Solar Thermal FAQs

1. Can you link a solar thermal system with a combination (combi) boiler?

Combi boilers, which provide instantaneous hot water and central heating without a storage tank, are now very popular in the UK and have been for some time in Europe. In short, the answer is yes, but it is not a typical approach.

It is possible to install a solar thermal system with an unvented cylinder or thermal store, suitable for providing pre-heated water to a combi boiler. Unfortunately the vast majority of existing combi boilers in the UK will not accept the pre-heated hot water from a solar system. If you are planning a new boiler then ATMOS or some Ferroli boilers can use solar pre-heated water. However, even with an old combi boiler there is a way around this problem by simply decommissioning the instantaneous hot water feature and installing a new dual-coil solar cylinder.

2. Can a solar thermal system provide space heating for the home?

For most buildings, the answer is no.

Whilst not as cost-effective as heating domestic hot water, space heating can be assisted by the use of a solar system, particularly in conjunction with a low temperature under floor heating system. However, a much larger area of panels is required and there is still a requirement for a back-up heating system. This works best on very well insulated new buildings and is still highly unusual.

3. Can a solar thermal system be used to heat a swimming pool?

Yes: Swimming pools are an ideal application for solar thermal systems (especially for outside pools) because they tend to be used more during sunny weather. The relatively low temperature required by a swimming pool means that the efficiency of the system is much higher.

4. Is it possible to run the pump from a photovoltaic (PV) panel?

Yes; the fitting of a small PV panel (638 x 278 mm) will provide enough electricity to operate an efficient control system and pump (3-watt DC pump and low power electronic controls). This ensures that the whole system is a totally renewable energy system.

5. My roof doesn't face south – what can I do?

Whilst a south-facing roof is ideal, it's not essential to have one. Anywehere between south east and south west is also quite acceptable. An east-facing roof collects solar energy mainly in the morning, whilst a west-facing roof collects solar energy in the afternoon. With solar thermal installed on east and west roofs you can either accept slightly lower efficiencies, or increase the collector area to compensate.

6. How much will I save?

A typical solar thermal system will save an average of 2,000 kWh of energy over a year. This could be from £60 - £100 per annum for those heating water with a gas boiler, or £200 - £240 for those heating water electrically. To put it another way, it can cover 20-25% of the heating bill in a postwar-constructed home – see question 11 for more information.

7. How long will it take to install a system?

Systems can usually be fitted within 1-2 days.

8. What affect does shading have on a system?

Whilst not ideal, as performance will be reduced, shading is not the major issue it can be for solar photovoltaics (PV – solar electricity generation)

9. What happens to the system in freezing weather?

The system will either have been installed with anti-freeze, or will operate a 'drain-back' process when not in use, meaning no liquid will be externally exposed during cold spells.

10. Is it possible to install your own system?

Yes: if you have a reasonable plumbing ability, the design and installation of a solar thermal system will not pose any great difficulties. Bear in mind that you will need to access your roof safely and scaffolding is recommended for this. Most suppliers will also offer over-the-phone assistance and some offer a 'semi-installation', whereby they install the collectors, leaving the homeowner to complete the plumbing.

DIY courses are also offered by Low Impact Living Initiative (LILI – and the Centre for Alternative Technology (CAT –

11. How do I work out financial 'paybacks'?

To work out if a solar water heating system is worth installing from a financial point of view, you need a rough idea of how much you spend on hot water throughout the year, and when you use it. The average UK home will spend around 20-25% of its heating fuel bill on hot water provision. This will be higher for homes built in the past few years (up to 40-50% for a recently constructed energy-efficient home).

You need to know the cost per kW of the fuel used to heat the water and the annual total – if unsure, consult your energy bills. You should also factor in a compound annual increase if you believe fuel bills will continue to rise. A 10% price increase per annum is often assumed. Taking this approach with the author's house (three bedrooms, mid-1980s, good insulation, gas water heating at 3p per kWh at the time of writing), the payback on an outlay of £3,500 would be 14 years, without a grant.

Oil, LPG and electric heating are more expensive than gas, so a solar water heating system will recover its costs more quickly if you heat your water with these fuels.

This approach does not allow for the increased longevity for boiler-based systems with solar thermal installed, as the boiler will be off over the summer months, saving on the wear and tear that can occur from operating at half outputs. This translates to an additional financial benefit.

12. Can I add the cost of installation to my mortgage?

Our understanding is that most mortgage companies will willingly increase their offer for the installation of solar technologies. This is a simple way of affording the upfront costs of a system and paying it back over time. It is an approach often taken for kitchen/bathroom upgrades and conservatories, and can be replicated for renewable energy installations.

13. Energy Performance Certificates (EPCs) will be needed in the future for all homes at point of sale and rental. How will installing a solar thermal system impact on my EPC?

Installing a solar thermal system is likely to have a major positive impact on the energy performance of your home and this will be reflected on the certificate. This will, in time, give the home a market edge over less well-performing properties, and add value to the installation in much the same way as having double glazing already does.

14. Are there grants available for solar thermal installation?

As of August 2007, the Government-funded Low Carbon Buildings Programme ( was providing £400 per householder and 30% installed costs to community and 'not-for-profit' groups. These grants are not expected to continue much past April 2008, although new grant schemes may emerge.

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