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Solar Thermal with Combi Boiler UK: Maximising Efficiency and Savings

Solar thermal systems are becoming an increasingly popular option for homeowners in the UK seeking to reduce their energy consumption and carbon footprint. These systems harness the sun's heat to provide hot water for a household, covering around 40%-60% of the total annual hot water demand, including hot water for showers and taps. While solar thermal technology provides numerous benefits, it is crucial to understand its compatibility with different types of boilers, such as combi boilers, before deciding on an appropriate system for your home.

Combi boilers are commonly used in the UK due to their compact size and energy efficiency. However, it is worth noting that solar thermal panels cannot be effectively used with combi boilers. To make the best use of solar thermal technology, a hot water store is required, which is typically achieved through a system boiler configuration comprising a boiler and a separate hot water cylinder. Nevertheless, solar thermal systems can be an excellent supplement to an existing heating system, as all major systems are designed to work in combination with solar technology.

When considering integrating solar thermal technology into your home heating setup, it is essential to weigh the pros and cons of different boiler types to ensure the most effective and efficient results. Homeowners with combi boilers may need to explore alternative options or consider transitioning to a system boiler configuration to fully utilise the benefits of solar thermal energy.

Overview of Solar Thermal and Combi Boiler Systems

Solar thermal systems take advantage of the sun's heat to provide domestic hot water (DHW) for households. They typically consist of solar collectors, a storage tank, a control system, an auxiliary heater, and a heat distribution system. These systems can supply DHW throughout the year, with a boiler supporting them to ensure hot water is available whenever needed.

Combi boilers, on the other hand, are a space-saving solution for heating and hot water provision in a home. They heat water directly from the mains, removing the need for a separate hot water cylinder or storage tank. While combi boilers are not typically compatible with solar thermal systems due to the absence of a hot water cylinder, some modern solutions have emerged to address this issue.

For example, solar thermal panels can be used in combination with a suitable solar-compatible hot water cylinder to store preheated water before it enters the combi boiler. Using solar energy in this way reduces the energy required from the boiler, leading to lower energy bills and a reduced carbon footprint.

When considering a solar thermal system with a combi boiler, it is essential to verify whether the existing hot water cylinder, if present, is solar-compatible and large enough to store the solar preheated water. If not, a new, appropriately-sized and solar-compatible cylinder may need to be installed alongside the solar thermal panels.

Types of Solar Thermal Collectors

Solar thermal collectors are designed to convert solar energy into heat, which can be used for various applications like heating water or buildings. There are two main types of solar thermal collectors used in residential settings: Flat Plate Collectors and Evacuated Tube Collectors. Both have their pros and cons, and each is suitable for different situations.

Flat Plate Collectors

Flat plate collectors are the most common type of solar thermal collector. They consist of a large, flat surface with tubes running through it to carry the fluid that will absorb the sun's heat. The flat panel absorbs sunlight and transfers the heat to the fluid inside the tubes, which can then be circulated through a heat exchanger to provide heating or hot water.

Some advantages of flat plate collectors include:

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    • Lower cost compared to evacuated tube collectors
    • Easy installation and maintenance
    • Higher efficiency in moderate climates

    However, flat plate collectors have some disadvantages:

    • Lower efficiency in colder climates or cloudy conditions
    • Larger surface area required for installation

    Evacuated Tube Collectors

    Evacuated tube collectors use a series of glass tubes, each containing a smaller fluid-filled tube that absorbs heat from the sun. The tubes are vacuum-sealed to reduce heat loss, making them highly efficient even in colder climates or under cloudy conditions.

    Some advantages of evacuated tube collectors include:

    • Higher efficiency in colder climates or cloudy conditions
    • Smaller surface area required for installation
    • Work well even at low temperatures

    On the other hand, evacuated tube collectors have some disadvantages:

    • Higher cost compared to flat plate collectors
    • More complex installation and maintenance

    Choosing the right type of solar thermal collector depends on factors such as climate, space availability, and budget. It is important to carefully consider these factors and consult with a professional to determine the best option for your specific needs.

    Integration of Solar Thermal with Combi Boilers

    Integrating solar thermal systems with combi boilers can be challenging, as most UK homes are heated by a combi boiler. A combi boiler is an all-in-one unit that supplies hot water on demand directly from the mains, which may complicate the installation of a solar thermal system. However, it is possible to combine these two systems in some cases.

    If you have a compatible combi boiler, it may be possible to feed pre-heated water from your solar thermal system into the boiler. This means your boiler won't have to work as hard to heat the water to the required temperature, which can lead to considerable savings on your energy bills.

