What is solar hot water?
Also known as solar thermal, a domestic solar hot water system is one which absorbs the sun’s energy and transfers it to a storage cylinder. It is different from photovoltaics; solar hot water panels do not produce electricity, they heat water directly.
Two solar hot water panels on a domestic roof; they are thicker than solar electric panels, but only protrude about 10cm from the roof.
In the UK it will not be the sole provider of hot water; it will complement a conventional system using gas, oil, electricity or solid fuel, but it will pre-heat water so that bills are drastically reduced. During summer months the system can provide all the hot water needed by a household.
Panels can either transfer heat to a separate pre-heat cylinder, or heat a twin-coil cylinder via the bottom coil.
In a direct system, the water that passes through the panels is the water that eventually comes out of the hot tap. In this type of system, there are issues around the water in the panels freezing in winter (so they need to be drained) and lime-scale build-up; in an indirect system, the water in the panels passes through a heat exchanger (coil) in the cylinder and then back to the panels in a continuous loop. Anti-freeze can be added, and there is no problem with lime-scale build-up.
A typical indirect solar hot water system: the gas boiler will kick in if the solar coil in the cylinder doesn’t raise the temperature of the water enough.
The two main types of collectors are flat-plate and evacuated tube. Flat-plate collectors heat the water directly, evacuated tubes contain a fluid which evaporates at low temperatures, and the resulting gas rises and condenses on a manifold, transferring its heat as it does so.
What are the benefits of solar hot water?
Solar hot water, along with photovoltaics, wind power, hydro, wave and tidal power and geothermal energy are renewable energy sources which don’t involve the burning of fossil fuels, and its associated problems.
Burning fossil fuels releases nitric oxides, nitrogen dioxide and sulphur dioxide into the atmosphere. This causes acid rain which damages forests, wildlife and human health; it also releases carbon monoxide, nitrous oxides, lead, particulates and hydrocarbons, which pollute the atmosphere, and cause damage to plants and ecosystems, and human health, especially respiratory problems.
An entire solar hot water kit, comprising panels, twin-coil cylinder, pump & control set, expansion vessel, air release set, filling bottle, anti-freeze, high-temperature pipe insulation and manual.
The burning of fossil fuels adds an extra 5 billion tonnes of carbon dioxide into the atmosphere each year. CO2 is an important greenhouse gas. In pre-industrial times there were 290ppm (parts per million) of CO2 in the atmosphere; it’s now over 380ppm, and rising by 2ppm every year.
Most scientists agree that the increase in the amount of greenhouse gases in the atmosphere is raising the earth’s temperature, and that it could rise between 1-4°C in the next 100 years (there is only a 5°C difference between now and the last ice age); this would mean lower overall global rainfall, global biomass reduction and extinction of many species, and for humans it would mean desertification, famine, forest fires, increase in tropical diseases, and flooding due to the melting of polar ice.
What can I do?
Solar hot water is probably the most cost-effective renewable energy technology that you can install in a domestic situation in this country, with the shortest payback time. A DTI investigation into solar hot water systems in the UK from 1970-2000 found that a typical system will provide 72% of a household’s hot water over the course of a year (c. 15% in winter and 100% in summer). This is assuming that the roof is south-facing – although if it faces south-east or south-west there will only be a 5% loss of efficiency.
The first thing to do is to choose either evacuated tubes or flat-plate collectors. Installed prices for both are typically in the range £2500 to £5000. If you choose flat-plate, make sure that the collectors have a selective surface – a special coating that maximizes the absorption of solar radiation and minimizes re-emission; in the UK’s climate, selective surface flat-plate collectors are only slightly less efficient than evacuated tubes. However, there is more to go wrong with evacuated tubes, and there have been quality issues. Try asking the salesman for a copy of the product’s EN12975 test report, and remember that cheap doesn’t always mean good value.
Fitting solar hot water panels to a roof; a scaffold tower is essential.
You can buy a system installed, self-build and install, or self-build and then have your system installed by a professional. There are often government incentives to install renewables. For solar thermal, the latest is the Renewable Heat Incentive.
There are (cheaper) special systems for swimming pools, consisting of a large area of black tubing.
NB: if you’re thinking of getting solar hot water at some point in the future, then don’t install a combination (combi) boiler – it’s not impossible, but it’s very difficult and expensive to combine solar hot water with a combi. Condensing boilers are fine.
See here for more detailed info on getting and using a solar hot water system.
The specialist(s) below will respond to queries on this topic. Please comment in the box at the bottom of the page.
David Thorpe of One Planet Life is the author of The Solar Energy Pocket Reference Book, The Earthscan Expert Guide to Solar Technology, The One Planet Life, and is a former manager of the publications department at CAT. He runs consultancy and workshops on aspects of solar thermal system design, commissioning and cost.
We'd love to hear your comments, tips and advice on this topic, and if you post a query, we'll try to get a specialist in our network to answer it for you.
