Solar Thermal Heating with a Boiler

Solar Thermal Heating with a Boiler


A solar thermal heating system is also known as solar water heating and uses the heat of the sun to heat water for your home. This hot water can be used to supply your wet central heating system (e.g. radiators, underfloor heating), taps and showers so you can use your boiler less often and buy less energy from your supplier.

Although solar thermal can significantly reduce your heating bills and carbon emissions, it’s unlikely to be able to meet your total hot water demand. On average solar thermal owners find that they can generate between 40 – 70% of their hot water with free solar energy. The rest will need to be topped up by your boiler.


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How Does Solar Thermal Work?

Solar thermal panels are similar to solar PV panels in appearance, but they turn the sun’s energy into heat rather than electricity. They are usually fitted to a roof where they will absorb the most sunlight. Each panel includes tubes of fluid which, when exposed to sunlight, absorb the sun’s heat. This fluid travels down into your home where the heat passes through an exchanger coil to heat water in your cylinder.

How Does Solar Thermal Work with a Boiler?

This process obviously happens during the daytime and is most effective on particularly sunny days although they will still work on cloudy days. However, on cloudy days, at night and during the winter when daylight hours are shorter your solar thermal panels may not be able to generate enough energy to heat your water to a high enough temperature*. That’s when you need to use the boiler (or immersion heater) to give the water the temperature boost it needs.

*Hot water should be stored at a minimum temperature of 60°C in order to kill legionella bacteria. A thermostat on your cylinder will enable you to monitor this.

Solar Thermal Hot Water Cylinders

A hot water cylinder is a must with solar thermal. While solar thermal panels will heat water during the day, most households use their hot water in the mornings and evenings. This is why you need a hot water cylinder to store the hot water produced during the day so it can be used at another time. The hot water cylinder will need to be solar compatible (with a solar heating coil) and ideally large enough to hold around 2 days worth of water.


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Will Solar Thermal Work with Your Boiler?

A solar thermal heating system is best suited to homes with System or Regular (conventional) boilers as these already use a hot water cylinder. However, unless your current cylinder is solar compatible and large enough, you may need to invest in a new one as part of the solar thermal installation.

Note: When incorporating solar thermal heating with a Regular boiler you will have a large feed and expansion tank in the loft which supplies your boiler with cold water. As these systems rely on gravity for the strength of the flow, you may need to install an extra valve or pump so the solar thermal system can work effectively when the heating is running at the same time.

Solar Thermal Heating with a Combi Boiler

If you have a Combi boiler (an all-one-unit which supplies hot water on demand directly from the mains) you may have difficulty installing a solar thermal system. Combi (Combination) boilers are the most popular choice of boiler in UK.

The issue is that most Combi boilers aren’t able to accept pre-heated water from a solar thermal system; they are designed to be supplied with cold water directly from the mains. In addition, as a Combi boiler supplies hot water on demand it doesn’t require a hot water cylinder meaning there is nowhere to store your solar generated water. It is possible to find some Combi boilers that are compatible with a solar thermal system but you need to check with the manufacturer and a professional heating engineer to be sure.


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What is a Solar Thermal Heating System?

Solar thermal means using free energy from the sun to heat your home’s water.

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Solar Thermal Heating: Pros, Cons and Costs

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Solar Thermal with an Immersion Heater

How to use solar thermal with your immersion heater to save energy.

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