This article presents a smart alternative to control units for solar thermal systems. There are several options on the market, from elementary devices to more advanced control systems. In this case a very simple system is used, consisting of two Sonoff intelligent switches and relative temperature sensor. This system, in addition to being easy to install and rather cheap, is also easy to manage through a very accurate interface. Furthermore, it allows remote management and to set temperatures directly from the smartphone application. By taking advantage of these new cheap smart devices it is possible with little to create a smart thermostat to improve the heating system.

Thermal plant

There are several reasons that led to the choice of a system of this type, which is atypical for the management of thermal plants. Among these it was fundamental to consider the physical structure of the plant. The thermal system originally includes a boiler that takes care of heating and domestic hot water. Clearly, there is a special thermostat for heating, while sanitary water is produced on request.

The system has been expanded by adding a solar panel with storage of 150 L. To correctly orient the solar panel, so as to maximize its efficiency, it has been installed away from the boiler. Clearly this posed a first limitation, that is, it was not possible to integrate the two systems. Both are connected to the system and, to avoid waste and further problems, there are valves that take care of closing the connection, leaving one system active at a time.

The most intelligent evolution, in terms of energy saving, of a system of this type involves using the solar panel as long as the water temperature in the storage tank is sufficiently high and then using the boiler. To optimize everything, when you switch to the boiler, you could also send the lukewarm water (not hot enough) from the accumulation of the panel into the boiler so as to reduce the energy required to heat it. Unfortunately in this case the latter option was to be excluded as it would have required major structural work as well as electrical and thermal systems.

Solar thermal

The solar thermal panel, also known as a solar collector, is a device of the thermal system that serves to convert the energy of solar radiation into thermal energy. In doing so, the energy is transferred to a thermal accumulator which is then used to produce both domestic and work hot water or to heat environments. The conception dates back to the Roman Empire where they exploited the irradiation through the greenhouse effect but only after the First World War did the first real panel capable of providing hot water spread.

The actual panel receives solar energy and exchanges it with the thermal fluid through an exchanger. Subsequently, the thermal fluid transfers heat to the tank where it is stored. The first important distinction comes precisely from the system with which the thermal fluid moves.


Circulation can be natural or forced. In natural circulation, convection is used. The liquid heats up in the panel and therefore expands moving away from the colder liquid found in the heat exchanger in the accumulation. The carrier fluid is typically water mixed with propylene glycol in suitable ratios to maintain frost resistance. Natural circulation is simpler than forced circulation and does not require pumps, however it is more sensitive to pressure drops.

In forced circulation, on the other hand, pumps are used that recirculate the fluid only when the temperature is high enough. Normally the circuit containing the thermal fluid is separated from that of the water that they go to heat. An electrical resistance can also be integrated into the storage tanks which can intervene when the irradiation is not sufficient, such as on very rainy days or at night.

There are several categories of thermal collectors. A first distinction is between flat solar panels and vacuum solar panels. The first system is used to obtain lower temperatures between 50 °C and 90 °C. It consists of a glass plate (if glazed panels), a copper absorber and thermal insulator that prevents dispersion.

Typically, flat glazed panels are used in which the structure around the absorber limits dispersions and, even if they have a slightly lower performance than non-glazed panels in optimal conditions. In reality they have a much higher yield in less optimal conditions so as to produce hot water from March to October. The vacuum solar panels, on the other hand, are able to maintain a greater heat input even in conditions of low irradiation.


The use instead of the boiler or the electric water heater leads first of all to reduce the consumption of electricity and hydrocarbons. There are also several advantages for the environment such as the total reduction of CO2 emissions, sulfur and nitrogen oxides and particulate matter also due to the exploitation of a renewable energy source such as the sun. The need for energy transport systems over large distances is reduced and energy independence is created in areas where fuel supply would be needed. It is an accessible, easy-to-build and cost-effective technology, with low construction and disposal costs.

energie rinnovabili europa
Evolution of renewable energy sources in Italy in the thermal sector – Credits: Gestore Servizi Energetici