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Paul evenden said on December 20, 2016
A key thing with solar hot water is the setting of the timer, if a boiler is allowed to be switched on by its thermostat in the water tank (for example after water has been drawn for showers first thing in the morning) the water in in tank will already be hot when the sun hits the panels and the solar pump won’t start.Our boiler has an additional timer for hot water only ,that only allows it to heat water for an hour in the evening, thus giving the solar maximum chance to work.Using a combined heating and water programmer is less fool proof as people tend to fiddle with them and allow the boiler to heat the water on demand thereby stopping the solar side of the system from working to maximum efficiency. If extra hot water is needed (for example you have guests staying and need extra hot water its a simple matter to press the override button on the additional timer to allow the boiler to take over from the solar.
Dave Darby said on December 20, 2016
I think that’s a really important point – maybe the most important point when it comes to solar hot water. You only want the boiler to come on in the evenings, so that the sun can have a good go at heating your water, without having to compete with the boiler. Otherwise you’re wasting all that solar energy and paying for gas that you don’t need.
villa_bruce said on November 30, 2017
I’m looking to heat and supply hot water to some out buildings. I have used solar tubes before so will make use of these but am looking for a system that also incorporates the heating of the buildings, any ideas ?
David Thorpe said on November 30, 2017
That’s a big topic. Correct sizing will be important, particularly of the heat store. Underfloor heating might be an option as the water doesn’t need to be so hot. The buildings should be well insulated to minimise demand. You’ll ant to consider multiple inputs to the heat store beside solar, e.g. biomass. Is it on a farm? If so, consider a mini AD system using those balloons to collect the gas and feeding it direct to an adapted boiler.
Henery KG Barr said on September 14, 2018
Hi How do I link a hot water cylinder to a combi system ?
David Thorpe said on September 16, 2018
Combi boilers yield heat on demand. They take a cold feed and apply heat, usually gas-fired. Combi boilers are designed to produce heat at a fixed temperature dependent on the water flow and demand. Far as I’ve ever seen, it is not controlled by a thermostat, so if the cold feed were to be replaced by pre-heated water from a heat store this could seriously upset the calibration and produce water that’s far too hot. So it’s very difficult; this is why solar water heating systems don’t usually include a combi boiler. It’s not impossible, but it does depend on your make of boiler and knowing a suitably qualified heating engineer prepared to customise the boiler – which could invalidate any guarantee. This page tells you how: http://www.yougen.co.uk/blog-entry/2522/How+to+benefit+from+solar+hot+water+with+a+combi+boiler/
Steve said on May 13, 2019
Hello, My Lili Solar hot water system has given trouble free service for many years 🙂 However today there is an error message “Delta T > 50K” . I think this means that the panels are more than 50 degrees hotter than the tank, which is about right because the whole tank is 66C and the panels are 117C. I’m worried that the panels might be damaged by getting too hot, but the pump won’t run to cool them because the tank is at the max temp. I don’t want the tank to get any hotter due to danger of scalding when using the water. We didn’t have a problem last summer but I think we were using more hot water. Any thoughts or suggestions would be greatly appreciated. Thanks
David Thorpe said on May 14, 2019
Check the thermostat. Is it functioning correctly? Is the pump ok? If yes to both, either use more hot water or install a larger, or second, tank. You’re lucky to have so much free heat! Presumably you claim RHI?
Steve said on May 14, 2019
Pump and thermostat working fine. Second tank is an interesting idea, or maybe a large jacuzzi….
I’m not claiming RHI, From Ofgem “If you’ve already installed a renewable heating system, you have up to 12 months to apply to the scheme from its commissioning date. That’s the date your installer tests and signs off the system….” As mine is a Lili self build system installed in 2002, I’ve missed the date and I don’t have a certified installer. Unless there is a work around ?
David Thorpe said on May 15, 2019
No workaround! Jacuzzi… (uses energy) … swimming pool 🙂 … radiators in greenhouse and grow pineapples!
Gavin Fredericks said on June 7, 2019
I have a evacuated tube collector which I plan to connect to a 40lrt hot water tank on my boat that also runs off electricity and the engine when running. Ive been told the collector is overkill for a 40 lrt tank so I’m planning to install 2 hot water radiators in the boat and when the tank is up to temp auto switch to the radiators. Is this do able ?
David Thorpe said on June 9, 2019
So the engine runs a dynamo that runs an immersion heater? The sensor-thermostats on the tank should prioritise the tube connection. If the tank is hot the dynamo is not needed. If not hot enough the dynamo feeds the tank. If the tank is hot because of the tubes alone and there is still surplus heat, it can be directed to the radiator. But why would you need it if it so hot outside? You need to be careful the system does not overheat. As the boat constantly changes direction, the fins can’t point at the sun. Better to use wind power I’d have thought, as this can be turned off when not needed. Tubes can’t be turned off (unless you cover them).