The Ultimate Guide to Solar Thermal


In this Ultimate Guide to Solar Thermal, we will delve into how we can harness the energy of the sun to meet your home’s heating needs, whilst also reducing your energy bills and carbon footprint.

Throughout this guide we will give you information on how solar thermal works, the different applications, how to install them and maintain them, and which safety factors need to be considered. We will also investigate the different components that fit into the different system types, how to size them, and understand their efficiency to help you choose the best solar thermal system for your home.

Browse the guide at your own pace or open the dropdown menu to click the links to jump to the sections you need the most.

Section 1 – What is Solar Thermal?

Unlike photovoltaic (PV) solar panels that convert sunlight into electricity, Solar thermal technology captures and uses the energy from the sun to produce heat. By producing this heat, solar thermal systems typically heat air, water, or other fluids, within a variety of applications.

There are three basic working principles to a solar thermal system, which involves three main components. These include:

Solar Collectors: These are classed as the heart of a solar thermal system. They are designed to absorb sunlight and convert it into heat. There are different types of solar collectors, including flat-plate collectors (For more information see Section 2).

Heat Transfer System: When the solar collectors have absorbed the sunlight and generated heat, the heat transfer system circulates a heat carrying fluid through the collectors. The fluid is usually a mixture of water and a heat transfer fluid, such as anti-freeze. This fluid then becomes hot as it absorbs the solar heat.

Storage or Utilisation System: The hot fluid is then directed to a storage tank (for water heating) or to the end use application (for space heating).

Section 2 – Solar Thermal Systems

There are various types of solar thermal systems, each has its own characteristics to fit specific needs, requirements, and applications. The main systems are:

Passive Solar Thermal Systems: These utilise the natural principles of heat transfer, and they do not use mechanical or electrical devices. They are usually low maintenance and are simpler when compared to active systems. These systems are most effective in buildings that have good solar orientation and design. Examples include:

  • Direct Gain: Directly exposing a thermal mass (e.g., masonry wall or concrete floor) to sunlight, allows it to absorb and store heat during the day to be released at night. This can help to regulate an indoor temperature.
  • Indirect Gain: Uses a thermal mass behind a transparent cover (e.g., sunspace or solarium) to collect and store solar heat. This is then circulated into the living space.
  • Isolated Gain: The thermal mass and living space is separated by a fluid/gas (e.g., water or air) that collects and transfers the solar heat to the living space.

Active Solar Thermal Systems: These systems use mechanical or electrical components that actively collect, circulate, and store solar energy. These can be controlled more easily, making them suitable for various applications. Examples include:

  • Flat-Plate Collectors: These are the most common type. They are a flat, insulated, and weatherproof box that has a dark coloured absorber plate and a transparent cover (e.g., glass or plastic). The sunlight passes through the cover and is absorbed by the plate. Heat is then transferred to the fluid/gas (e.g., water or gas) which is in the tubes of the absorber. The heated fluid is then pumped into a storage or utilisation system.
  • Evacuated Tube Collectors: They are similar to flat-plate collectors; however, they have rows of glass tubes that contain an absorber. The vacuum-sealed tubes reduce heat loss, making them more efficient in colder climates or on cloudy days.
Section 3 – Solar Thermal Applications

Solar Water Heating: The most common use of a solar thermal system is to heat water for use within homes. Using solar water heaters can significantly reduce the consumption of energy sources typically used for heating water. This leads to savings on energy bills and reduced greenhouse gas emissions.

Space Heating: Space heating can be provided in colder months with solar thermal energy. The solar energy is collected to heat air or a heat-transfer fluid. This is then circulated through radiators or underfloor heating systems.

Solar Pool Heating: These use solar collectors to capture sunlight and transfer the heat to the pool water. This helps to maintain a comfortable water temperature for swimming without relying on conventional methods.

Solar Desalination: Desalination systems convert seawater or brackish water into freshwater.

Solar Cooling: Advanced systems can be used for cooling purposes through absorption chillers. The collected thermal energy drives a refrigeration cycle, which is then used to cool buildings.

Solar Cooking: Solar thermal energy can be used to cook food in solar cookers or ovens, to reduce the need for conventional fuels.