    One way to achieve integration of solar thermal panels with a combi boiler is through the use of a heat exchanger. The heat exchanger separates the solar system from the potable water side, but it is important to dose the solar side with potent chemicals to maintain system efficiency.

    Before opting for a solar thermal and combi boiler integration, it is essential to consult with a professional heating engineer, who can assess your specific setup and provide the most appropriate solution. Additionally, companies like Viessmann UK offer solar thermal heating systems designed to work in combination with various heating systems, including condensing boilers, heating systems for wood, and heat pumps.

    Advantages of Solar Thermal and Combi Boiler Systems

    Solar thermal and combi boiler systems offer several benefits for homeowners in the UK who are looking to utilise renewable energy sources for heating and hot water needs. This section will highlight the key advantages.

    Energy and Cost Saving: Installing solar thermal panels in combination with a combi boiler can lead to considerable energy savings. Solar hot water panels require a cylinder, but when working together with a combi boiler, the system can still provide energy savings by reducing the boiler's operation time. A solar thermal heating system reduces the need for using your boiler as it harnesses the sun's heat to warm water for your home.

    Reduced Carbon Emissions: By utilising solar thermal and combi boiler systems, homeowners can lower their carbon emissions. Solar energy is a clean and renewable energy source that helps contribute to a lower carbon footprint.

    Year-round Functionality: Solar water heating systems can work throughout the year, although they may require additional heating from a boiler or immersion heater during winter months when solar energy is less available. For increased sustainability, thermodynamic panels can be added to complement the hot water system.

    Low Maintenance: Solar thermal systems generally require minimal maintenance, making them a reliable and cost-effective option for homeowners, who can expect a long lifespan from their installation.

    Government Incentives: In the UK, homeowners who install solar thermal panels may be eligible for government grants and incentives, such as the Renewable Heat Incentive (RHI), which offers financial rewards for using greener sources of energy.

    In summary, solar thermal and combi boiler systems benefit homeowners by offering energy and cost savings, reducing carbon emissions, providing year-round functionality, requiring low maintenance, and potentially qualifying for government incentives.

    Cost and Savings

    Initial Investment

    Installing a solar thermal system with a combi boiler in the UK typically costs between £3,000 and £5,000 for an average home, such as a family of four. This price range is influenced by factors such as the complexity of the pipe runs, roofing materials, and the type of solar panels used (e.g., evacuated tubes). In cases where a larger system is required due to higher hot water demand, installation costs may rise to £7,000.

    Running Costs

    Once a solar thermal system is installed, the running costs tend to be relatively low, as the primary energy source, the sun, is freely available. The main expense is the operation of the circulation pump, which is typically around 8% of savings. Regular maintenance and servicing of the system should also be accounted for in the running costs.


    Solar thermal systems can significantly reduce reliance on boilers for hot water, resulting in energy savings and lower utility bills. In the UK, solar collectors combined with a domestic hot water cylinder can provide a compelling alternative to boiler operation, particularly during the summer months. However, it is important to note that solar thermal systems alone are unlikely to meet the total hot water demand for a home, and a boiler will still be needed to supplement the system.

    Government Incentives and Funding

    The UK government has introduced several incentives and funding schemes to support the adoption of renewable energy technologies, including solar thermal systems combined with combi boilers. One such initiative is the Renewable Heat Incentive (RHI), which offers financial support for eligible renewable heat technologies. Homeowners who install solar thermal panels can receive payments from RHI based on their system's output, thereby lowering energy bills and reducing carbon emissions.

    In addition to RHI, the government has also invested in the Help to Heat initiative, aiming to make homes warmer and cheaper to heat. The initiative includes programmes such as the Boiler Upgrade Scheme and the Home Upgrade Grant phase 2, which can potentially benefit homeowners with solar thermal systems and combi boilers. More details about these schemes can be found on the GOV.UK website.

    Furthermore, households installing rooftop solar panels can save on installation costs and energy expenses. A typical household can expect to save over £1,000 in installation costs and around £300 per year on energy bills, according to the UK Government. The residential solar panel market also enjoyed a temporary VAT-free period, set to end in March 2027, when the VAT rate returns to 5%.

    To summarise, homeowners in the UK considering the installation of solar thermal systems with combi boilers can benefit from various government incentives and funding schemes, such as the Renewable Heat Incentive and the Help to Heat programmes. These initiatives aim to encourage the adoption of renewable energy technologies and contribute to a cleaner, greener future.

    Installation Steps and Requirements

    When considering solar thermal installation with a combi boiler in the UK, there are several steps and requirements to be aware of. The first stage of solar thermal installation is to obtain multiple quotes from different MCS registered installers. Each one will assess your home and advise on the type and size of system that would be most suitable. This includes measuring the roof's orientation, inclination, and assessing any shading caused by trees or other properties.

