x SV / 2 x SH1 x 3  m²
4120250/3002 x SV / 2 x SH1 x 3  m²
5150300/4002 x SV / 2 x SH1 x 3 m², 1 x 1.5 m²
61804003 x SV / 3 x SH1 x 3 m², 1 x 1.5 m²
82405004 x SV / 4 x SH2 x 3  m²
103005004 x SV / 4 x SH2 x 3  m²
123605005 x SV / 5 x SH2 x 3 m², 1 x 1.5 m²
154505006 x SV / 6 x SH3 x 3  m²

Design assumptions:
Consumption of 30  litres per person at 60  °C. If the consumption per person is significantly higher, selection should be according to litres per day.

Standard value

A family of four needs a collector area of around five square metres (m²) and a DHW cylinder with a capacity of about 300  litres for domestic hot water heating.

Size and design for central heating backup

The following sizing overview can be used as a guide for central heating backup:  

OccupantsDHW demand per day (60  °C) in lBuffer cylinder capacity in lCollector, number of Vitosol-FM/-FCollector, Vitosol-TM area
2607504 x SV / 4 x SH2 x 3  m²
3907504 x SV / 4 x SH
2 x 3  m²
4120750/9004 x SV / 4 x SH
2 x 3  m²
4 x SV / 4 x SH
2 x 3 m², 1 x 1.5 m²
4 x SV / 4 x SH
2 x 3 m², 1 x 1.5 m²
72109506 x SV / 6 x SH3 x 3  m²
82409506 x SV / 6 x SH3 x 3  m²

Standard value

A family of four typically needs a collector area of 10 to 12  square metres (m²) and a cylinder with a volume of 60 to 80  litres per m² for central heating backup.

FAQ – frequently asked questions about calculating solar thermal systems

Is a solar thermal system suitable for everyone? What requirements must be met? And does a solar thermal system require annual maintenance? In this section, we answer the most important questions about solar thermal systems. In addition, we clarify some technical terms that relate to solar thermal systems and that must be taken into account in advance when calculating a system.

In principle, any roof is suitable for a solar thermal system. Nevertheless, the roof pitch and orientation need to be right. A south facing roof surface forms a good basis, and should be unshaded. Minor losses in the morning or evening hours due to the low position of the sun can be tolerated. Otherwise, installation is possible on both flat and pitched roofs. A Viessmann solar thermal system can also be fitted directly to the façade of a building, however.

As a rule, these systems do not require special permits. The only exceptions are listed buildings and buildings in areas with environmental protection rules. If you are unsure, contact the office responsible before doing anything.


The solar collectors collect the energy whenever the sun's rays hit their surface. In winter, the only thing to keep in mind is that the sun is lower on the horizon. In addition, the number of sunshine hours is usually also lower. This should always be taken into account before installing a solar thermal system. Viessmann solar collectors offer several installation options. If the roof position is unfavourable, façade mounting can be a solution. In this case, however, the section of the façade that is exposed to the most sun during the day should be chosen as the installation site. Basically, the inclination of the collectors as well as the size of the collector surfaces can be adjusted so that you also have a comparatively high yield in winter. However, solar thermal as the sole heating system in winter is not really possible in Germany. A supplementary system, e.g. a condensing boiler, should be used as well.

The maintenance effort required for a solar thermal system is low compared to other heating systems. Nevertheless, it is advisable to have an annual inspection carried out. During an inspection, things like the system pressure and the pumps are checked. In addition, a visual inspection of the main components should be carried out every three to five years. During this inspection, the contractor will not only look at the collectors, but at all other pipelines, fittings and other components.

Every time a heating system is filled, air is introduced into the collector circuit. This air is largely displaced by the heat transfer medium. A small part remains and is also in the medium itself, but only gets released at higher temperatures. Air in the collector circuit causes noise and can significantly affect proper flow through the solar collectors. Unlike with other heating systems, it is not possible to vent the system at its highest point (i.e. at the collector) while it is operational. Air vent valves are used, which are opened or closed either manually or automatically. These should ideally be located in the flow and upstream of the cylinder inlet.

In the sunnier months of the year, the buffer cylinder of the solar thermal system may become saturated so that it is no longer able to absorb heat from the collectors. Supply exceeds demand. However, provided that the control unit recognises this, it will interrupt the solar circuit in time. Thermal stagnation is a normal operating process in a solar thermal system – the solar fluid begins to evaporate at a temperature of about 140 to 150  degrees Celsius. The pressure in the system continues to rise due to the increasing volume and the collector fluid is forced into a diaphragm expansion vessel (DEV). Little or no liquid remains in the collectors. These only fill up again when the temperatures fall. Due to the condensation of the solar fluid, the pressure in the system falls again and the fluid temporarily stored in the DEV flows back. If heat is needed, the solar thermal system starts up again. It is important that the solar thermal system is designed for stagnation and is regularly maintained. This allows the condition of the collector fluid to be checked, as it can be affected by the process.

When it comes to solar energy, the interesting question is how high, in real terms, the proportion of solar radiation that can be used actually is. Of the 1367  W/m² absolute radiation level (solar constant), a maximum of approximately 1000  W/m² reaches the ground, due to the Earth's atmosphere. The part of radiation that hits the ground when the sky is clear is called direct radiation. If sunlight passes through clouds, it is scattered and is termed diffuse radiation. The sum of diffuse and direct radiation is called global radiation.

Solar coverage describes the ratio between the energy required for heat generation and the usable solar heat. The higher the solar coverage, the lower the amount of energy that has to be provided by the conventional system. The calculation basis for solar coverage is always the amount of heat provided by the respective heat generators per year (and not their output).

Efficiency describes the ratio of irradiated energy to usable solar heat. High temperatures and long idle periods reduce the efficiency. Efficiency has a direct influence on the specific yield of the collector system. It states how much usable heat can be produced by the solar thermal system per year for every square metre of collector area. As a rule, the greater the specific yield, the higher the system efficiency.

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