Solar hot water faqs questions, solar thermal, solar panels

Water is circulated between the solar collector on the roof to heat your hot water cylinder (usually in the airing cupboard). Once installed, it's fully automatic and its performance can be monitored with the controller display.

The vacuum tube solar collector concentrates the sun's energy to the manifold at the top of the collector. The controller continuously compares the temperature in the manifold against that in the hot water cylinder. When the water temperature in the manifold is hotter than the water in the cylinder, a pump circulates water through the manifold, down well insulated pipe-work and through a second heat-exchange coil in your hot water cylinder (referred to as a solar cylinder or twin-coil cylinder). The water + antifreeze flowing around the solar system is used to heat the water in the cylinder indirectly. This means that no water in the Solar Heating system will come into contact with water in your hot water cylinder. The heat is transferred, not the water.

The slightly cooled water then returns to the solar panel. As long as the water in the hot water cylinder is at the required temperature, your existing boiler will not switch on.

More comprehensive details of the technology can be found here.

From April 6 2008, homeowners have been able to install solar panels, without needing to get planning permission, as long as there is no impact on others. Size limitations have been set to reduce impact on neighbours. Solar panels attached to the building must not protrude more than 200 millimetres from the roof slope. The only exception is if your property is a listed building or is in a conservation area, in which case you will need to consult with your council planning department.

A solar system will provide typically between 50-70% of your hot water annually. This ranges from 95% in summer to 10-20% in winter. Your existing boiler will provide the rest. Solar systems can be used effectively with gas or oil boilers, Agas, Rayburns, solid fuel boilers, and many combi-boilers.

Replacing your existing cylinder with an efficient purpose-built, twin-coil solar cylinder will result in the most efficient system, so this is recommended. However a lower-cost option is to use a "retro-fit" solar coil, which can be fitted in place of the immersion heater in your existing cylinder. This will reduce the initial outlay, and if required the system can be upgraded by replacing the cylinder some time later. Another option is the use of an external heat exchanger (Willis Solasyphon). This can be used with pressurized and unpressurized tanks.

In many cases, it is recommended to increase the size of the hot water cylinder by ~25% due to the higher position of the boiler coil in a solar cylinder. Since in winter, when the solar contribution is less, some water below the boiler coil will not always be heated.

Any solar hot water system requires a tank somewhere in the system to store the large quantities of heat generated. When connecting to a combi boiler, a tank (referred to as a "Thermal Store") is situated upstream of the combi-boiler, so that the water going into the combi is pre-heated. For all but the most recent "solar-ready" combis, it is necessary to install a mixer valve to limit the temperature of the water on the inlet to the combi to about 40°C-45°C. This prevents the combi from cycling. For most of the summer the combi can be switched off completely and the mixer valve temperature can be increased to ~60°C.

I would recommend the following: get a conventional condensing boiler (not a combi) and a solar twin coil water cylinder. A twin coil cylinder has one heating coil that is connected to your boiler and the other (the lower one) connects to the solar panel. Check out the prices here for our vented and unvented twin coil cylinders. Unvented (mains pressure) cylinders give better water pressure for showers etc, but the cylinders are more expensive since they have to withstand high pressure. If you already have a vented cylinder (with a header tank) you may want to simply replace this with a vented twin coil cylinder. A solar twin coil cylinder is not much more expensive than a single coil cylinder and it gives you the option of adding the solar panel very easily at a later stage. So since you plan to add solar at a later stage you should definitely ensure that you get the right cylinder now. he cost of your solar kit will of course be less when you come to buy it, since you will already have a suitable cylinder.

Unvented cylinders are connected directly from the mains supply of water, so work at main pressure (about 4 bar). Vented cylinders are connected to a header tank in the loft, and the pressure in the tank (and hence the pressure/flow from your hot water tap) is less since it is just due to the height of the header tank. Because unvented cylinders are fed directly from the mains supply of water, the water pressure and flow rate is much stronger, providing hot water at mains pressure to all outlets within the home. The greater water pressure can be a benefit, especially if you like a strong shower, but it will of course mean you will tend to use more hot water.

