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Solar Thermal Heating with a Boiler

Nick Geary
By: Nick Geary
Updated: 6th July 2022

A solar thermal heating system uses the sun’s heat to warm water for your home.

This hot water can be used to supply your wet central heating system, taps and showers. So you won’t be using your boiler so much which will save you money.

However, solar thermal alone is unlikely to be able to meet your total hot water demand. On average, solar thermal owners find they get between 40-70% of their hot water with free solar energy. The rest needs to be topped up by the boiler.

How does solar thermal work?

Solar thermal panels look very similar to solar PV panels.

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. And 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 a cylinder.

Solar thermal with a boiler

Solar thermal panels only work during the day. And are most effective on 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.

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Will solar thermal work with your boiler?

A solar thermal heating system is best suited to homes with a system or regular (conventional) boiler as these 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 fitting solar thermal with a regular boiler you will have a large feed and expansion tank in the loft. This 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. That way, the solar thermal system can work effectively when the heating is running at the same time.

Solar thermal heating with a combi boiler

Most homes in the UK are heated by 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.

The issue is that most combi boilers aren't able to accept pre-heated water from a solar thermal system. Instead, they’re 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. This means that there’s 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.

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.

Interested in Solar Thermal?



What is a Solar Thermal Heating System?

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

Find out more

Solar Thermal Heating: Pros, Cons and Costs

Find out if a solar thermal system is the right choice for your home.

Find out more

Solar Thermal with an Immersion Heater

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

Find out more


Nick Geary

About the author

Nick Geary

Nick has a wide range of experience writing about conventional and renewable heating solutions, and is always increasing his knowledge by researching any new heating technologies on the horizon.

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