May 21, 2019


Solar heating systems for space and hot water.

Solar heat is an alternative green heating and hot water supply, but we can also save huge amounts of our house’s expenses.

In recent years, with the large increase in fuel prices and the introduction of the Energy Efficiency Regulation of Buildings, the need to save and reduce primary energy consumption has begun to become conscious. Solar heat is a method by which solar energy is utilized, producing hot water and meeting some of the heating requirements of buildings. The method has been applied for many years in countries of Europe, but in our country it is much less well known. Most of the solar systems installed in Germany and Austria, countries with low sunshine and high heating needs, are combined space and water heating systems. Throughout the Mediterranean, the prices of solar radiation favor the use of solar systems throughout the year. Especially in Greece, the climate is ideal for such facilities.

How does it work

The solar combined heating (DHW) and domestic hot water (DHW) system is a system that takes advantage of the thermal energy generated by the solar panels. This will heat the water and the water circulating in the heating system. It consists of three main parts: the solar panels and two hot water storage tanks, one of the water and one of the space heating water (Figure 1). These two cans can be placed in one another (container in a container), thus saving space, piping and automation (Figure 2). As the containers can be placed in any available space in the building, the solar panels are aesthetically better integrated into the building. In addition, they can be placed in places that are heated, thus reducing the heat loss of the water. Flat solar panels absorb diffused sunlight, collecting solar energy even in cloudy days and converting 2/3 of solar radiation into beneficial energy. Most market brands have antireflex high absorption glass collectors to ensure maximum heat transfer. The peculiarity of solar thermal is that it works by contributing to the heating produced by using other fuels rather than by removing it. They can be combined with any conventional energy source (oil or gas burners) or a renewable energy source (biomass burners), and are integrated into an existing system as long as space is available for the installation of hot water storage tanks and containers. They can also be combined with any heating medium, but it is preferable to use them with low temperature heating media, such as fancoils or underfloor heating. This is because water as a heating medium is circulating at low temperatures, which even with minimal sunshine is easy to achieve.

The system takes priority of full coverage of hot water needs and then, if there is excess energy, the space heating water heats up. If excess energy is not enough, the solar system is bypassed and space heating is done by the burner, as in conventional heating systems.

How much does sunlight contribute to existing heating?

The degree of solar thermal contribution and hence the reduction in fuel use depends on climatic conditions, altitude, geographic location, plant size and other factors. Depending on the local climate and location of the building, the solar panel can capture more or less energy. Depending on the size of the installation, solar heating covers up to 80% of the annual heating needs. Of course, very high coverage is not economically viable solutions. The ideal cost-efficiency ratio is to achieve a coverage of 40-60% of the total heat load (DHW and DHW).

Example for contribution rate of solar thermal:

A solar thermal system applied and combined with floor heating in Crete, will be able to take almost all of the heat loads by zeroing the use of oil. The contribution rate will be significantly lower in a corresponding region of Macedonia, where the solar radiation is reduced, but it will certainly exceed 50%, a reduction that is very important for fuel consumption and the daily economy.

Volume and installation area

To achieve 40-60% coverage of the thermal load, based on average sunshine in Greece and the average required heating energy of buildings, the surface of the solar energy collectors should account for 20% of the surface of the heated space, the required storage volume is about ten times, calculated in liters. For a house of 100 square meters, 20 square meters is required, flat collectors and 1000lt. (about 200 liters for hot water and 800 liters for heating water). In the case of a roof, the required available surface should be about 1.5 times the surface of the collectors,  about 30 square meters, and about 3-4 square meters   for the installation of the storage containers.

Advantages of Solar Thermal

  1. Fuel saving
  2. Quick depreciation of the investment
  3. Reduced maintenance
  4. Decrease of pollutants
  5. Aesthetic effect
  6. The existing heating system is not abolished
  7. Very small interventions in existing homes

Another advantage of such a system is that its size (and consequently its cost) can be adapted to user requirements and can be easily changed. For example, it is possible to initially install a system that takes 30% of the heating load and after one year to expand by installing additional solar panels to cover 60% of the heating. Particularly in the case of solar thermal installation in a newly constructed building, saving money is greater: Because the percentage of the thermal power required by the solar thermal system is the size of the conventional equipment (boiler and burner) required to be installed is smaller. Part of the money invested in solar heaters is saved from day one when buying basic equipment. For example, it may be preferable to purchase a 20KW boiler instead of 50KW if a 60% solar thermal system is installed during the initial installation (newly built building).


The cost of a 20 square metres system as in the example above, is approximately 10,000-12,000 euros. The savings it can make are about 1.3 tons of oil per year. 10% of installation costs are deducted from income tax. This means that the cost of installing the current oil price is saved in about 8 years. The heating and motion oil price equation is already being studied, hence building heating through solar thermal is becoming more attractive.