Unvented cylinders are pressure vessels and hence require safety features and controls such as pressure relief valves and temperature controls.

Vented cylinders are the more traditional option, popular within many households as they are simpler to install and they are very effective in supplying with hot water, smoothly and efficiently. These types of cylinders can be used with most outlets whereas unvented cylinders have the disadvantage of not being compatible with some mixer taps and power showers.

An unvented cylinder installation must be done by an appropriately qualified plumber/heating engineer. In simple terms, vented cylinders are less complicated, meaning they are easier to install and maintain. Unvented cylinders are more expensive option initially but give better water pressure. Choosing the right option for your property is essential and can potentially reduce your utility bills and provide you with a long-lasting energy efficient solution.

Our 30 tube panel produces 1.75kW under standard conditions (1000W/m² irradiation level). Note solar thermal panels are very efficient; it would take seven 250W PV panels to produce the same power.

Technical specifications of our panels can be found here.

In most installations the system is designed for water heating. This reduces the load on the central heating boiler and the improved insulation of the new cylinder makes the conventional water heating system more efficient. Towel rails can be added to system, however winter space heating is not a recommended application. If a company tells you otherwise, seek a second opinion. The reason for this is that when the energy demand is at its peak (e.g. cold long winter evenings) the supply of solar energy is at a minimum.)

For a successful installation, a solar hot water system requires plumbing, roofing and to a lesser degree, electrical skills. Some solar training is recommended, but many competent DIYers have successfully installed their own systems. What is most important is an appreciation of the high temperatures that these solar collectors can achieve.

Our solar thermal kits come with a comprehensive installation guide. Have a look at our preview instructions. This should give you a good idea of what is required.

From customer feedback it seems that the most challenging aspect is usually the plumbing. If you are installing a cylinder it is advisable to hire a plumber. A registered plumber is mandatory if an unvented cylinder is being installed.

If there are still problems please get in touch and we will get back to you.

On a roof or wall, ideally facing South, or anywhere between South-East and South-West for best performance. If your roof ridge is oriented north-south, you can install two smaller panels, one on each side of the roof (a dual-aspect installation). This works well since the east facing panel will take advantage of the morning sun and the west facing panel the afternoon sun. The controller can be programmed to take account of dual panel installations such as this.

Usually when mounting on a roof just use the roof angle. Angles between 25°-90° (i.e. wall mounted) are fine. If you are not constrained by a roof angle (such as with a flat-roof installation) then the simple answer, is to use the latitude. Using a south facing panel in Southern England as the example (latitude 51° North). At the spring/autumn equinox the ideal angle for the panel is 51° to the horizontal. The earth's axis is tilted 23°, so at the height of summer the ideal angle would be (51-23)=28°, and in mid winter (51+23)=74°. Some people prefer to set the optimum angle for winter (~60°) since there is a surplus of energy in the summer. If you specify larger panel size but use a winter-optimized panel angle, this is one way of flattening the supply/demand curve, and giving more contribution out of season.

As water is heated, it expands and becomes less dense, so hot water rises. If the solar panel is positioned below the hot water tank, this natural convection effect will automatically circulate the working fluid around the system. The top of the panel should be at least one metre below the hot water tank, and it is best if it is positioned close, but 2-3 metres below. A thermosyphon is simple and elegant since it does not require a pump, or electronic controller.

Swimming pools are a very good application for solar heating. Our collectors can be roof-mounted or pool-side. Its possible to design a system that will provide hot water for your home as well as your pool. Heating a pool using conventional fuels is an expensive business. Solar will make significant energy savings and enable you to maximize the season.

The electronic control unit provides information on the performance of the system via its LCD display. Many people find that in the summer they can switch off their fossil fuel boiler and leave it off for several months.

