Solar Hot Water and Electric Systems  


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Most enquiries we receive can be answered by resources to be found on this site. If you cannot find what you are looking for, we will be pleased to receive your enquiry using the form at the bottom of this page,

In the first instance please help us by first checking out the sources of information in the box on the left.


On the Downloads page you will find many useful documents in printable pdf format.

2. FAQs

Seek your answer from the popular questions listed here…


Please complete our enquiry form here…




Frequently asked questions








Q: Why should I buy an ISL solar system?

A: At ISL it not our aim to trick customers into buying a solar system by high pressure sales techniques, gimmicks or unjustifiable claims about savings to be made. The ISL system has been independently tested and the performance figures are freely available for comparison. They show the system to perform very well in its class, which is for a typical Dutch system with a panel area of 2.7 square metres. Now taking into account the fact that is is PV powered and the minimal maintenance requirements we believe that the ISL panel has the edge on its competitors. However the ‘bottom line’ as a potential customer is how much does the ISL system cost compared to other systems in its class? Our answer is that we believe we are providing one of the lowest-cost and best value solar systems in the UK and we therefore encourage you to compare our prices with any other suppliers. We believe you will come back to order your system from ISL.

See our Datasheet “Reasons to go Solar with Imagination


Q: What guarantees do you supply?

A: We offer a six year manufacturer's guarantee on the solar panels and two years on the pump and controls. The guarantee on the cylinder depends on the manufacturer, which varies depending on which system is chosen. This relates to defects in the manufacture of the components and not any subsequent damage. If we install the system ourselves, we offer a one year guarantee on our workmanship. In the event of any dispute over whether the guarantee applies, we would use the arbitration service of the Solar Trade Association to resolve any disputes.


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Q: How does the ISL flat plate collector compare to evacuated tubes?

A: There is a lot of conflicting information available about the relative performance of flat plate and evacuated tubes. For example comparing evacuated tubes to unglazed flat plate collectors or those with non selective absorbers, or comparing at temperatures which are not representative of normal domestic hot water usage. This has led some people to believe that a much lower area of solar collectors are required to be used for evacuated tubes, which is simply not the case. The truth is that there are good solar collectors and bad solar collectors and many good flat plate collectors perform better than poor evacuated tubes for a given collector area. Also a lot depends on the design of rest of the solar system, such as the pump energy consumption, control system and hot water cylinder. We believe that the ISL solar system is one of the better flat plate systems, having a low water volume panel, a selective absorber surface, a low wattage pump. Therefore until more conclusive test data is available we suggest that you obtain comparative quotes for the same area of mains powered evacuated tubes as our panels.

Q: Do I need planning permission to install a solar systems?

A: Planning permission may be required from you local authority in some cases, depending on where you live. Certain types of minor development (e.g. roof windows and solar panels) in some areas are known as permitted development and are given general permission by the Town and Country Planning General Permitted Development Order 1995. However local authorities have the powers to override this locally, in which case a planning application will be required, as is also the case for new buildings. Also be careful in National Parks, conservation areas and in listed buildings, where planning regulations are must tighter. In any case always check with your local planning authority who can quickly tell you.

See our datasheet "Planning Permission Guidance"

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Q: What are the advantages of the ISL drain back system?

A: The drain-back system, which is very common on the continent, has a number of advantages over the pressurized system, which is common in the UK. Assuming the unit is installed as per the installation guides, then:

  • At night, when there is no sun or in other very cold weather there is no water in the panel to freeze, so plain tap water can be used in the system.
  • No chemical antifreeze is required, so there is no maintenance to check and maintain antifreeze protection for the system.
  • In very hot weather with no water draw-off [e.g. whilst away on summer holidays] the system is protected from boiling by draining back the water when the cylinder exceeds 80 degrees centigrade.
  • If you choose a mains powered system, then in the event of a power failure the system always fails safe, with water draining back out of the panel, where it will neither freeze nor boil.
  • Drainback systems can be ‘switched off’ without harming them, so we can offer high limit shutdown at 80C or 65C, which make the system ideal for use with un-vented cylinders.
  • Plain tap water is a better heat transfer medium than antifreeze, so carries a greater density of heat from the panel into the hot water cylinder.

