How do solar water heating systems work?

Solar thermal systems work all year round, though you'll probably need to heat the water further with a boiler or immersion heater during the winter months.

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Technical information made simple

A solar thermal system usually consists of a set of solar collectors on the roof connected to pipework containing a carrier fluid which transfers the heat from the collectors to your domestic hot water.

  1. The solar collectors (the panels) on the roof collect heat from the sun.
  2. The heat is absorbed by the fluid (typically water mixed with antifreeze) and then transferred away from the collector using a pump.
  3. This heated fluid is pumped through a coil in the hot water cylinder, heating the hot water as it passes through.
  4. This hot water is then available for use for heating or washing. If it is not hot enough an immersion heater or boiler can be used to top up the heat before use.

The two main types of solar thermal collectors

  • Flat plate 
  • Evacuated tube

Evacuated tubes are more efficient than flat-plate collectors, so are often smaller but generate equal amounts of hot water. Some panels have a drain-back mechanism which drains water from inside the solar panel when the pump is switched off. This prevents the water freezing or boiling inside the solar panel.

Flat plate collectors
These collectors are similar in appearance to solar PV panels. They are plates with a base layer of thin pipes carrying the carrier fluid, covered by light absorbent material which is then covered with a transparent layer to allow the sun’s energy to reach the absorbent layer.

Evacuated tubes
Evacuated tube collectors consist of a set of double layered tubes side by side and connected at one end by a ‘manifold box’. These tubes have a vacuum between their layers, with the inner layer coated in an absorbent coating to collect the sun’s energy. This heat is concentrated onto a central collection point.

There are two different types of evacuated tube:

Heat pipe
These contain a copper heat pipe filled with a volatile fluid that when heated by the sun, vaporises and rises up to reach the manifold box. The heat is then transferred through the box to the carrier fluid.

Direct flow
As the name suggests, these collectors have tubes that contain the carrier fluid, meaning that the heat is directly transferred from the tube’s absorbent layer to the fluid.

The Solar Cylinder
As part of installing a solar thermal system, you will need to replace your current hot water cylinder with one designed for solar systems, which is 30-50% larger. If you have a large hot water cylinder you may instead be able to retrofit the coil of carrier fluid piping into the cylinder. If you have a combi boiler, and therefore no hot water cylinder, it is likely that a solar thermal system is unsuitable for you.



In order to qualify for the Renewable Heat Incentive Premium Payment the panels must be fitted by a MCS accredited installer. Find out more about the Installation of Solar Thermal Panels here.


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Do I need to replace my hot water cylinder to get a solar thermal panels installed?
If your hot water cylinder is not large enough for a second heating coil it will need to be replaced with a larger 'solar cylinder' specially designed for use with solar thermal panels. The cylinder needs to be larger because it has to accommodate your boiler/immersion heater's heating coil as well as the heating coil from the solar thermal system.
Does my roof need to face south to be suitable for solar thermal?
A south facing roofs at a 30° angle offers the best annual output for a solar thermal system. However, east or west facing roofs can function very well as the available solar radiation only reduces by about 20% in these orientations. The size of the panels can be increased to compensate for this reduction.
How much space do I need on my roof for a solar thermal installation?
Typical installations have panels occupying 4.6 - 6.9m2 of roof area.
Are solar thermal collectors or panels fragile?
Not typically - they are designed to withstand the elements so they should be quite robust. All MCS accredited products should last their predicted 20 year lifetime.
How hot does solar heated water get?
Very hot! The water can easily exceed 100°C, therefore it is important that the components are able to withstand these temperatures. All MCS certified products will be able to do so.
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