    Combi boilers can be compatible with solar thermal systems depending on the model. While some types of combi boilers accept pre-heated water, others do not. It is essential to consult with a professional to determine whether your combi boiler will work with a solar thermal system.

    Once a suitable solar thermal system has been chosen and your combi boiler's compatibility has been confirmed, the following steps need to be followed:

    1. Acquire necessary permissions and approvals, such as building regulations and planning permissions, as required by local authorities.
    2. Prepare the roof area where the solar thermal panels will be installed, making sure it is strong enough to support the system's weight.
    3. Install the solar thermal collectors on the roof, ensuring they are securely fastened to the roof structure.
    4. Connect the collectors to the interior pipework and install a heat exchanger that links to the combi boiler.
    5. Install and set up the necessary controls, such as thermostats and pump stations, to ensure the system functions efficiently.
    6. Fill the system with a heat transfer fluid, typically a glycol-water mixture, and ensure it is adequately pressurised and free from leaks.
    7. Test the system to ensure proper operation and make any necessary adjustments.
    8. Provide the homeowner with instructions and guidance on how to operate and maintain the solar thermal system.

    The cost of installing a solar thermal system can vary depending on factors such as the complexity of the pipework and roofing materials. A two or three-panel solar thermal system that can supply an average 200 to 300-litre cylinder may cost between £4,000 to £7,000.

    Maintenance and Troubleshooting

    Regular maintenance is essential for maintaining the efficiency and longevity of a solar thermal system combined with a combi boiler. Inspecting and maintaining the components, such as the solar panels, hot water cylinder, and blending valve, can help prevent potential issues and keep the system running smoothly.

    Conducting periodic inspections of the solar panels is crucial. Ensure they are clean and free of debris, as dirt and dust can reduce their efficiency. Also, check for any signs of damage, such as cracks or broken seals, which may require professional attention. Remember to adhere to safety precautions when accessing the roof.

    Hot water cylinders should also be inspected regularly. Look for any leaks or signs of corrosion and assess the condition of the insulation. In addition, inspect the blending valve to ensure it is functioning correctly and directing the pre-heated water to the combi boiler without any issues.

    When troubleshooting problems, consider the following common issues:

    • Low hot water temperature: This could be due to an issue with the blending valve or a malfunctioning solar panel. Check these components for any signs of damage or incorrect settings. If necessary, consult a professional for further diagnosis and repairs.
    • Reduced system efficiency: Inspect the solar panels for dirt or debris, as this can have a significant impact on their performance. Cleaning the panels using a non-abrasive soap solution and a soft brush should improve their efficiency.
    • No hot water: If the solar thermal system is not producing any hot water, there could be an issue with the combi boiler itself. Check the boiler for error codes and refer to the manufacturer's manual for guidance on resolving these issues. If needed, contact a qualified technician to inspect and repair your boiler.

    Performing regular maintenance and addressing any issues promptly will help ensure that your solar thermal system with a combi boiler operates efficiently and reliably.


    Solar thermal systems, when combined with combi boilers, can prove to be an effective and sustainable way to provide hot water and heating for homes in the UK. These systems utilise the sun's heat to warm water, reducing the need for fossil fuels and potentially saving homeowners money on energy bills. However, it is important to keep in mind that solar thermal alone might not be able to meet the total hot water demand, especially during winter months when sunlight is limited.

    In the UK, investing in solar thermal systems can provide long-term financial benefits, as the investment tends to pay for itself within a few years. Moreover, with the current focus on reducing carbon emissions and promoting renewable energy sources, adopting solar thermal technology contributes to a greener and cleaner environment.

    Using solar thermal panels along with combi boilers needs careful consideration and planning. Compatibility between the two systems is crucial for optimal performance. Homeowners need to evaluate their hot water usage patterns, geographical location, and roof orientation to maximise the benefits of solar thermal systems.

    In summary, switching to solar thermal heating with a combi boiler is a commendable option for UK households seeking energy-efficient solutions. It is essential to conduct thorough research, consult professionals, and assess individual requirements to ensure the seamless integration of both systems and maximise the potential benefits.

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    Mark McShane
    Content Writer
    Mark McShane is a seasoned professional in the solar industry and the owner of Skills Training Group, a leading provider of solar training and education. With years of experience in the field, Mark has developed a deep understanding of the latest trends, technologies and best practices in the industry. His expertise and passion for solar energy have helped countless professionals develop the skills and knowledge needed to succeed in the fast-growing field of solar energy. Whether you’re an aspiring solar professional or an experienced industry veteran, Mark is an excellent resource for anyone looking to enhance their skills and knowledge of the solar industry.
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