Agriculture Applications: These can be used in greenhouses to regulate temperatures and improve crop growth.

District Heating: This can supply heat to multiple buildings or a whole community from a central solar thermal plant.

Section 4 – System Components

Solar Collectors: These are devices that capture the sunlight and convert it into heat. There are several different types including flat-plate collectors and evacuated tube collectors. Choosing which is right for you depends on the specific application and efficiency requirements.

Heat Transfer Fluid: This is used to absorb the solar heat that is collected. The fluid can be water or another specialised heat-transfer fluid. The fluid carries the heat from the collector to the storage or utilisation system.

Heat Exchanger: This is responsible for transferring the heat from the heat transfer fluid to the water or air that needs heating. In solar water heating systems, they are used within the storage tank to heat the water.

Storage Tank: Insulated storage tanks store the heated water or other fluid. This allows excess heat to be stored in warmer periods, to make it available when it is cloudy or at night.

Circulation Pump: This circulates the heat transfer fluid through the solar collectors and the heat exchanger. It ensures the fluid is always circulated for effective and continuous heat transfer.

Controller: The central component that monitors and manages the solar thermal system’s operation. It controls the circulation pump, the temperature settings, and it protects the system from overheating or freezing.

Expansion Tank: In closed-loop solar thermal systems, they are used to accommodate the expanding and contracting of the heat transfer fluid as it heats up and cools down.

Piping and Fittings: These connect the components of the system together, to allow the heat transfer fluid to flow effectively throughout the system.

Insulation: This will help to minimise heat loss and maintain the temperature of the heat transfer fluid. Overall, it helps to increase the system’s efficiency.

Backup Heating System: This is optional and can be an electric or gas heater. They are usually used to provide heat during periods of insufficient solar radiation.

Section 5 – Installation and Maintenance

Installing and maintaining a solar thermal system correctly is crucial to ensure effective and efficient operations. It can enhance the system’s performance, extend its lifespan, and reduce the potential for any breakdowns. Whilst also helping you to be more sustainable and helping to reduce your energy bills.


Site Assessment: A thorough site assessment must be carried out to find the best location for the solar collectors. Solar exposure, shading, orientation, and roof angle must be considered.

Proper Sizing: The size of the system depends on the demand for hot water or heating, solar irradiance in the area, and if the space that is available for the collectors and storage tank.

Quality Components: High quality components will ensure that the system is reliable and efficient.

Correct Installation: Follow the manufacturer’s guidelines or hire a professional for proper installation of all the components.

Insulation: To minimise heat loss and maintain the temperature of the heat transfer fluid ensure that the pipes and the storage tank are properly insulated.

Safety Measures: These measures should be put in place to protect against scalding or other potential hazards that are associated with hot water systems.


Regular Inspection: Ensure you schedule regular inspections with a qualified technician to check for leaks, worn-out components, and the overall performance of the system.

Cleaning: The solar collectors must be kept clean, and free from dust, dirt, and debris.

Fluid Check: The level and condition of the heat transfer fluid should be monitored. If using water, make sure it doesn’t freeze during colder months.

Controller Calibration: Calibrate and check the controller settings regularly to make sure the system is operating correctly, and that the temperature is controlled.

Pump Maintenance: Check the circulation pump to ensure efficient operation and that there are no obstructions.

Antifreeze Replacement: If using antifreeze in the heat transfer fluid, make sure it is replaced regularly. Check the manufacturer’s guidelines to check how often this needs to be done.

Backup System Check: If there is a backup heating system, ensure that it is working correctly and ready to kick in if needed.

Check for Air Locks: These can obstruct fluid circulation and reduce the system’s efficiency.

Monitor System Performance: Check the system’s performance regularly, including checking the temperature of the water and the energy output. This will help to identify any issues.

Section 6 – Efficiency and Performance

The efficiency and performance of a solar thermal system are important as it determines how effectively the system harnesses the sun’s energy and converts it into usable heat. There are several factors that influence the system’s efficiency and performance including:

Solar Irradiance: This is the amount of solar energy that is available at the site, this directly affects the system’s performance. Locations with more sunlight have a higher solar irradiance, meaning they will have better system efficiency.

Collector Type: Evacuated tube collectors are usually more efficient in capturing solar energy when compared to flat-plate collectors.

