There’s nothing better than good planning, and this is especially true when calculating a solar thermal system. It’s essential to clarify a number of aspects upfront. It’s recommended to clarify a few key points so you can calculate the right size and design for a solar thermal system to suit your needs.
You can install a solar thermal system with flat plate or tube collectors. Evacuated tube collectors are typically used in areas with limited roof space. They are more efficient than flat plate collectors and offer particularly good thermal insulation due to the vacuum.
There’s another aspect to consider: do you want to heat only domestic hot water or also relieve the heating system? Whatever your decision, the rest of the system must be sized accordingly. If the system will be used exclusively for heating domestic hot water, you really need to consider the DHW cylinder. This, in turn, affects the collector area, as well as the size of the cylinder, which ultimately depends on the number of people living in the house. In any case, individual framework conditions should be discussed with a Viessmann sales partner and included in the calculation. Key points include:
The following sizing overview can be used as a guide for DHW heating:
| Residents | Daily DHW requirement (60°C) in l | Cylinder volume in liters | Collector, quantity Vitosol-FM/-F | Collector, Vitosol-TM area |
|---|---|---|---|---|
| 2 | 60 | 250/300 | 2 x SV / 2 x SH | 1 x 3 m² |
| 3 | 90 | 250/300 | 2 x SV / 2 x SH | 1 x 3 m² |
| 4 | 120 | 250/300 | 2 x SV / 2 x SH | 1 x 3 m² |
| 5 | 150 | 300/400 | 2 x SV / 2 x SH | 1 x 3 m², 1 x 1.5 m² |
| 6 | 180 | 400 | 3 x SV / 3 x SH | 1 x 3 m², 1 x 1.5 m² |
| 8 | 240 | 500 | 4 x SV / 4 x SH | 2 x 3 m² |
| 10 | 300 | 500 | 4 x SV / 4 x SH | 2 x 3 m² |
| 12 | 360 | 500 | 5 x SV / 5 x SH | 2 x 3 m², 1 x 1.5 m² |
| 15 | 450 | 500 | 6 x SV / 6 x SH | 3 x 3 m² |
Design assumptions:
Consumption is 30 liters per person at 60°C. If per person consumption is significantly higher, the selection should be based on liters per day.
Standard value
A family of four requires a collector area of approximately five square metres (m²) and a DHW cylinder with a capacity of approximately 300 litres to heat domestic hot water.
The following sizing overview can be used as a guide for backup central heating:
| Residents | Daily DHW requirement (60°C) in l | Volume of the buffer cylinder in l | Collector, quantity Vitosol-FM/-F | Collector, Vitosol-TM area |
|---|---|---|---|---|
| 2 | 60 | 750 | 4 x SV / 4 x SH | 2 x 3 m² |
| 3 | 90 | 750 | 4 x SV / 4 x SH | 2 x 3 m² |
| 4 | 120 | 750/900 | 4 x SV / 4 x SH | 2 x 3 m² |
| 5 | 150 | 750/900 | 4 x SV / 4 x SH | 2 x 3 m², 1 x 1.5 m² |
| 6 | 180 | 750/900 | 4 x SV / 4 x SH | 2 x 3 m², 1 x 1.5 m² |
| 7 | 210 | 950 | 6 x SV / 6 x SH | 3 x 3 m² |
| 8 | 240 | 950 | 6 x SV / 6 x SH | 3 x 3 m² |
Standard value
A family of four typically requires a collector area of 10-12 square metres (m²) and a cylinder capacity of 60-80 litres per m² for backup central heating.
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. We also explain some technical terms related to solar thermal systems that should be considered beforehand when calculating system costs.
In principle, any roof is suitable for installing a solar thermal system. However, the roof’s pitch and orientation must be correct. A south-facing roof surface provides a good base and should be unshaded. Minor losses in the morning or evening hours due to the low sun position are tolerable. Otherwise, installation is possible on both flat and pitched roofs. A Viessmann solar thermal system can also be installed directly on the building’s façade.
As a rule, these systems do not require special permits. Exceptions include buildings listed on the register and buildings in areas with environmental protection regulations. If you are unsure, contact the relevant authority before undertaking any action.