Remember that the sun is an inexhaustible source of energy for us. With a surface greater than one million times that of the earth, it reaches temperatures of millions of degrees, constantly emitting about 400,000 billion kW of electromagnetic radiation. On earth this power is significantly lower due to the screen due to the atmosphere and we find about 1000 W / m2 which drops to about 100 ÷ 150 W / m2 in the presence of clouds. All this energy can be used for solar thermal as well as for other solar systems, such as photovoltaics.

energie rinnovabili europa
Energy consumption by source in Europe – Credits: Gestore Servizi Energetici

To solar thermal for the production of hot water are also added passive systems that make use of various devices integrated in buildings that use solar energy. These include skylights, greenhouses, reflective surfaces, high thermal inertia structures, phase change walls and Trombe walls.

Solar circuit

The integration with the heating system can be done in various ways, however all follow some basic principles. The tank outlet pipe is connected to a thermostatic valve that mixes the water from the storage tank with cold water, keeping the outlet at a constant temperature. This avoids the danger of burns, leakage due to dispersion and damage to the devices placed in series. Then there is a three-way valve which can, depending on the temperatures, activate the different outputs. If the temperature is high enough, the water is introduced directly into the circulation, otherwise it is sent to the inlet of a boiler or an accumulation that heats it up to the necessary temperature. In other cases the boiler directly heats the storage when the solar thermal energy is not sufficient.

In a simplified model of heat exchange we can consider:

\Gamma c_p\left(T_{out}-T_{in}\right)=(\tau \alpha)GA-\varepsilon \sigma A(T_p^4-T_a^4)-UA(T_p-T_a)

The terms of which are respectively:

  • Energy transferred to the fluid
  • Component of energy absorbed by solar radiation
  • Component of energy lost by radiation
  • Conductive and convective losses

In the steady state we have that:

Q_{utile}=Q_{solare}-Q_{perso}=A\left[(\tau \alpha)G-U_L(T_p-T_a)\right]

Where τ is the transmission coefficient of the roof, α the absorption coefficient of the thermal plate, A the surface, G the incident radiation and U L the coefficient of total loss.

That is efficiency:

\eta ={Q_u\over AG_{tilt}} 

Generally manufacturers provide efficiency considering the average temperature of the liquid (Tm) and the ambient temperature (Ta), in the form:

 \eta=\eta_0-a_1{(T_m-T_a)\over G}-a_2G\left[(T_m-T_a)\over G\right]^2

Where generally the performance of the collector is described by a curve that represents its efficiency as the operating temperature varies. Typically in summer η = 0.6 while in winter η = 0.35.

Ta = 30°C e irraggiamento 800 W/m2 a sx; Ta = 20°C e irraggiamento 500 W/m2 a dx.


Solar energy is not always available and in fact tanks are used that allow the hot water to be used continuously. There are different types but the most common are the serpentine ones that provide both the production of hot water and to conserve heat. There are also various configurations for the tanks, from the simplest to the combined ones that integrate heating to the production of hot water.

The regulation and their control are based on differential thermoregulators or a regulator that allows you to set the desired temperature difference and different probes that measure the temperature in the panels and in the accumulation. There are different configurations and each requires its own optimized control system.


The panels must be installed on surfaces capable of maintaining good insolation by coping with the formation of possible shaded areas, taking into account the orientation angle and the angle of inclination with respect to the horizontal. In our hemisphere, the thermal collectors are ideally arranged to the south but variations of ± 30 ° C induce a loss of only 2.5%. On the other hand, it is more important to consider the angle of inclination and typically 20 ÷ 40 ° are used for systems with summer operation, 50 ÷ 65 ° for systems with winter operation and 40 ÷ 60 ° for systems with annual operation.

posizionamento solare termico
Solar diagram. Source: Enea – Atlante italiano della radiazione solare

IoT and thermoregulation

To the reasons listed above is added the desire to have an intelligent system. At the simple mechanical request to close and open the valves to select the device that produces hot water, it is possible to integrate a temperature threshold. By constantly checking the temperature, it is possible to understand what the real needs of the system and users are, as well as selecting the best device, avoiding waste of energy. Clearly the system must be automated. To these possibilities is added the desire to have a remote control and the possibility to intervene manually, and in a comfortable way, to adjust the thresholds if necessary.

