How do you choose the best, most reliable and long-lasting solar system ?

Our solar thermal system guide helps you to understand and find the best in solar thermal systems. You can also find out about solid solar thermal systems designed to last, with the LONGTIME® label.

Which solar thermal panel to choose?

Good heating and energy savings are essential these days.

Technical solutions have diversified, and the arrival of solar thermal collectors and combined systems offers new prospects. Whether you're looking to reduce your electricity bill, are planning to renovate your home or are building a new one, solar thermal energy can be an economical, sustainable and carbon-free solution for your heating and domestic hot water production. Combined solar thermal systems can easily replace an oil or gas boiler or back up any other heating or hot water production system.

Have you decided to replace your old oil-fired boiler? Would you like to heat your home with an efficient, environmentally-friendly system, but you're not interested in a heat pump? Would you like to buy reliable, high-performance solar thermal panels and enjoy peace of mind for many years to come? We're here to help;

The more you know about solar thermal energy, the better. So in the rest of this article, we'll go into a little more technical detail. If the rest seems a little complex and you simply want to have an opinion on the best solar panels and the best combined solar thermal systems, then simply trust the LONGTIME® label.

Solar collectors and combined solar thermal systems have been awarded LONGTIME® certification after being assessed against our 41 criteria for repairability and reliability by an independent inspection body.

And what's more, they're 100% French!

What is a solar thermal panel?

A solar thermal panel, also known as a solar thermal collector, is a type of solar panel designed to capture the sun's heat and convert it into energy that can be used to heat water or air. Unlike solar panels with photosensitive cells, which produce electricity from sunlight, thermal solar panels produce heat directly from the sun's energy.

These panels generally consist of a frame, a protective pane, a circuit filled with heat transfer fluid, an absorber and an insulator. The absorber is generally a dark, heat-conducting material that absorbs solar radiation;

  • How does a solar thermal panel work?

Their operation is relatively simple. A fluid capable of storing energy travels in the circuit through the panel and into contact with the absorber. The fluid is charged with energy (heat) by the absorber. Depending on the purpose, it is then pumped to a heat exchanger, which transfers the heat from the heat transfer liquid to the water or air that will be used to heat the space or hot water.

The panel's positioning is optimized to receive as much solar radiation as possible.

To simplify and conceptualise the principle, you may have already noticed that a bottle of water placed behind a windscreen and on a black dashboard can become very, very hot! Thermal solar collectors work in a similar way.

  • What is a solar panel used for?

They can be used to heat domestic hot water, water radiators, underfloor heating, ambient air heating or swimming pool heating. Solar heating systems can be used in combination with traditional heating systems to reduce energy bills and environmental impact.

In combined systems, the energy is stored in a reservoir for later use when demand is higher or sunshine is low.

  • What's the difference between a solar thermal panel and a solar photovoltaic panel?

Be careful not to confuse thermal and photovoltaic energy. These two collectors have completely different functions:

=> Solar thermal panel generates energy by heating a heat-transfer liquid
=> Photovoltaic solar panel that generates electricity by exciting photosensitive cells.

Thermal energy has about as much in common with photovoltaics as a microwave oven!

Which solar thermal system is right for you?

To choose the right system for your home, you need to take stock of your needs and consider the size of your home, its location and the composition of your household.

What is a solar combi-system?

An "SSC" solar combisystem or individual solar water heater (CESI) consists of 4 sub-assemblies:

  1. The collector (solar panel)
  2. The storage tank
  3. The circulator
  4. The solar controller (control system

How does a solar thermal system work?

The operation of an SSC is fairly straightforward: the panel captures the heat from the sun's rays and transfers it to the storage tank via the circulation of a heat transfer fluid, thanks to the circulator and the control system.

A solar combi-system lets you benefit from the sun's energy when you really need it, i.e. in the evenings and mornings, when the sun's rays aren't exactly optimal!

Producing hot water for 2 people in Nice does not require the same installation as covering the needs of 4 people in Lille.

What are the different types of solar thermal collectors?

There are many different solar thermal technologies.