Collector Orientation and Tilt: Make sure the solar collectors are aligned to face the sun to optimise energy capture. The tilt angle should be adjusted based on the latitude of the installation site.

Shading: Avoid any shading on the solar collectors, as shadows can reduce the amount of light that the collector can absorb.

Heat Transfer Fluid: Select the appropriate fluid, such as water or a heat-transfer fluid like glycol, as this can affect the efficiency of heat transfer.

Insulation: Adequate insulation will prevent heat loss during the movement and storage of the heated fluid.

Tracking Systems: Some systems incorporate tracking systems that move the solar collectors to follow the sun’s path. This can improve energy capture.

System Sizing: Properly sizing the system according to the demand for hot water and heating ensures that the required load is met without wasting excess energy.

Heat Exchanger Efficiency: A properly functioning and well-designed heat exchanger ensures that there is an efficient transfer of heat from the heat transfer fluid to the water or air being heated.

Maintenance: This is crucial for maintaining the efficiency and performance of the system.

Measurement and Monitoring

The efficiency of a solar thermal system is usually measured as a ratio of useful heat output to the incident solar energy. This is expressed as a percentage. The efficiency of a solar thermal system ranges from 20% to 80%. This depends on several factors including the design of the system, its location, and the components within it.

Potential issues can be flagged up through continuous monitoring and performance analysis. This monitoring and analysis can also show if there is any opportunity to improve and upgrade, which can increase optimisation. This can help to further reduce your energy costs, lower emissions, and increase energy independence.

Section 7 – Sizing and Design

It is important that your solar thermal system is sized and designed properly, to meet your hot water and heating needs. Doing this can optimise performance which can lead to cost savings, reduce greenhouse gas emissions, and an increased reliance on renewable energy sources.

It is recommended that you consult a professional and utilise software tools to help accurately design an efficient system. Solar irradiance, climate, hot water consumption, and system components need to be considered.

Key steps to sizing and designing a solar thermal system include:

Hot Water Demand Assessment: Estimate the daily hot water consumption based on how many people live in your home, as well as their hot water usage patterns.

Solar Resource Assessment: The solar irradiance needs to be evaluated at the site to estimate the amount of solar energy that is available. This information is important for sizing the needed solar collector.

Collector Selection: The collector type should be chosen based on the available solar resource and the required hot water temperature. For more information see Section 2.

System Sizing: The size of the system should be based on the demand for hot water and the solar irradiance. The size should also be determined by the collector area and the storage tank capacity. Undersizing and oversizing can lead to a reduced efficiency.

Storage Tank Capacity: The storage tank should hold enough heated water to meet your daily hot water demands. Take seasonal variations and potential cloudy days into account.

Orientation and Tilt: When designing the system make sure the solar collectors are positioned at a tilt angle that will maximise solar energy capture.

Piping and Circulation: The layout of the pipes and circulation system needs to be planned to ensure efficient heat transfer between the different components.

Heat Exchange Design: Choose an appropriate heat exchanger that can efficiently transfer heat from the collector fluid to the water in the storage tank.

Freeze Protection: When designing the system make sure freeze protection is put into place, such as using anti-freeze.

Controller and Monitoring: Install a reliable controller to monitor and control the operations, temperatures, and efficiency of the system.

Safety Measures: Add in safety measures when designing the system to prevent scalding or overheating.

Compliance and Regulations: Make sure that the system design complies with local building codes and regulations.

Section 8 – Cost and Incentive

The cost of a solar thermal system can vary depending on the size, collector type, complexity of the installation, and the location of the property. Usually, a solar thermal system would cost between £3,000 and £8,000. It is important to remember, however, that these systems can provide significant long-term benefits including cost savings on your energy bills and a reduction in greenhouse gas emissions.

Many see the initial upfront cost as a major consideration, however, there are various incentives and financial support for those wishing to choose more sustainable and renewable energy options. These include:

Domestic Renewable Heat Incentive (RHI): Now closed for new applicants. For those that have applied, this scheme provides financial support to homeowners who install an eligible renewable heating technology, including solar thermal systems. Under the RHI, participants receive payments for generating and using renewable heat over a period of seven years for residential installations. The RHI scheme has different tariff rates based on the technology and heat demand.

