Written by : Andreas Gavrielatos , Mechanical & Aeronautical Engineer

και and Dimitris Tsiatsios, Chemical Engineer NTUA


Solar thermal systems, known as COMBI solar systems, assist in space heating and domestic hot water (DHW) production. They are very popular at several European countries including Austria, Germany, Italy, France, Denmark etc. According to the CRES in 2001 the total area of solar collectors involving Combi systems in Europe was approximately 340.000m2.2.

 Greece is particularly favored by the weather. Despite this factor, Combi systems are not very widespread and only in recent years, mainly because of soaring oil prices, are taking steps to exploit this very environmentally friendly technology. The sunshine in our country is the largest in Europe (3.000 hours per year). The average daily energy given by the sun in Greece is approximately 4,6kWh/m2.2.



The combined operation of solar thermal systems for DHW and space heating can meet on an annual basis by 10% - 50% the needs of home heating and hot water production, depending on the size of the solar collecting area to be installed, the volume of buffer tank, the meteorological data in the region and the characteristics of the house (size, quality of insulation, thermal requirements). The ideal application of the system is in houses with good insulation and low temperature heating (underfloor heating, fan coils), while in houses with heating radiators a reduction in yield by 10%-15% is expected. A big advantage of these systems is that they can be installed both in new and in existing houses without major modifications.


Generally these systems comprise the circuit of solar panels (energy production), the buffer tank (energy storage), an auxiliary power system (heat pump, oil or gas or electric boiler), a heating system (underfloor circuit) and a control system (PLC).




The solar energy collected in solar panels is converted into heat and transferred to a specially designed buffer tank, which heats the water for the heating and the domestic hot water circuits.

If solar energy is not enough, the auxiliary power source is automatically turned on and completes the required action. With this method high fuel economy is achieved. Space heating and hot water heating is achieved in a manner which is environmentally friendly, particularly in a country like Greece, where the need for saving energy and reducing greenhouse gas emissions becomes increasingly urgent.

For the proper functioning of such a system and in order to maximize energy savings, both the proper sizing of the system and also the selection of appropriate materials is required.


The solar panels should be high efficiency selective type, since the system requires the maximum energy collection during the winter. ενέργειας κατά την χειμερινή περίοδο.

One of the key elements of a proper system of solar heating is the buffer tank, which is the "heart" of the system and must be specially designed and constructed for this purpose. The buffer tank should be well insulated and in particular to assist in stratification of the water temperature inside.

The stratification of the reservoir leads to maximum system performance, minimum heat losses and maximum energy collected by solar panels.

  The buffer tank COMBI of the Italian factory SICC spa, is specially designed and constructed for this purpose. It comprises two separate containers, one inside the other (tank in tank). The external tank, which operates as buffer tank, circulates the central heating water. Inside the container there is a second vessel, the container of hot water, which is treated with high-tech material VITROFLEX HI-TECH 2, which makes it suitable for potable water. A solar heat exchanger is immersed into the external vessel. The whole construction is strong insulated by 70mm hard foam to minimize heat loss.

The inner hot water container is equipped with electronic anodic protection of titanium anode. This system ensures the anti-corrosion protection due to electrolysis, it needs no maintenance and it is absolutely efficient with low power consumption.


The transfer of solar hot water from the collectors to the Combi tank is accomplished via a pre-assembled solar kit, which includes all necessary plumbing fittings for the safe and proper circulation of the closed circuit solar water and allows precise adjustment of flow, safe ventilation, filling and emptying the system from the boiler room.


Especially during the summer months it is likely that the temperature of hot water inside the domestic hot water container reaches such high values that direct use can be dangerous for burns.

For this reason the use of special thermostatic mixing valve MMV-SOLAR is required, which is specially designed for solar systems with high flow and high water temperatures. It ensures a constant temperature flow of hot water at the outlet, which can be adjusted from 30 to 60oC, to protect the end user.




The digital controller SIEMENS is the brain of the system. It controls the heating circuit with three-way progressive valve operation and the circuit of the domestic hot water production. It also controls the two power sources: the boiler and the solar. As long as there is available solar energy, the controller charges first the domestic hot water tank and after the buffer tank used for heating circuit.

 If there is not enough available energy from the sun, then the boiler produces the hot water needed for all purposes. It is also possible to heat up a swimming pool via an extra heat exchanger. A digital display on the controller indicates all the temperatures and functions. Connectivity to network LPB to control more complex applications is also available

YDROENERGIA , a technical company based in New Port Corfu, being a leader at systems involving renewable and alternative energy sources, undertakes the study, supply and installation of the appropriate heating system for each application.


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