  • The flat-glass collector :

Appreciated for its sturdiness, combining simplicity and technical features, its excellent price/performance ratio makes it the most popular model. It is discreet and easy to integrate.

Important details that differentiate models are the quality of the casing, insulation, glazing, seals and assembly.

  • The flat-plate, unglazed collector:

This is a panel with no body and no glazing, so it is much cheaper to manufacture. It is much less efficient because it is very sensitive to air temperature. It is efficient in summer, when demand for hot water and heating is low, and performs poorly in winter, when demand for hot water and heating is high...

  • Solar thermal tube panel :

Still relatively new to the French market, these collectors consist of a juxtaposition of transparent tubes. Each tube is evacuated and filled with a gas (Xenon, for example) to insulate the absorber inside the tube and improve energy production.

Evacuated tube collectors make it possible to take advantage of periods of limited sunlight, such as mornings and evenings, by increasing the angle of solar absorption. There are different types of evacuated tubes, such as direct flow tubes, heat pipe tubes and Sydney/CPC tubes;

These sensors are renowned for their suitability for extreme conditions.

  • Hybrid photovoltaic/thermal solar panel :

Also known as hybrid solar panels or mixed solar collectors, these collectors have the ability to generate electricity using photoelectric cells, as well as producing heat. There are two types of collector in this family: air-source (aerovoltaic) and water-source hybrid solar panels.

  • Solar thermal collector, self-storage or thermosiphon :

The thermosiphon operating principle is based on the difference in density between hot and cold water, and requires no pumps or mechanical systems.

It consists of an absorber and a storage tank, and works by heating the heat transfer fluid in the absorber using solar energy, which causes the hot water to rise towards the storage tank and the cold water to fall, creating a convection movement. This system is used to heat domestic water or for domestic and commercial heating, and has the advantage of being simple to install, inexpensive and requiring no electrical power to operate.

NB: Unlike photovoltaics, it is difficult to find a plug-and-play kit. This type of kit is not widely available for these technologies;

What are the advantages and disadvantages of solar thermal panels?

Like all systems, solar thermal has its advantages and disadvantages.
The main advantage lies in their ability to convert solar energy directly into usable heat, and to do so with excellent efficiency.
They are particularly efficient for producing hot water and heating domestic or industrial spaces, reducing heating costs.

  • 5 advantages of solar thermal energy

- Energy savings and solar thermal :

Thermal solar panels can significantly reduce energy consumption and, consequently, energy bills for hot water and space heating.

- Solar power, a renewable energy source :

They use a renewable, clean and a priori inexhaustible energy source - the sun - to produce heat, reducing dependence on non-renewable energy sources such as fossil fuels.

- Low maintenance :

These products generally have few moving parts and therefore require little maintenance. They generally last well over 30 years if they are of good quality, properly installed and well maintained. It may be necessary to clean them periodically to ensure they are working properly, and to monitor the condition of the heat transfer fluid and the sacrificial anode in the tank.

- Wide range of applications :

Solar thermal systems can be used in a wide range of applications, including hot water production, pool and spa heating, and space heating.

- Reducing GHG emissions :

Producing energy from the sun reduces emissions that are harmful to the environment and helps combat climate change. The panels produce no particulate emissions and require very little electricity to pump and circulate the heat transfer fluid.

Although solar thermal systems have many advantages, there are a few points to bear in mind.

  • Here are 5 disadvantages of solar thermal energy

- Dependence on sunlight :

Solar thermal collectors require direct exposure to the sun to operate efficiently. This means that their output can be affected by clouds, snow, shade or low light levels. So it's important to take into account the particularities of your climate when sizing the installation.

- Initial cost of solar thermal :

The initial costs of installing solar thermal panels can be relatively high, which may limit accessibility for some users. However, these costs can be amortised over the lifetime of the system, and above all the installation of a solar thermal system can benefit from state aid, making it very attractive.

- Physical space required to install solar collectors :

Physical space is inevitably required to install the sensors, which can pose problems in urban areas or where space is limited. If you live in a flat, it's difficult to opt for this solution on an individual basis.