Our modern hi-tech panels work brilliantly in the UK climate. They work best in direct sunlight, but still work effectively on diffused solar radiation, and hence contribute well to water heating even on cloudy days. They are more efficient than conventional flat panel solar systems, and hence can provide more of a contribution to water heating outside of the peak summer season. For the level of solar insolation in your locality check our solar map.

The expansion vessel / kit allows the fluid in the system to expand as it heats. This makes the system 'sealed' and virtually maintenance-free. It also keeps the working fluid under pressure, which raises the boiling point so the water/antifreeze can circulate at temperatures in excess of 100°C.

We recommend the following sizes for panel selection:

No, the manifold assembly is well insulated, and the solar heating circuit is filled with 40% antifreeze, specifically made for solar installations. More about antifreeze. Our performance page shows the panel performing covered in snow in mid winter.

System life is approximately 25 years. The are very few moving parts. The tubes are tested to withstand hail up to 25 mm in diameter. If individual tubes are damaged the system will continue to function normally but at a slightly reduced capacity until such time as the tubes can be replaced (a simple process).

The high temperatures generated by our solar panels are far greater than those encountered in normal heating systems. Hence normal pipe insulation cannot cope with the high temperatures of a solar system and in many cases will melt. We recommend the use of Armaflex HT insulation which is specifically designed to withstand high temperatures.

The thermal kits we provide are covered by a manufacturer's warranty of 24 months.

You can view our full Terms and Conditions HERE.

In mid summer the panel(s) will be absorbing a great deal of energy. If the system is in use this presents no problem. If the system is left unattended (for instance if you go for a 2 week holiday) then obviously the weather at home in the UK will be fantastic, and the panel will heat the cylinder up to a nice hot temperature. If this were left to compound over several days with no safety system to keep the temperature in check the cylinder could overheat. The options available to protect the system are:

1. Use the recycling "holiday programme" on controller. The controller detects when the cylinder reaches a certain temperature and runs the pump at night to dissipate excess heat. This brings the cylinder temperature down and prevents overheating. This feature is available on our advanced controller.

2. Use heat dump circuit. At its simplest this involves installing a radiator or towel radiator in parallel in the solar circuit. An electrically actuated solenoid valve can be used to switch the radiator on if the cylinder reaches a certain temperature. Alternatively you can simply use a manual valve and turn it on when leaving the system unattended for any length of time.

3. Covering the panel when leaving the system unattended is another basic option. This is often used in systems where the panels are used primarily for heating (i.e. they are not required during summer).

Now that you effectively have free hot water, you need to make the most of it. There are many household appliances which use electricity to heat water. So you can save electricity by filling these using the hot water from your solar panels. Fill your kettle with HOT water. Connect your dishwashers and washing machine to a HOT feed pipe so that the water going in is from your free solar supply. Otherwise the dishwasher/washing machine wastes electricity by heating cold water, when you already have an abundant free supply of hot water from your solar panel. This will save on your electricity bill. Also it is no longer necessary to use low temperature "eco" cycles on the dishwasher and washing machine, which in turn means that the amount of detergent used can be reduced.

Normally you wouldn't need to change anything on your existing controller. It will react to the temperature in the the thermal store as currently set up.

The Solar controller controls the pumpstation and circulation of the fluid around the solar circuit by comparing the temperature difference between the panel and the cylinder, so there is not usually any electrical connection between the boiler controller and the solar controller.

The only change I would recommend to the boiler controller is to alter the program settings so that it does not heat up the hot water cylinder in the morning.

Also, if your cylinder doesn't have a spare pocket you could insert the thermostat and the NTC10K sensor into the same pocket in the cylinder.

Our kits are Solar Keymark registered and thus approved by MCS.

If the kits are installed by an MCS registered installer they will be eligible for RHI.

We use six stainless steel mounting straps for each panel. They are very light but exceedingly strong. Each one could take the weight of a panel on its own!

