Solar Hot Water Panels FAQ

General

What is Solar Thermal Heating? - How is it different from photovoltaic?

Solar thermal panels absorb heat from sunlight. Typically this can be used to provide domestic hot water.

Solar thermal panels are generally cheaper to install, and produce more energy than photovoltaic panels (which generate electricity) of the same size. However, hot water cannot be exported to a 'grid' as electricity can, therefore it is only sensible to generate as much as a household can reasonably use.

What are the benefits?

This can benefit you financially in two ways

  • a significant reduction to your fuel bills - a field trial by the energy saving trust found savings of 60% for hot water energy needs, on system which were correctly sized and installed, and used sensibly. The median saving was only 39% but the best was 98%.
  • via the government's Renewable Heat Incentive, which is expected to give you a significant return on your installation cost.

But in addition to saving yourself money, you will be benefiting the environment by burning less fossilfuel.

Do I need to change the way I use hot water?

To get the most benefit you should time your heating system to top up the water temperature in the early evening if necessary, after the sun has done what it can. The worst thing you can do is to top up the water temperature in the morning, so when the sun comes up the cylinder is already hot.

However this doesn't mean you need to change you habits to use the hot water in the evening. Provided your cylinder and pipes are well insulated the tank will still be hot enough the next morning. After all you donít need water at 60°C or even 50°C to run a shower or bath.

What types of panel are available?

Flat plates - These are generally considered aesthetically pleasing, looking a bit like dormer windows. A glass panel at the front traps the sun's rays to heat a working fluid (typically, water with antifreeze) circulating through pipes behind the glass. This fluid is pumped through one coil of your water cylinder, transferring its heat to the water.

Evacuated tubes - The panel consists of an array of glass tubes, evacuated to improve thermal insulation. Inside each tube is a heat pipe (typically containing a small amount of alcohol), which transfers the solar energy to a heat exchanger at the top of the array. The hot fluid from the heat exchanger is pumped to your cylinder, as for the flat panel type.

You can also get evacuated tubes hidden by flat glass, as shown in the picture.

These panels are evacuated tubes under flat glass (Vaillant)

Can I use solar heating alongside conventional heating?

A solar thermal installation is unlikely to meet the total domestic hot water need, especially in the winter; therefore it is more normal to combine it with a conventional (e.g. gas fired) heating system.

If you have a conventional (not combi boiler)

  • Your hot water cylinder needs dual coils Ė but most suppliers can also provide a suitable replacement cylinder.
  • Some solar thermal systems require a vented hot water system. If your existing system is pressurised, check for compatibility with your supplier.

If you have a combi boiler

  • You will need to add a hot water cylinder for the panels to heat up.
  • If your combi boiler accepts pre-heated water (check with the manufacturer) then this cylinder can feed into the combi.
  • if not, then you need a system such as Viridian Solar Pod. This connects to the combi water input with a bypass valve
    • When the stored water is hot enough it is fed directly to the taps, bypassing the boiler altogether
    • If it isn't then cold water is mixed in to make it cool enough for the boiler to accept - typically 30C. So you still get some benefit from the stored solar heat.

Adding solar heating to a conventional boiler will reduce the fuel usage to the extent that during the summer, many users find that they do not need to use their boilers at all.

Can I use it to heat my house?

Solar thermal is not generally recommended for space heating, as it is too variable and would require a large reservoir of heat if it is to be useful at night. Also there is little requirement for space heating except in winter and you would need an awful lot of solar panels to make much difference - giving you far more heat than you need in the summer.

Performance

What happens when it's cloudy?

You will still get some heat, even during the winter.

What temperature will it heat the water to?

On a sunny day, it's theoretically possible to get 95°C - but the control system will limit the output to a safe temperature. The hotter you run your tank the more heat you can store but there is a risk that it will scale up more quickly. Also, you should have a thermostatic mixer valve to add some cold water before it runs to the taps to avoid risk of scalding. You only need this if you allow the system to heat the water above about 65°C. On days when there is not enough sun your boiler will top up the heat as necessary.

Evacuated tubes usually have a built-in thermal isolator for extra safety, to prevent the water from boiling.

Installation

How much roof-space do I need?