- Performance limited by temperature :

Thermal panels can be affected in very hot environments. In periods of strong sunlight and high heat, the heat transfer fluid that stagnates can reach critical temperatures and deteriorate.

En conclusionSolar thermal panels have many advantages, such as energy savings, a renewable energy source, low maintenance and reduced greenhouse gas emissions. However, they also have disadvantages, such as their dependence on sunlight, their high initial cost, their need for space, their need for regular maintenance and their temperature-limited performance. So it's important to assess these factors carefully before deciding to invest.

How do you check the reliability and robustness of a solar thermal system?:

What are the best solar panels? How do you choose a solar collector that's reliable, repairable and durable? To ensure you make a safe purchase, certain features and points need to be checked closely. Here we focus on the product most widely distributed in Europe, the flat-plate glass collector.

Below you'll find a number of points to watch out for, to make sure you're buying robust, reliable, top-quality equipment.

Pay close attention to corrosion risks

Solar thermal systems are highly susceptible to corrosion.

We recommend that you carefully assess the quality of the accumulator, sensor and connections by checking two crucial points:

  1. The entire system is protected against the risk of galvanic corrosion by the use of compatible materials or separators.
  2. The accumulator features a high degree of resistance to corrosion and high water temperatures (corrosion-resistant materials, sacrificial anode with maintenance schedule), stainless steel, enamelled steel, copper.
  3. Plumbing fittings are corrosion-resistant.

Checkcheck the reliability of a solar thermal panel

The solar collector is not an item of equipment that can be repaired outright. In the event of failure, the number of interventions will be relatively limited. Reliability and robustness are therefore essential.

Sensors are subject to strict European standards, but we recommend that you pay particular attention to the following points:

  • Reliable sealing for glazed flat-plate collectors
  • Sturdiness of the chassis and resistance to corrosion by choosing aluminum or stainless steel sensor housings.
  • Quality of glass cover with choice of tempered glass
  • Solid mounting of heat transfer coils on the panel absorber with laser-welded integration
  • Absorber coating by PVD (Physical Vapour Deposition), Sputtering or PECVD (Plasma Enhanced Chemical Vapour Deposition) for improved efficiency

Choosing a robust rechargeable battery

Here are the 4 main families of accumulators used in solar power systems.

Galvanized storage tanks :

Highly inadvisable for use with solar systems, due to the lack of heat stability of the zinc used in the galvanizing process at high temperatures.

Enamelled balloons :

Commonly used to protect the tank against internal corrosion, enamel coatings are combined with the tank steel. Resistance to corrosion is good, but the slightest defect in the coating during the manufacturing phase means that the risk of corrosion returns with a vengeance.

Plastic-coated balloons :

Often more economical, they also have the advantage of being impervious to corrosion. If they are well designed, they can be an excellent choice.batteries that have been awarded the LONGTIME® label are guaranteed for life by the manufacturer HelioFrance!!!!
Not all plastic batteries are created equal.

Stainless steel balloons :

Resistant to corrosion thanks to their high nickel and chromium content, but remain sensitive to chlorine depending on the concentrations in the water. These are the most expensive products, but they are also virtually indestructible.

NB: If you opt for a steel battery, check for the presence of a sacrificial anode. This device is useful for protecting the metal in your battery from corrosion, as the anode corrodes instead of the metal it protects. It should be changed periodically;

The internal heat exchanger

The storage tank can be fitted with one or more internal heat exchangers.

If your accumulator has one, make sure it is made of 316L stainless steel.

Electric boiler backup

It's very likely that your storage tank is fitted with an electric heater to compensate for the lack of sunlight on certain days. If possible, opt for a storage tank with an electrical complement mounted on a standard thread.

Evaluating solar circulator quality

Make sure that the circulator on your system is of the highest quality, and that it is designed and sized for solar thermal systems.

The circulator is a device used to ensure that the heat transfer fluid circulates through the collector and accumulator.

Logically, your individual solar water heater or solar system should be equipped with a glandless circulator, capable of pumping liquids in excess of 100 degrees without suffering damage.

