What is Solar Water Heating?

Solar Water Heating (SWH) is a system for heating water using energy from the sun. Solar energy is collected by a panel, which is connected by pipes to a hot water storage device such as a hot water cylinder. Systems can be installed for domestic hot water, swimming pools, caravans and similar applications.

The main focus in this website, is on domestic water heating, but the same principles apply to all solar systems. A description of a typical domestic hot water heating system is described here.

Why install Solar Water Heating?

• It can save you money using free energy from the sun to heat your water
• It helps the environment by reducing greenhouse gas emissions
• It takes your hot water system into the 21st century
• It could add to the value of your house
• If installed properly, it should be virtually maintenance free

What happens in different seasons of the year?

SWH depends on radiation not direct sunlight so it works even on dull days. However, in winter, although a panel can be effective in helping warm your water, you will still need your existing hot water heater to provide most of your water heating needs. You should note that these systems are not designed to work in conjunction with your central heating radiators - they can only heat your water.

However, in the Autumn, Spring and Summer, a solar panel can yield surprising results, often not requiring any support from your boiler. Depending upon your system and how and when you use your hot water, you could find that almost all your needs are met by a SWH panel at these times of the year.

Installation

• You need either a southerly facing roof or two roofs facing east and west
• The panel usually sits on top of existing tiles/slates
• You can carry out a DIY installation or can use an approved contractor

Disclaimer: Whilst many people have successfully fitted their own solar panels, it is important to recognise the dangers involved, especially when working on a roof. This information pack does not attempt to deal with issues surrounding health and safety for DIY installations of systems. As such, the CEI urge extreme caution if you intend to carry out the work yourself.

Technical Details

The diagram above shows a typical SWH sealed system used to supplement a conventional domestic hot water system.

The collector (solar panel) should face south or close to south and lie on a pitched roof that will provide the natural angle to face the sun.

A conventional central heating pump forces water through a coiled pipe in the solar panel where it is heated by the sun. The heated water then flows down and through a second (lower) coil in your hot water cylinder, referred to in the diagram as a solar cylinder. The hot water passing through this coil heats the water in the cylinder.

The slightly cooled water is then returned back to the solar panel via the pump. The controller box continuously compares the temperature in the panel against that in the hot water cylinder (see dotted lines). It switches the pump on when the water temperature in the panel is hotter than that in the cylinder and switches it off when the reverse conditions apply. As long as the water in the hot water cylinder is at the required temperature, your existing boiler will not switch on.

The water flowing around the solar system is used to heat the water in the cylinder indirectly. This means that no water in the SWH system will come into contact with water in your hot water cylinder. The heat is transferred, not the water.

This is beneficial because:

1. it keeps the total amount of water flowing in the solar system to a minimum, making the system more efficient at bringing heat down from the panel
2. anti-freeze needs to be added to the water in the solar system circuit as it is partly outside the house and could freeze in winter. As an alternative to using anti-freeze, a drain back system can be used. In this the panel water drains back into a special bottle when the pump switches off.

An extra coil in your hot water tank

Normally the solar coil in the hot water cylinder is additional to the standard cylinder coil connected to your boiler. It is possible to fit a second solar coil into your existing cylinder, but as this can be a very tricky operation, it is generally better to purchase a new twin coil hot water cylinder. These can be purchased with an optional immersion heater socket at the top if required. 

The air vent shown on the diagram is installed at the highest point and is required to prevent air locks. As this is a sealed system, an expansion vessel is also shown which allows for expansion and contraction within the system as it heats up or cools down.

The system is filled using a one-way valve (not shown). Care must be taken to ensure that water contaminated by anti-freeze does not get into the water feed supply. As an alternative, systems may be filled up from a small water tank as in a conventional central heating system. In these systems the tank needs to be higher than the top of the panel, so that water drains back into the system. In this case an expansion vessel is not needed and the water supply will be automatically isolated.

You can also buy an electronic display board which can be mounted in a convenient inside location which gives both roof panel and cylinder water temperatures. This is fun when showing guests how marvellous it is to heat your water from the sun!

Panels

Solar panels of the type described above are called flat collectors. A flat collector is basically a piece of copper tube, bent to form a serpentine shape and then soldered to a sheet of copper. The tube and sheet are painted black and are mounted in an insulated box with a suitable glass or plastic lid.

You can even make them yourself but, if you do not wish to do so, they are available at increasingly competitive prices from various manufacturers. They are very effective and require little maintenance.

An alternative is a vacuum tube collector that contains a heat sensitive liquid. Vacuum tube collectors are more expensive than flat collectors but are more effective in winter.

The table below shows suggested collector area and cylinder capacity needed for hot water provision for various sized households. This can only be a rough guide as it depends on whether residents are in or out most of the day, prefer a shower to a bath, use a washing machine regularly and so on.

Household size	Total collector area (m2)	Cylinder Capacity (litres)
1-2	2.5	160
3-4	3.5	195
4-5	4.5	245
5-6	5.5	294