Solar collectors collect energy whenever sunlight hits their surface. In winter, the only thing to keep in mind is that the sun is lower on the horizon. Furthermore, the number of hours of sunshine is typically also reduced. This should always be taken into account before installing a solar thermal system. Viessmann solar collectors offer several installation options. If a rooftop location is unfavorable, façade mounting may be a solution. However, in this case, the installation location should be chosen on the part of the façade that receives the most sunlight throughout the day. In principle, the inclination of the collectors, as well as their surface area, can be adjusted to maintain relatively high performance even in winter. However, using solar thermal energy as the sole heating system in winter in Germany is not feasible. A supplementary system, such as a condensing boiler, must be used.
Solar thermal systems require minimal maintenance compared to other heating systems. However, an annual inspection is recommended. This includes checking system pressure and pumps. Additionally, a visual inspection of the main components should be performed every three to five years. During this inspection, the contractor will inspect not only the collectors but also all other piping, fittings, and other components.
Each time the heating system is filled, air is introduced into the collector circuit. This air is largely displaced by the coolant. A small amount remains and is contained within the coolant itself, but is released only at higher temperatures. Air in the collector circuit causes noise and can significantly impede proper flow through the solar collectors. Unlike other heating systems, it is impossible to bleed the air at the highest point of the system (i.e., the collector) while it is operating. This is achieved by using air bleed valves, which are opened or closed manually or automatically. Ideally, they should be located in the flow and upstream of the cylinder inlet.
During the sunnier months of the year, the solar thermal system’s buffer capacity may become saturated to the point where it can no longer absorb heat from the collectors. Supply exceeds demand. However, if the control unit detects this, it will interrupt the solar circuit in a timely manner. Thermal stagnation is a normal operating process in a solar thermal system—the solar fluid begins to evaporate at a temperature of approximately 140-150 degrees Celsius. The pressure in the system continues to rise due to the increase in volume, and the fluid from the collector is forced into the membrane expansion tank (DEV). Little or no liquid remains in the collectors. They are refilled only when the temperature drops. Due to condensation of the solar fluid, the pressure in the system drops again, and the liquid temporarily stored in the DEV flows back out. When heat is required, the solar thermal system restarts. It is important that the solar thermal system be designed for stagnation and regularly serviced. This allows for the condition of the fluid in the collector to be checked, as it can be affected by the process.
When discussing solar energy, an interesting question is how much of the solar radiation is actually usable. Of the 1,367 W/m² of absolute radiation (the solar constant), a maximum of about 1,000 W/m² reaches the earth via the atmosphere. The portion of the radiation that reaches the earth under clear skies is called direct radiation. If sunlight passes through clouds, it is scattered and is called diffuse radiation. The sum of diffuse and direct radiation is called global radiation.
Solar coverage describes the ratio between the energy required to produce heat and the useful solar heat. The higher the solar coverage, the less energy a traditional system must provide. Solar coverage calculations are always based on the annual heat output of the respective heat generators (not their capacity).
Efficiency characterizes the ratio of irradiated energy to useful solar heat. High temperatures and long periods of inactivity reduce efficiency. Efficiency directly impacts the specific output of a collector system. It determines how much useful heat a solar thermal system can produce per year per square meter of collector area. Generally, the higher the specific output, the higher the system’s efficiency.
Installing a solar thermal system is a complex task and should always be performed by a specialized company. However, it’s helpful to understand the installation steps, the components involved, and what you, as the system owner, should pay attention to during the installation process. As a general rule, you should consult with your solar system installer and clarify any unanswered questions.
Roughly speaking, there are four stages that a specialized company implements:
Before handing over the acceptance certificate, a solar panel technician will instruct you on the operation of the system.
Yes. Here, the solar panel installer uses height-adjustable roof hooks to compensate for any unevenness. Other preparatory work may be required, but the installer will discuss this with you in advance.
This is not recommended. As already noted, installation is complex, and especially the connections to the hot water system and heating circuit require plumbing experience. However, you can discuss with your specialist company in advance whether you can begin preparatory work on your roof. However, don’t rush into things. Your specialist company will be happy to advise you on what can be done if you have the necessary technical skills. However, you should not attempt to connect or install a solar thermal system yourself.
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