In addition, we invite you to check the electrical consumption of the circulator. The LONGTIME® solar thermal system standard requires glandless circulators to comply with an Energy Efficiency Index EEI<or equal to 0.21. Finally, a degumming screw is a must!

NB: Circulators are subject to European ecodesign regulations.

Watch the durability of the durable solar controller :

Pressure sensors, temperature probes, level probes... To operate correctly, your system requires regulation.

Virtually all systems therefore incorporate a set of electronic components controlled by the main electronic board, the solar controller.

The solar controller is the brain of your installation, controlling the circulation of the fluid, for example, according to the information collected by the NTC sensors. It understands and processes the information from the probes and sensors, as well as all the operating and safety phases.

For all these parts, the selection of quality materials and the company's knowledge of design and integration will have a major influence on the lifespan of the solar thermal system and its comfort of use. So beware of low-cost equipment, which may still represent an investment, but which will give you absolutely no satisfaction in the long term;

So make up your own mind, and don't hesitate to ask dealers questions about the card's origin.

NB: It is important to bear in mind that even with the best electronic board equipped with the most reliable components, if the internal software is not well developed, you may not be fully satisfied with your product. It is therefore essential to take into account the company's know-how in this area.

It is also advisable to check carefully the company's policy regarding the electronic part of the card. This may include the length of time cards are made available, the possibility of component repairs, the use of reconditioned electronic cards, and measures to protect against overcurrents;

Which is the most efficient solar thermal system?

A solar system consists of a solar thermal collector and an accumulator. The accumulator is usually a solar water heater. It's hard to know which solar thermal system is the most efficient, because it depends on a number of factors, including yield, your region and the purpose of your solution (heating, DHW, swimming pool, etc.).

How do you size a solar thermal system?

Before you do anything else, check with independent professionals such as Espaces Conseil France Rénov' that this is the right solution for your home and your needs. The best energy is the energy you don't use, so before you start thinking about your heating system, we recommend that you take stock of your home's energy losses.

The sizing of your system, i.e. the collector surface area required, depends on several criteria:

  • The technology of your system and the yields associated with the collectors
  • Climatic conditions where you live (temperature, days of sunshine)
  • The characteristics of your home (size, insulation, number of people)
  • The purpose of the solution (heating, DHW, swimming pool, etc.)

How efficient is a solar thermal collector?

For a glazed collector, we recommend a minimum efficiency in relation to the overall surface area of the charging system n0 of 75%, with conduction loss A1 less than or equal to 5 and convection loss A2 less than or equal to 0.1.

The energy performance of thermal solar collectors is generally much higher than that of photoelectric collectors. However, within a given panel category, the differences in performance can be very significant. Before buying a panel, be sure to check and compare performance.

Solar thermal systems and technical feasibility

Once again, it's essential to carry out a detailed technical feasibility study to assess the viability of a solar thermal installation in a specific context: your own.

The technical constraints associated with installation are variable and depend on geographical region, system size, specific application and technologies used, but overall they are relatively low.

The 3 points to bear in mind when assessing the technical feasibility of a solar thermal system

  • Sunshine conditions:

The availability and intensity of sunlight are key factors in the performance of the installation. Regions with low or limited sunshine can have reduced heat production, which can affect the efficiency and profitability of the system and require back-up solutions when intermittency is important. This is the case in most French regions.

  • Space required :

To be effective, solar collectors necessarily need to be installed on the roof or facade. The space available on the roof or in other installation areas can be a constraint, particularly in urban areas where space is limited or if you live in a flat. Your roof and roof timbers also need to be in good condition to safely accommodate the panels, and at the right angle and inclination to catch the sun's rays. Rest assured, this is generally the case.

  • Heat storage :

Storing the heat produced by collectors can pose technical challenges. The heat must be stored efficiently for later use, which may require specific thermal storage tanks, such as hot water tanks or high-temperature storage tanks, and therefore dedicated physical space.

How is a solar thermal panel made?

The manufacture of a solar thermal panel varies according to collector category, but here are the general steps in the manufacturing process for a glazed flat-plate collector:

Materials collection :

The materials needed to manufacture a panel include metal frames for the chassis, glass plates, copper tubes, absorbers, insulating materials, gaskets, connectors and other electronic components.