Local Authority Grants: Check with your local authority to see whether they offer grants or financial incentives to those installing renewable energy systems. This includes solar thermal systems. Many local authorities do this to promote sustainable energy use.

Energy Efficiency Financing: Some financial institutions offer green or energy efficiency financing options. These may have favourable terms for those installing renewable energy technologies. These options can help to spread the upfront cost over time, making renewable technologies more accessible and affordable.

Before choosing a financial option, research and stay updated with the latest incentives and grants within your location. Government policies and support programmes may change, or new opportunities may become available.

Section 9 – Environmental Impact

Reduction in Greenhouse Gas Emissions: The systems use renewable solar energy which reduces the need for fossil fuels. By moving away from these carbon-intensive energy sources, your greenhouse gas emissions will lower.

Energy Conservation: Solar thermal systems can help to minimise energy waste and reduce the demand for non-renewable resources.

Air Quality Improvement: The systems produce heat without emitting harmful pollutants or particulate matter, which can lead to negative impacts on human health, including reducing respiratory and cardiovascular health issues.

Water Conservation: Using solar energy to heat up water can help to conserve traditional water heating fuels, such as electricity or gas.

Sustainable Resource Utilisation: Solar energy is derived from the sun, which is a renewable and abundant resource. It is not going to become depleted or face depletion costs, like fossil fuels.

Low Water Usage: Compared to fossil-fuel based systems, solar thermal systems usually have a significantly lower water usage.

Local Energy Generation: The systems can be installed close to the point of use, which reduces the need for long distance energy transmission and distribution. This can also increase energy security and grid resilience.

Long Lifespan: These systems have a long operational lifespan with relatively low maintenance requirements. Which results in a reduced environmental impact throughout its lifecycle.

Section 10 – Safety Considerations

Scalding Risk: As the system involves heating water or other fluids to high temperatures, there is a risk of scalding. Ensure the temperature controls are set properly and check for any malfunctioning components. Thermostatic mixing valves can help prevent accidents.

Freeze Protection: To prevent damage to the system, freeze protection should be put in place. Protection includes antifreeze or drain back systems that drains the fluid from the collector during freezing conditions.

Pressure Relief: High pressures can be generated when heated fluid expands. Make sure the system is properly sized and that pressure relief valves are functioning properly to prevent overpressure and ensure safe operation.

Ventilation: Adequate ventilation is required to allow for any potential harmful gases to be released.

Electrical Safety: If the system contains electrical components, proper grounding and electrical safety measures must be in place.

Roof Load: Consider the additional weight of the collectors and storage tank if the system is going to be roof mounted. Ensure that the roof can support the load safely.

High temperatures: Solar collectors can become very hot when they are exposed to direct sunlight. Make sure proper warnings are in place to prevent accidental contact.

Installation and Maintenance: A qualified professional should install the system to ensure safe operation. Regular maintenance and inspections should be carried out to identify any safety issues.

Proper Signage: Clear and informative signs need to be placed near the system to indicate areas of potential risk (hot surfaces, high-voltage areas).

Compliance with Regulations: Ensure the system complies with local building codes and safety regulations.

Training and User Education: Users and owners should be trained on how to operate and maintain the system safely.

Section 11 – Solar iBoost

Solar iBoost is a device that is designed to enhance the efficiency of a solar PV (photovoltaic) system, by diverting excess electricity generated during the day, to heat water. Either in a hot water cylinder or an immersion heater.

The purpose of the Solar iBoost is to effectively use the surplus solar electricity generated, instead of sending it back to the grid or the utility company for a feed-in tariff. The electricity, however, is redirected to your water heating system, allowing you to increase your self-consumption of solar energy.

How they work:

Monitoring: The device is connected to your solar PV system’s generation meter or smart meter. This is to monitor the amount of electricity being generated.

Diversion: Excess electricity is detected and directed to an immersion heater or the hot water cylinder.

Water Heating: The diverted electricity heats up the water, therefore, the excess solar energy isn’t wasted.

Grid Export Reduction: This reduces the amount of energy you export back to the grid, which increases your self-sufficiency and reduces your electricity bills.