Solar thermal heating

Although solar thermal is most often used to make hot water, solar thermal technology can also be used to heat a home and the most common way to implement this is to use a heat store.

In the UK, solar thermal panels will rarely provide enough hot water to meet a typical home’s heating demands on their own in the middle of winter, so they need to be used in conjunction with an alternative heat source. That said, if you have enough roof space for a large system you may be able to reduce your heating costs considerably because solar energy is free and can reduce the amount you need to use your boiler considerably.

If you’re looking to reduce your carbon footprint and have some roof space which faces roughly south, installing a solar thermal system makes a lot of sense and will save you money. In all honesty it doesn’t make a lot of sense to use solar thermal for space heating, especially in the UK, because the energy available in the winter will never be sufficient to heat a home. Instead it makes more sense to use solar thermal to heat hot water for washing and bathing – and a solar thermal system will produce between 40-70% of the hot water needed by an average family of 4 annually. In the summer, a good system should provide around 90% of your hot water requirements, dropping to around 25% in the winter. One of our testers lived in a house with an old flat plate solar hot water system and did not use the boiler between the beginning of March and the end of October.

How does solar thermal heating work?

Solar thermal panels, or tubes, are fitted to a home’s roof and absorb energy from the sun, so they work best on sunny days. The heated fluid is then transferred to a hot water tank or heat store. The heated water remains in the tank until it’s required meaning the boiler does less (or no) work to bring it up to temperature.

A heat store is required for space heating from solar thermal energy and this differs from a conventional hot water cylinder because the water in the store is not the water that will come out of the tap. Instead the store is filled with a mixture of water and additives to prevent corrosion. The heat store has to be kept at a higher temperature than a hot water cylinder would be (typically 80°C as opposed to 60°C), so they have to be better insulated if they are not to suffer from higher heat losses. The heated fluid in the heat store is circulated around the home’s heating system (either underfloor heating or radiators) when heat is required.

Solar thermal heating efficiency

Solar panels are very efficient at turning solar energy into hot water – and solar thermal tubes are even more efficient. Solar thermal systems can absorb as much as 70% of the solar energy which strikes the panels of tubes but, keep in mind that solar energy is completely free, so energy conversion factors are less important than for other types of heating systems. Put another way, given enough roof or land space, you can capture as much solar energy as you want for free. So the main factors to consider when assessing efficiency are installation costs and roof space.

In general, evacuated tubes perform better in colder, cloudier conditions than flat panel systems, but they are also more expensive. The vacuum in the glass tubes helps tubes catch and convert a high percentage of the sun’s energy into heat. Evacuated tube systems can even work in freezing conditions, especially when mounted at angles above 45° to avoid a build up of snow.

Advantages

The main advantage of solar heating systems is the lack of running costs. You should plan to get a solar system serviced every year or two, but apart from that there are not really any running costs. Solar is also completely zero carbon. Any energy you capture comes straight from the sun without a whiff of fossil fuels.

Disadvantages

Solar thermal heating systems can’t meet 100% of a UK home’s heating, or hot water requirements, so a secondary heating system is always required to back up a solar thermal system. If you want to try and use solar for space heating you will need a large system and a heat store, which will take up more space than a regular hot water tank. For space heating systems there are fairly high installation costs but the costs for a regular solar system for hot water are not prohibitive. Lastly, solar heating does rely on solar energy, so you will get less hot water on dull, cloudy days.

Solar thermal heating costs

You should plan to spend about £75 getting a solar hot water system serviced every year or so, to swap out the antifreeze and make sure everything is running as intended, but apart from that there are not really any running costs.

Is solar thermal heating worth it?

Solar thermal systems are worth installing, even in the UK. To be honest, there is nothing quite as satisfying as taking a hot shower in water you know was heated from the sun without any carbon emission. The satisfaction alone makes solar thermal worth it in our opinion.

But, if you’re less concerned about feeling good or cost is the primary factor, installing a solar thermal system can still be worthwhile. Given that a solar hot water system can cost as little as £3000 installed and will save roughly £220/year a new system will pay for itself in 13.5 years.

However, that’s at today’s gas price of 10.3p/unit and we can probably expect gas prices to go up. At 15p/kWh for gas the payback on a £3000 solar thermal system would be 9 years.

Plus, don’t forget that installing a solar thermal system will add value to your home so, if you have the roof space, it still makes sense to install solar thermal even if you don’t think you will be living in the same house in 10 years.

Assumptions for the calculations below:

A solar thermal system should provide about 90% of a home’s hot water, and hot water accounts for roughly 20% of a typical home’s gas bill.
90% of 20% of 12,000 kWh of gas = 2160 kWh gas used for hot water per year
2160kWh x 10.3p/kWh = £222.48 saved per year
£3000 / £222.24 = 13.5 years payback

To find an accredited installer in your area, pick one that is part of the Microgeneration Certification Scheme from the list on the MCS website.

Check out the other alternatives to gas boilers in our energy guide.