Chassis assembly :

The frames house the insulation, the absorber, the heat transfer circuit and the glass. Each section is welded together to form the frame.

Absorber assembly :

The absorber is a metal plate that absorbs the sun's heat. It is generally made up of several layers of materials, such as reflective metals and heat absorbers. These layers are assembled and fixed inside the box.

Joining copper tubes :

Copper tubes are used to transport water or other coolant through the absorber. The tubes are placed on the absorber and fixed in place using laser welding.

Assembly of other components :

Other components, such as connectors and insulators, are then assembled in place.

Glass installation The glass plate is then installed on top of the enclosure to protect the internal components and help trap heat inside the panel. It is usually sealed with mastics.

Ultimately, the manufacturing process for a solar thermal panel can vary between models and manufacturers, but these general steps give you an idea of the basic fabrication.

How long does a solar thermal panel last?

These products must be robust and designed to withstand repeated climatic hazards (snow, hail, UV radiation, wind, frost...).
If you follow the manufacturer's recommendations for installation, use and maintenance, the lifespan should not be less than 30 years.

LONGTIME solar collectors are certified reliable and robust.

How to extend the life of your solar thermal panels:

Two factors will play a decisive role in extending the life of your system: installation and maintenance.

Installing a solar thermal system to the highest professional standards:

For the safety of your home and your comfort, the installation must comply with strict technical specifications.

Sizing the collector surface, installing and securing the solar panels on the roof, connecting and filling the circuit... all these steps require solid skills and are not for the first DIY enthusiast.

Manufacturers usually have an authorized network of distributors and installers trained in their products, both for optimum installation and for efficient technical support in the event of a fault. Builders often require your system to be installed or inspected by a Qualisol-certified professional to activate guarantees in the event of a problem. Home insurance policies may also contain restrictive clauses concerning these appliances, so be careful.

A poorly installed system won't work properly, with the risk of overheating, leaks or reduced performance, resulting in premature wear and tear on your system.

Maintaining a solar thermal system

Maintenance and servicing are really necessary and play a major role in extending the useful life of your equipment.

A watchword: "Read the use and care instructions".

Periodic maintenance is required, depending on the sensors and the purpose of your installation. The brazier, ashtray and glass should be serviced daily or weekly. Annual maintenance is also required, to ensure thorough cleaning and sweeping of the flues. Good maintenance will prevent the stove from clogging up.

Heat transfer fluid

When your collector has a fluid-based heat transfer circuit, we recommend filling the circuit with a fluid that complies with the manufacturer's specifications.
The fluid has a decisive influence on both the system's performance and its lifespan.

  • During periods of maximum sunshine, such as heatwaves, the fluid can reach extreme temperatures. In these situations it can "caramelise", which means that the fluid loses its properties and risks damaging your circuit.
  • In periods of extreme cold, if the control system deems it unwise to circulate the fluid, there's a risk of freezing and bursting.

So remember to have both the quality and quantity of the fluid in your system checked.

image of a check being given

What installation subsidies are available for solar thermal systems?

To speed up the thermal renovation of buildings and meet its carbon emission commitments, France has decided to encourage the sector.In France, there are a number of grants and subsidies to encourage installation and bring down the price. Here are the most common support schemes:

  • MaPrimeRénov'
  • Boiler Conversion Bonus
  • Tax credit for energy transition (CITE)
  • Zero interest eco-loan (éco-PTZ)
  • Energy Savings Certificates
  • Local aid for collector installation

It's important to note that grants are subject to change, and we recommend that you contact the relevant bodies, such as the Agence Nationale de l'Habitat (ANAH), France Rénov' or local authorities, to find out about current grants and the eligibility conditions specific to your situation. Above all, beware of abusive canvassing and don't sign anything without checking your real rights with an independent body.

Where possible, you should also consider getting several quotes. This will also allow you to cross-check the information provided by the professionals.

How to repair your solar thermal system

For this product family, repairs will focus mainly on the accumulator, circulator and control components.