From a market research, among the dedicated devices, the high cost of the control systems immediately emerged. This rises considerably when looking for something connected to the internet, with the possibility of saving a sensor history, a practical interface, remote control, the possibility of controlling the valves without requiring direct wiring, etc. The experience in the home automation field has made it possible to create the desired system with a very low cost by integrating all the functions from an Internet of objects perspective.

Clearly this system is a far cry from a professional control system and requires some basic installation skills. However, the functions provided fully satisfy the wishes of the installer and the user. The system uses commercial hardware typically sold to make home appliances intelligent for integration domotica .

Smart thermostat

To make the thermostat, the brain of the control system, a Sonoff TH16 is used with relative temperature probe. Added to this is the configuration of the scenarios that activate / deactivate the output on the basis of a target temperature. The activation of the output allows to control the first electro-actuated valve. By configuring some scenarios, it is also possible to configure the switching on of additional smart switches connected for example to an additional solenoid valve.


The device was installed near the solar panel storage using the probe well. The temperature probe is a DS18B20, a waterproof temperature sensor well known in the Internet of Things field. It is a digital thermometer that provides the temperature in degrees Celsius with a resolution of 9 to 12 bits. It communicates on a 1-Wire bus which requires only one data line besides the power supply. Furthermore, it has a 64-bit serial code that allows you to integrate multiple probes on the same bus.

It is used in various fields including HVAC (Heat, Ventilation and Air Conditioning) systems, temperature monitoring in buildings or machinery and process controllers. Used in both consumer products and professional products.

To the practicality of the 1-Wire protocol is added a rather wide range of measurement, from -55 ° C to + 125 ° C. Furthermore, it allows you to read the temperature in less than 1 second in the highest resolution and in less than 100ms if used at 9bit with a maximum accuracy of 0.5 ° C.

sensore di temperatura per il termostato
Typical performance curve – Datasheet

Thermostat configuration

The configuration is quite simple and is guided by the eWeLink application. After installing the application and creating an account, simply add a new device and register it by associating it with a name. The thermostat function can then be configured by setting the temperature at which it turns on/off and possibly a certain offset.

From the application it is possible to check the temperature at any moment. Furthermore, through the scenarios it is also possible to configure the switching on of other devices following the thermostat, useful for example to control additional valves or pumps.


Integration with home-automation

Of course, this system can also be easily integrated with Alexa. Just install the eWeLink skill, log in and then scan for devices. At this point, just ask the smart assistant what the temperature is.

Alexa, how many degrees are there in the solar panel?


Other application scenarios

It is clear that this system can be used wherever a thermostat is needed. Furthermore, if sensor waterproofing is not required, it is possible to take advantage of the other sensor in the kit, namely AM2301. This sensor allows to read both the temperature and the humidity and therefore to configure different application scenarios.

The applications are potentially endless. The classic scenario of the thermostat could be to control the activation of the heating based on the ambient temperature. Of course this can also be used for cooling or controlling a refrigerator. With humidity it is also possible to control a dehumidifier or a fan. It is also possible to apply this system in greenhouses, farms, irrigation systems or storage systems at controlled temperatures, etc.


Temperature and humidity sensor

The temperature and humidity sensor, AM2301, works in a similar way. It is a digital sensor that guarantees high reliability and good long-term stability. The sensor includes a capacitive humidity sensor and a temperature measuring device with an 8-bit controller. The sensor uses the 1-Wire protocol with a distance of up to 20 meters.

AM2301 Datasheet

The low consumption, the small size, the good performance and the simplicity of connection make it widely used in HVAC systems, dehumidifiers, test and inspection equipment, the automotive world, control systems, weather stations, medical devices, etc.


All the electrical and plumbing connections have been made together with a specialized company. It is not recommended to work on the heating or electrical system if you are not competent. Both systems must follow their respective standards.

The information provided is for information only and should not be considered as advice. Furthermore, the accuracy verified at the publication of the article could change with the updates of the platforms mentioned. We therefore decline all responsibility for any problems or damage caused by errors or omissions, even if such errors or omissions result from negligence, unforeseeable circumstances or other causes.

It is always recommended to call a qualified technician.

TCS thermostat