Solar thermal water heating systems use infrared light to heat water, alongside your conventional domestic hot water heater. The technology is well developed with a large choice of equipment to suit many applications.
How solar thermal works
Solar water heating systems use solar panels called ‘collectors’. They are normally fitted to your roof but can be ground or wall-mounted. These convert infrared light from the sun to heat water which is stored in a hot water cylinder. A boiler or immersion heater, or both, can be used as back up to heat the water further to reach the temperature you want when there is not enough sun to heat it fully. (This is also important to kill bacteria that can thrive in luke warm water.)
There are two main types of solar water heating panels:
Solar thermal systems can provide almost all of your hot water during the summer months and about 50% year round.
Is my property suitable?
You will need around 3-5m2 of southeast to southwest facing roof, un-shaded for the main part of the day. You'll also need space nearby, say within 10m, to locate a water cylinder, probably larger than any existing one, in turn within a reasonable distance from the bathroom (to avoid large heat losses in the pipes).
Panels don't have to be mounted on a roof; they can be fixed to a frame on a flat roof or on the ground, or fixed to a wall. Ground-mounted panels are more vulnerable to damage.
The way hot water is used in a building, and how the main heating system is controlled, has a great effect on how cost-effective a solar thermal system will be. For example, if a boiler is set to heat the tank in the morning, when the main demand for hot water is not until the evening, then the solar panels will not operate however sunny it is, as the water in the tank will already be hot. To maximise savings the back-up heating, when required, should be timed to do its work and go off again just before the normal times of highest demand so that there is usually cool water in the tank before the middle of the day. Evening bathers should save more than morning bathers as heat is lost overnight.
Solar thermal systems are generally sized on the basis of the number of bedrooms in the house and therefore the possible demand for hot water. If, however, you know that during the 20 year life of the system there will mostly only be, say, a couple in the house, then it will be more cost-effective to apply the rule of 1m2 of panel per person and ‘one for the house’ and try to source a competitively-priced system of this size. Collectors that are too large will spend much of the summer doing nothing but ‘cooking’ on the roof as the heated water is not used up so stays hot and does not circulate.
Cost and maintenance
Total costs for installing a typical solar water heating system range from £3,000 to £5,000.
Solar hot water systems generally come with a 5 or 10-year warranty and require very little maintenance. You should check the pressure gauge occasionally and have the system checked by a professional installer every 3-5 years, but consult your system supplier for exact maintenance requirements.
You can fit or build the system yourself. It may be cheaper but you'll need a certain level of skill.
Solar thermal systems are supported under the Government’s Renewable Heat Incentive, which comes to an end in March 2021, which would eventually pay off your capital cost. Note, however, that DIY systems are not eligible for Renewable Heat Incentive.
For information about the domestic RHI scheme, see www.ofgem.gov.uk/environmentalprogrammes