Solar Thermal Hot Water

Welcome to a series of four short video clips designed to show what you can do in your home to produce electricity and/or hot water for your use.

The information and views provided are based on actual use by Chew Magna residents who have installed these technologies and have been using them for years.  This means you are getting actual operational experience and not just marketing views from the equipment supplier!

Some of the issues raised will be specific to the location and not associated directly with the technology and these will be highlighted.

  • In addition to using PV panels or tiles to generate electricity, another way to use the suns energy is to heat water directly, to provide your domestic hot water.  STHW systems use energy from the sun to warm water for storage in a hot water cylinder or thermal store.  As the amount of available solar energy varies throughout the year, a STHW system won’t provide 100% of the hot water required throughout the year and a conventional boiler or electric immersion heater is typically used to make up the difference.
  • Larger STHW arrays can also be arranged to provide some contribution to heating your home but the amount of heat provided is generally small (less than 10% of the home’s heating requirement), so it is not usually considered worthwhile.  Most STHW water systems are just designed to provide the hot water you use for bathing, showering and hot taps. 
  • The process is very simple:  solar thermal collector panels contain small tubes of fluid heated by the sun and are (usually) fitted on your roof.  There are two types of solar water heating collectors:
  • Evacuated tubes:  a bank of glass tubes mounted on the roof.  The evacuated tube comprises a smaller glass tube suspended within a larger glass tube and the air is pumped out of the space between the two tubes, creating a vacuum thermal insulation layer that reduces heat loss from the solar collector.  A sealed copper heat pipe transfers the solar heat via convection of its internal heat transfer fluid to a “hot bulb” that indirectly heats a copper manifold within the header tank.  The diagram shows the basic principle of how the solar energy unit works, followed by a typical roof top installation.  
  • Flat plate collectors:  can be fixed on the roof tiles, or integrated into the roof.  Sunlight passes through the glazing and strikes the absorber plate, which heats up, changing solar energy into heat energy.  The heat is transferred to liquid passing through pipes attached to the absorber plate.
  • The differences between a flat plate collector and evacuated tube system are that flat plate collectors are heavier, take up more room and can be cumbersome to install on some roofs.  Evacuated tubes tend to have lighter components and are easier to manage on the roof but tend to be more fragile than flat plates.  Evacuated tubes also work in winter, even mid-winter, producing hot water and saving the owner money.  Even with an ambient temperature of -15°C, a STHW system can continue to produce 80°C hot water.
  • Energy is transferred from the sun to the water-glycol fluid used to heat water stored in a hot water cylinder.  Inside the hot water cylinder, a base coil is connected to the solar collectors.  Typically, one cylinder is used, with either an immersion heater or another coil connected to your boiler, near the top of the cylinder.  The top immersion heater or coil will heat the water to a higher temperature when needed.  If a dedicated solar hot water cylinder is not already installed, you will usually need to replace the existing cylinder.
  • Sizing the cylinder is important, as you’ll usually need one almost double the size of a standard cylinder. This is because a taller cylinder allows you to store more solar-heated water and meet more of your needs.  A large cylinder gives more buffer to get you through cloudy spells, so less backup heating is needed.
  • As with PV panels, STHW collectors are best placed on south facing roofs, or between east to west but not north facing.  Panels don’t have to be roof mounted but can be fixed to a frame on a flat roof, hung from a wall, or mounted on the ground.
  • As a general rule, about one square metre per person is needed to provide enough hot water in summer.  For flat plate panels this means one panel for a small household, or two panels for a large one.  For evacuated tube collectors, perhaps 20 or 30 tubes are needed, depending on the household and hot water use.  About 5m2 of collectors receiving direct sunlight for the main part of the day is sufficient for the average house in this area.  
  • STHW hot water systems are normally designed to provide about half your annual hot water requirement.  The typical output of a domestic STHW installation in the UK is between 1,000kWh and 2,500kWh but depends on individual factors, so the savings will vary throughout the year, with systems providing nearly all your hot water in the summer but less during the winter months.  The cost of installing a typical 4m2 solar water heating system is around £3,000 to £5,000 but costs will depend on whether you choose evacuated tube or flat plate collectors, as well as the size of the system. 
  • The heated water can reach up to 900C and is passed down into your home, where the heat is transferred to water in a storage tank or cylinder, via a coil.  Your hot water system then transports the hot water for use in your home.  The figure shows how a SLTH system fits withing your hot water system.
  • Most home STHW heating systems are considered ‘permitted developments’, meaning they don’t require planning permission.  However, exceptions apply and you should check with your local planning office.  If your home is a listed building, or in a conservation area or national park, you may have more restrictions.
  • When installing a STHW system, find a qualified professional installer to ensure the system is properly installed and the control system set up.  Also get three quotes from different installers to compare.  The Microgeneration Certification Scheme (  lists accredited installers and products.  The previous government incentive scheme has ended but try the Government’s simple energy advice website (
  • Recent increases in fuel prices will improve the savings from using solar water heating, but without incentives to boost the market the financial payback could be many years.  A family-size STHW system may contribute 1,500kWh of heat per year and with gas prices rising to about 8p per unit (kilowatt-hour, kWh), equates to a saving of about £120 per year and with gas prices expected to rise again in Autumn 2022, the picture will change again.
  • If your STHW unit replaces direct electric heating, with electricity prices approaching 30p per kWh the saving could be over £400 per year, giving a potential payback of less than 15 years.  However, much will depend on future increases and decreases in electricity prices, so it’s hard to give a good estimate of payback at the moment.
  • Further information on STHW technology and costs are available at websites such as, and
  • In summary the Pros and Cons of a STHW system are:
  • Pros:
  • hot water throughout the year;
  • reduced energy bills;
  • low maintenance;
  • lower carbon footprint
  • Cons: 
  • upfront installation costs;
  • only provides hot water, not electricity;
  • if you have a combi boiler installed in your home to take advantage of the hot water you produce you will need to install a hot water storage tank;
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