For a house with a South facing roof, 2-4 square metres should be a rule of thumb. This should provide 100% of an average family's hot water requirement during the summer, and maybe 60% of the requirement averaged over the year. You can of course choose a smaller or bigger installation depending on your need.

N.B. With an evacuated tube system the actual collector area is less than the roof-space occupied, because there are gaps between the tubes; however this is more than compensated by the collectors being more efficient than flat panels.

Also note that an evacuated tube system is normally one large unit, making them suitable for large roofs. By contrast, a flat panel system can be made up of smaller units which can distributed to make better use of awkwardly shaped roofs.

What if I don't have a South facing roof?

You will need more roof space to give a similar amount of heat. For example, some houses have panels on both East and West facing roofs (in such cases, each panel will be pumped independently so that the heat collected on the sunward side is not simply re-radiated on the cold side).

More generally, the efficiency of the panels varies with orientation, in much the same way as for photovoltaic panels. Our PV FAQ contains graphs showing this effect.

Can the panels go on a walls or flat roof?

Ideally (in England) they should be mounted on a 40 degree slope; deviation from the ideal angle will result in less heat being collected. Many suppliers can provide brackets for mounting at this angle on flat or vertical surfaces.

What else do I need?

You will need a hot water cylinder, to store the heat gathered during the day. For a stand-alone solar thermal system, this will need one heat exchange coil.

However it is more likely that the cylinder is part of a conventional (e.g. gas-fired) hot water systems; in this case, it must have two coils (one for the boiler, one for the solar panel). For many existing hot water systems, this will require an upgrade.

Can I 'do it myself'?

To receive any RHI or other benefits, installation should be done by an MCS certified installer.

Do I need planning permission?

Solar panels are normally a permitted development if you are putting them on your roof, even in a conservation area. But if your house is listed, check with your local authority. The planning portal gives more detailed advice.

Maintenance

How long will the system last?

Systems generally carry a warranty of about 10 years but should last 25 years.

What maintenance is required?

Occasional topping up of antifreeze should be the only thing required on most systems. However it is important to check regularly that it is working. You may not notice otherwise, since your boiler will top up the heat anyway. The easiest way is usually to check the temperature of the tank in the late afternoon, after the sun has been shining but before the boiler cuts in to top the heat up if necessary (assuming you have configured the boiler to top up in the evening, if the tank isn't already hot enough).

Evacuated tubes look rather delicate; what happens if they break?

In fact breakages are rare, but each tube is self-contained so if it does break, the rest of the system can still function. Replacement of a broken tube is easy.

Can I connect it to my computer, to monitor the performance?

Some systems allow this, but it may need a controller upgrade.

Finance

How much will it cost?

For a normal semi-detatched house, expect to pay about £3000 to £5000 plus VAT (but solar panels qualify for the reduced rate of 5%). Although this may be a large outlay, the RHI (see below) is expected to make it a reasonable financial investment.

Are there any grants to cover the cost?

The Renewable Heat Incentive (RHI) is a government scheme to help you pay for your panels. It gives you a guaranteed income for 7 years, based on the energy your panels provide according to your assessed hot water needs. The Energy Saving Trust has estimated typical payments to be:

Household sizeRHI payments
2 persons£195/year
3 persons£265/year
4 persons£335/year
5 persons£435/year
6 persons£470/year

You will need a Green Deal Advice Report and if this recommends loft insulation or cavity wall insulation you need to get that done first.

To qualify for the RHI, your installed must be MCS certified. They will give you the documentation you need to apply. There is more information on the RHI on the OFGEM website.

How much will I save?

If you heat your water with a gas boiler which 80% efficient (boilers are not usually quite so efficient in the summer) then assuming you pay 4p/kWh that is an additional 5p/kWh saving. Assuming the same usage that the EST used in the above table, your savings would be

Household sizeGas bill savings
2 persons£50/year
3 persons£70/year
4 persons£90/year
5 persons£110/year
6 persons£120/year

Links

Who are the local suppliers?

Here is a list of some of the local companies that can supply and install Solar Thermal systems in the Cambridgeshire area:

Also we have a personal recommendation for an installation/maintenance engineer:

  • Roland Parker 01206 853460

Where can I find more information?