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 4.8 stars

1,566 reviews

Solar Thermal Heating with an Immersion Heater

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

Solar thermal or solar water heating systems use the sun's heat to produce hot water for your radiators, taps and showers.

Installing a solar thermal system is an effective way to lower both your heating bills and carbon emissions as you'll need use your usual heating system less, reducing how much gas, oil or electricity you buy from a supplier.

The average homeowner gets between 40-70% of their hot water with the free energy generated from solar thermal. The rest needs to be supplied by a boiler or the immersion heater in your hot water cylinder.

How does solar thermal work?

Solar thermal panels include tubes of fluid; this fluid heats up when exposed to sunlight during the day and then travels down to your hot water cylinder. The heat is passed to the water in your cylinder via a heat exchanger coil.

A hot water cylinder is crucial to the solar thermal set up as most households use the majority of their hot water in the mornings and evenings when the solar thermal panels aren't generating heat. A hot water storage cylinder will store the hot water generated during the day for when it's needed. The cylinder needs to be solar compatible (with a solar heating coil) and ideally large enough to hold around 2 days worth of water.

When there isn't enough sunlight to heat enough hot water or get it to the right temperature*, you will need to top it up. This could the case on a particularly cloudy day or during winter when daylight hours are shorter. This is when your immersion heater comes in.

*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 with an immersion heater

Hot water storage cylinders usually include an immersion heater.

This is a metal element immersed in water inside the tank. When powered by electricity it heats the water around it.

This immersion heater is isolated with a switch so you can turn it on and off as needed. Or, you can fit a timer so it will heat water to suit your routine or when your energy tariff is cheapest e.g. at night.

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More 'free' hot water with solar PV

Immersion heaters are powered by electricity which is expensive.

However if you have or plan to have a solar PV system installed in addition to a solar thermal system, you can reduce your water heating costs even further.

A solar PV (photovoltaic) system converts the sun's energy into usable electricity for your appliances. Like solar thermal panels, solar PV panels are fitted to the roof where they can
generate energy in daylight hours. However, as many people aren't home during the day to use this free energy, solar PV panels often produce more energy than the average household can use during the day.

Usually, this surplus solar energy is automatically sent to the National Grid. However, there are 2 ways in which you can keep hold of more of your solar electricity and potentially use it to power your immersion heater.

Solar power diverters

This gadget monitors the amount of electricity you're generating and compares it to how much you're using.

When it detects a surplus (spare energy which would usually be sent to the grid), it automatically diverts it to your immersion heater. This means your water is being heated by free solar electricity and should mean you need to buy even less from a supplier.

Solar storage batteries

A solar battery stores any extra energy generated by your solar PV panels during the day so you can use it at night. Or even when there’s a power cut depending on the model.

Both of these options could enable you to reduce your water heating costs even further.

What about solar PVT?

A solar PVT (photovoltaic thermal) system is a hybrid of solar PV and solar thermal.

These panels are fitted to your roof and generate electricity for appliances and heat for your hot water.

Simply put, PVT panels are like solar PV but they have an additional unit which removes the heat from the PV panel. This both cools the panel down to keep it working at optimum temperature (less than 25°C) and extracts heat for your hot water cylinder.

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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.

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 a Boiler

How to reduce your heating bills with solar thermal AND keep your boiler.

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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