Q: Why not use a fully filled system?

A: The ISL system has a number of advantages:

  • There is no expansion vessel to regularly service. We simply have a small air pocket that absorbs any water expansion as the system becomes hot.
  • Very low maintenance, as the system does not need servicing or re-pressurising if there are any leaks or a release of water from the pressure relief valve.

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Q: Can I use the ISL solar system with a Combi boiler?

A: Combi boilers, which provide instantaneous hot water and central heating, are now very popular in the UK. For this reason we offer solar systems with an un-vented cylinder or thermal store, suitable for providing pre-heated water to a combi. Unfortunately 99% of existing combi boilers in the UK will not accept the pre-heated hot water from a solar system. Therefore we do not provide specific advice on which makes and models are solar compatible, as the answer is usually 'no'. Please consult the technical department for your boiler manufacturer if you suspect yours may be compatible (e.g. Eco Hometec). If you are planning a new boiler we can recommend ATMOS boilers which we have successfully used with solar pre-heat. 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. Please see data sheet below for details.

See our “Combi Boiler Datasheet”


Q: Can an ISL solar system be used to provide space heating for the home?

A: 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. Of course a much larger area of panels are required and there is still a requirement for a back-up heating system. This works best on very well insulated new build houses.

See a schematic diagram of solar underfloor heating


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Q: Can an ISL solar system be used to heat a swimming pool?

A: This is an ideal application for solar systems [especially for outside pools] because they tend to be used more during sunny weather. The low temperature required by a swimming pool means that the efficiency of the system is much higher.

See our datasheet - Large Scale & Swimming Pool Systems - Design and Budget Prices

Q: Is it possible to run the pump from a photovoltaic (PV) panel?

A: Yes, we can supply as standard a small PV panel (638 x 278mm) which will provide enough electricity to operate the control system and pump. This is because we use only a 3 watt DC pump and low power electronic controls, rather than a more typical 50 watt mains electric pump. This makes our solar system very suitable for remote locations with no mains power supply - or just because you want to be ‘super green’.

See our Datasheet “Solar Electric (PV) Power Supply

Q: My roof doesn’t face south, what can I do?

A: Whilst a south facing roof is ideal, it’s not essential to have one. Between south east and south west is also quite good. An east facing roof, will collect solar energy mainly in the morning, whilst an west facing roof will collect solar energy in the afternoon. With east and west roofs, we can supply two solar panels and control systems, which will typically collect more energy than a single south facing panel.

See our Datasheet “Locating Your Collectors - Tilt Angle & Orientation

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Q: Why use a polycarbonate cover for the ISL solar panel?

A: The one piece injection moulded polycarbonate cover is preferred to the typical ‘box’ construction panel for a number of reasons:

  • A polycarbonate cover is virtually unbreakable, unlike glass covered panels or evacuated tubes and the polycarbonate has been specially treated to maintain light transmission over many years to avoid yellowing, brittleness and deterioration. Ideal if you suffer from vandalism, including those with four legs or two wings!
  • There is no need for aluminium side pieces for the panel which add to both cost and weight of most solar systems. For those interested in the environment, the embodied energy (energy used in manufacture) is therefore much lower for the Imagination Solar system, than for heavy aluminium ‘box’ panels, which typically weigh three times as much.

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Q: Why do you prefer to roof integrate the ISL solar panel?

