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Solar thermal

Introduction to solar heat and hot water

Solar Panels

Solar Panels

Solar water heating (known as solar thermal) systems capture the free heat from the sun and use it to heat up water for use in the home. It’s a simple process:

  • panels on your roof absorb heat from the sun – they are known as the collector
  • the water in the panels heats up
  • this hot water is pumped through a coil in your cylinder
  • which transfers the heat to the water in the cylinder

Is solar thermal suitable for my home?

solar thermal evacuated tubes

solar thermal evacuated tubes

The ideal situation for solar panels is facing due south, although they are effective facing anywhere between south east and south west. As a rule of thumb you need between 1 and 2 m2 of collector (solar panels) per person living in the house. Shade on the panels at any time of day will reduce the performance.

Most panels are mounted on a roof, but they can also be mounted at ground level. It is important that they get direct sunlight. To get the best results they should be at an angle between 20 and 50 degrees from horizontal (most pitched roofs fall within this bracket).

If you have an electric shower it won’t use your solar hot water. Similarly cold-fill dishwashers and / or washing machines heat the water they use. In this situation, solar water heating is not suitable unless you use the bath for most of your washing and bathing, as you won't be able to use much of the solar hot water you generate.

Solar panels are compatible with most existing hot water systems. However, you will need a new cylinder with two coils. Ideally it should be big enough to hold two days worth of hot water. 

Solar hot water with combi boilers is more difficult, but still possible. If you have a combi boiler it is important to check with the manufacturer that it will accept pre-heated water.

If your present system is gravity fed, it will need a control (such as a valve and pump) for the hot water circuit so the panels can work effectively in winter when the boiler is running for central heating.

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How much hot water from solar thermal panels?

Solar Panels

Solar Panels

Solar thermal panels should provide most of your hot water from April to September, and make a worthwhile contribution in the months on either side of that period. Outside of that estimates vary depending on who you ask. The  Energy Saving Trust field trials found that solar thermal panels will provide about 60 per cent of a household's hot water needs, if well-installed and properly used.

How much you benefit will depend on a variety of factors:

  • How much hot water your household or business uses. The higher the usage, the more benefit you get from a solar thermal system. 
  • How much interest you take in how the system works and adapt to make the most of the free hot water (ie having showers in the evening rather than the morning). The sun isn’t as reliable as a timer clock.
  • The size of your cylinder. Many cylinders only hold enough water for a day’s supply of hot water, so a day or two of cloud and rain will mean you have to turn on the boiler or immersion heater.
  • How you programme your back up heating. If your control panel does not allow you to programme the hot water and central heating separately, you may not get the maximum benefit from the solar panels when the heating is turned on. By only boosting the hot water once the sun has gone down, you maximise opportunity for solar heating.
  • Adequate insulation of both cylinder and pipes carrying hot water.
  • Allowing hot water temperature to vary. If you do not need high temperatures all the time, you will have less need for back-up heating. You will also reduce heat loss. However, it is important to make sure your cylinder reaches more than 60 degrees centigrade at least once a week to avoid risk of Legionella.

The Energy Saving Trust identified a huge range of performance, with the best system producing 98 per cent of the household's hot water, and the worst just 9 per cent. The median across all systems was 39 per cent, so it is important to take action to maximise your solar gain.

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What type of solar thermal panel is best?

There are two types of solar thermal panel: flat plate panels and evacuated tubes.

Flat plate panels consist of an absorber plate in an insulated metal box. The top of the box is glass or plastic, to let the sun’s energy through, while the insulation minimises heat loss. Lots of thin tubes carry water through the absorber plate heating it up as it passes through.

Instead of a plate, evacuated tube collectors have glass tubes containing metal absorber tubes, through which water is pumped. Each tube is a vacuum (the air is ‘evacuated’ hence the name), which minimises heat loss.  

The Energy Saving Trust field trial found little difference in performance between the two. For many people the decision is a matter of aesthetics.

Research from Swiss-based Solartechnik Prufung Forschung shows that the best performing collectors are more than twice as efficient as the worst ones. It found that the most effective  flat plate collectors are made in Austria and Germany and the best evacuated tubes in Switzerland and Northern Ireland. Six of the 10 worst performing evacuated tube collectors are made in China.

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What does solar thermal cost?

The cost of installing solar a solar thermal system will depend on the type and quality of the panels, whether you need scaffolding, and how easy it is to integrate into your existing plumbing system. 

The cost of scaffolding and the new cylinder are both significant, so a cost- effective time to install would be when you need a new hot water cylinder, or when you are having repairs done on the roof (or at the same time as a solar PV installation).

Panels are available for DIY installation, but buying them this way means that you have to pay the full rate of VAT (instead of the 5 per cent rate) and, more significantly, won't be eligible for the renewable heat incentive. Some people make their own from old radiators and the like. CAT publishes a book Solar Water Heating: a DIY Guide to help the enterprising.

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Renewable heat incentive

Solar thermal systems are eligible for the Renewable Heat Incentive. Find out more about this in our Financial Incentives section. 

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Planning permission for solar panels?

The installation of solar panels and equipment on residential properties is likely to be classed as 'permitted development', meaning there is no need to apply to the Local Planning Authority for planning permission.

However, there are certain exemptions which must be met to benefit from these permitted development rights.

You should discuss with the Local Planning Authority for your area whether all of the limits and conditions will be met.

See our blog on installing solar panels on a listed building.

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More information on solar thermal

From the blog:

Before you install

Solar hot water panels: 7 things to check before you install


Choosing an installer 

Five tips for finding a reputable installer

Practical advice-maintenance

Seven steps for servicing your solar thermal system

Solar Thermal - A guide to life expectancy and maintenance 

Servicing your solar water heating panels

How do I know if my solar thermal panels are working correctly? (Sept 2013) 

More practical advice  

Solar hot water: readers' tip of the month 

5 key ways to get the best from your solar hot water system 

Getting the best out of your solar thermal system

Solar thermal panels are possible with a combi boiler

Solar hot water without changing your cylinder 

How to avoid bugs in your solar hot water

Storing your solar heat 

User control is vital for best performing solar water heating 

Your questions answered

How much does solar thermal really save?

Which household would benefit from solar thermal?

Thermodynamic panels: your questions answered 

Is my solar thermal system safe while I'm away on holiday?

Can solar PV and solar thermal be successfully combined?

Do I need a new cylinder for solar hot water?

Can I use solar thermal panels to heat my house?

Solar thermal panels: flat plate or evacuated tube?

Can I have solar hot water with a combi boiler? 

How much of my hot water can I get from the sun?

Can I use solar thermal for space heating?

Case studies

Solar hot water: Its the best thing I ever did

Getting the best out of solar panels

Planning for solar panels

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