Flat-glass collectors are not, in themselves, highly repairable equipment, given their performance and atmospheric resistance. To avoid breakdowns and problems, there are a few basic rules to follow:

  • Choose a collector surface and technology suited to your needs, your home and your region.
  • Prefer installation by a certified professional approved by the manufacturer.
  • Simple yet precise system control
  • Comply with instructions for use and, above all, for maintenance and upkeep.

If you observe these few basic rules and have chosen a solar system that is durable, i.e. robust, reliable and repairable, you should be relatively safe.

However, no brand is immune to failure, and no product is infallible.

What should I do if my solar system breaks down?

In the event of a breakdown, and as a first step, check the warranty status of your system:

  • Is your system less than two years old? Contact the distributor or installer who sold and installed your system. In France, the person who sold you the equipment is legally responsible for the garantie légale de conformité (GLC), which lasts for 24 months.
  • Is your installation more than two years old? Check whether the manufacturer offers an extended warranty over the two-year GLC.
    Also take a look at your home insurance and/or credit card contracts. You may have additional insurance or extended cover without even knowing it.

Your system is no longer under warranty, don't panic! Solar thermal systems and CESI are products that can be repaired in the event of a breakdown:

  • In all cases, first contact your installer or the after-sales service of your product brand, either directly with the manufacturer or through your dealer.
  • Before undertaking any repairs, always check that your system is correctly supplied with electricity, especially the circulator and controller.

What are the most common faults in solar thermal systems?

As with any piece of equipment, faults can occur in your installation. Here is an exhaustive list of the most common faults:

Sensor failure :

  • Impaired sealing (internal condensation, water stagnation)
  • Frame corrosion due to inadequate material or paint quality, or scratches during installation
  • Pressure loss due to leakage at a fitting (overpressure, freezing, galvanic corrosion)
  • Freezing of the heat transfer circuit due to liquid stagnation or use of an unsuitable liquid
  • Cover breakage for flat-glass collectors (exceptionally large hailstones, asteroids!)

Control or solar controller failure :

  • Internal electronic board faulty (overvoltage, component failure)
  • NTC sensor faulty

Accumulator (storage tank) failure :

  • Expansion vessel failure (diaphragm, leaky connection)
  • Faulty auxiliary resistor (broken resistance wire, loss of insulation)
  • Corrosion of tank walls and/or internal coil exchanger

Circulator failure :

  • Circulator motor faulty (seal, power supply, bearing)
  • Internal electronic board faulty (overvoltage, component failure)

In conclusion, it's important to note that, as with any hydraulic system, leaks are likely to occur over time, either at fittings or through punctures caused by overpressure or corrosion.

DIY* solar thermal systems

For the more do-it-yourselfers among you, some problems can be solved by DIY (self-repair). The Internet is full of explanatory videos, and there are many groups on forums.

You'll learn how to dismantle your circulator and check the safety electrode, but be careful! As with all EEE products, make sure you have a solid grounding so that you can work on your appliance in complete safety. We sincerely recommend that any work on these products is carried out by qualified professionals;

Reminder: An essential point when it comes to durability, make sure that the manufacturer of your solar system, collector or CESI provides high-quality technical support and after-sales service. All products with the LONGTIME® label are repairable products, so think about it!

In summary, to choose the best solar thermal collector or solar thermal system :

  • Check not only the availability of spare parts over time, but also the lead times and prices of these parts.


  • Choose brands with genuine technical support and a training program for installers.
  • Find out about the robustness of the collector and accumulator, its efficiency and the reliability and repairability of the solar controller and on-board electronics.


  • Make sure the various priority parts are accessible: solar controller circuit board, auxiliary resistor, circulator, solar collector...

Would you like to buy a reliable solar thermal panel, a sustainable thermal system without a degree in HVAC engineering and without poring over 15 blogs and 4 consumer magazines?

Take a look at our product directory and you're sure to find what you're looking for!

Votre voix compte !

Votre voix compte !

Participez à la consultation pour l’évolution du référentiel LONGTIME® en moins de 5 min !

Je participe

Merci d'avoir participer