We use stainless steel mounting straps, which work very well on slate roofs with minimal disruption of the roof. The only penetration is for the two pipes (flow and return).

Evacuated tubes are more efficient than flat plate collectors (10% to 30% more efficient depending on which expert is being quoted). The increased efficiency is due passive solar tracking. The cylindrical shape of the tube means that it absorbs the sun's energy from dawn to dusk, not just when there is direct sunlight.

In the summer months the differences may not be too great. In the winter months, however, the evacuated tubes will be much better at heating water. There will be benefits in spring and autumn too.

More detailed information about solar evacuated tube technology can be found here.

Notwithstanding all of this, considerations of efficiency do not take into account relative costs or aesthetics. Some people just prefer the cleaner lines of a flat panel and are prepared to sacrifice some efficiency as a result.

30 tube collector is 103 kg

18 tube collector is 62 kg

More detailed specifications can be found here.

We would recommend using copper pipe plus 'Armaflex outdoor' insulation. Then put that into a upvc plastic waste pipe before burying it.

You go on total surface area, in this case 75m² As a rule of thumb you should plan on having solar panels of between 1/4 and 1/3 of the area of the pool. Our 30 tube panels have gross area of roughly 5m² so we would recommend 4 or 5 of our 30 tube panels.

If the panels are to be fitted to East and West facing roofs, aim to have the same number on each side, in this case 3 + 3. Each side would need it's own pumpstation and controller.

The hotter you like your pool temperature the more panels you will need.

We recommend 10mm pipe work for the following reasons:

a) By using 10mm pipe the volume of water in the solar circuit is relatively small. This ensures that the heat transfer fluid warms up faster and hence the transfer of heat to the cylinder starts sooner and is more efficient. The rate of flow with 10mm pipe is more than adequate (especially for an 18 tube panel). Many central heating systems use 10mm pipe to even fairly large radiators.

b) With 10mm pipe the surface area of the pipe is relatively less, thus reducing heat loss.

c) The manifold and cylinder heat exchange coils use 22mm pipe so that the fluid remains in contact with the heat exchange surfaces for longer. The heat transfer fluid lingers in the manifold (22mm pipe) to gain heat, then flows quickly (using 10mm or 15mm pipe) to the heat exchange coil in the cylinder. It then lingers in the heat exchange coil where it transfers it's heat to the water in the cylinder. If a large diameter pipe and a high flow rate are used the transfer fluid will not heat up so much and the transfer of heat will be less efficient.

d) 10mm pipe is supplied in 25m coil lengths and usually the entire run can be done in a single run with no joins required. It makes the installation very much more straighforward.

You can of course use other sizes of pipes. It's just that we have just found 10mm to be the best (for single panels) all things considered. 15mm would work fine. 22mm would work but is over engineering the installation quite considerably and may not be as efficient. (If you have 4 panels in series that would be a different matter!)

Yes you can use the solar thermal panels as part of an underfloor heating (UFH) system. It is recommended that you connect it as follows:

Keep the hot water cylinder separate from the UFH, otherwise the UFH system will suck all the heat out of it and you will never have any hot water.

We recommend having two separate cylinders, say one ~200-250 litre one for the hot water, and another ~300 litre thermal store one for the UFH. Use 3x 30 tube panels and connect these in series. Connect the solar hot water feed to the hot water cylinder heat exchange coil first then, in series, to the UFH cylinder. That way the hot water cylinder gets heated up first. Once that is hot the UFH will heat up.

This is the simplest configuration. If you want to make it more flexible you could add some valves to allow isolating each cylinder so that you could switch the solar heated water between the hot water cylinder and the thermal store.

Our controller allows the UFH circulation pump to be controlled separately (that is, starting to circulate when the thermal store reaches a set temperature).

For all specifications and costs visit our solar thermal kits webpage.

Solar Water Heating FAQs