A: Most solar systems either rest directly on top of the roof or are suspended just above the roof on brackets. We can mount on top of a roof, but prefer to fully integrate the panels into the roof because:

  • Our roof integrated panel provides a very attractive low profile when integrated into the roof covering. Being sunken below tile level it’s actually lower than a typical rooflight.
  • The streamlined design helps keep the panel in place and minimise any potential noise in high winds.
  • There are no tiles or slates hidden under the panel which are inaccessible for maintenance or renewal.
  • In new build there is a considerable saving of tiles or slates where the panel is fitted.
  • In existing roofs many tiles and slates become available for repairs elsewhere
  • All plumbing is safe and dry inside the roof space.
  • No control sensor wires are fitted on the roof, very often fitted by drilling holes through roof tiles and sealing with mastic.
  • Fitting the panel to the roof normally only takes one day.
  • The cover has an special UV anti-aging treatment which prevents reduction of light transmission by more than 6% in over 10 years. In the unlikley event of a damaged cover (virtually unbreakable) a separate a replacement is available at low cost.

See some photos of roof integration

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Q: Why control the ISL solar system on light levels rather than water temperatures?

A: We prefer the ISL control system to a differential temperature controller because it’s simplicity. Also with a differential controller (comparing temperatures between panel and cylinder) it is quite difficult to find positions in either the solar panel or the stratified hot water cylinder which are truly representative for use in a differential controller. Therefore we measure the amount of solar radiation and start the pump running when the sun is providing 150-180 watts per square metre, which is enough to exceed the heat losses in the pipe work.

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Q: How safe is the ISL solar system?

A: Extremely safe. The ISL systems has a safety cut out which will operate when the hot water cylinder reaches 80 degrees centigrade or 65C according to your preference. If PV powered then there is no risk of mains electrocution. Pressures are much lower than in fully filled sealed systems. We strongly recommend fitting a mixing valve to the hot water supply, to eliminate the risk of scalding in sunny weather unless you have a controller limited to 65C.


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Q: How much energy does the ISL solar system save?

A: All solar systems save far more energy in use than in their manufacture and operation. The actual amount of energy saved depends on the household hot water usage, but is generally between 30 and 70% of the total hot water consumption. The financial saving will depend on the cost of fuel and efficiency of the existing boiler. Tests by the Netherlands building research organization TNO have shown that a single panel system should provide 4 GJ [giga joules] per year or 1112 kWh [kilo watt hours] under standard circumstances, at the taps (much more from the collector into the cylinder). Of course the more panels that are fitted the more energy that will be saved. Of course with no electricity running costs or antifreeze to top up, there are no running costs to offset against these savings.


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Q: How cost effective are ISL solar systems?

A: Everyone always asks how much will a solar system save and how long will it be before I recoup the costs? The difficulty in answering is because it is a complex calculation which is different in each case. The more hot water you use, for example with more people in the house, the more you will save by having a solar system. Also if you use more hot water in sunny weather the more you will save, because that’s when free hot water is available. The other factor is the cost of your existing fuel, which may range from an efficient condensing gas boiler to an electric immersion heater. Each case is different but we would suggest carrying out cost effective measures such as loft insulation and cavity wall insulation before considering purchasing a solar system. However a solar system is probably more cost effective than double glazing your home and is one of the vary few measures you can do in your home which actually contributes energy rather than merely reduce the amount used. If you do decide to get a solar system the most cost effective one is that which is efficient and at the lowest cost which is why we have done as much as possible at ISL to bring you a high quality low cost solar system.


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Q: How environmentally friendly [‘green’] are ISL solar systems?

A: A number of features have been incorporated to make ISL solar systems very environmentally friendly:

  • The system can run on plain tap water and requires no chemical anti-freeze, when installed as per the installation guides.
  • The system is PV powered and consumes no high carbon electricity to run the pump or controls.
  • Low embodied energy collector design. The ISL solar panel design uses small amounts of material and minimises the use of aluminium, a material requiring high energy use for manufacture.
  • Our standard hot water cylinder is CFC and HCFC free, and has zero ozone depletion potential.
  • Solar panel insulation is CFC and HCFC free glass wool with CFC and HCFC free polyurethane insulation behind.
  • The ISL solar system displaces many times more carbon dioxide over its lifetime than is used in its manufacture or operation.

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