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FAQs on solar water heating

What is solar water heating?

A solar thermal system uses collectors on your roof to convert daylight and radiation from the sun into heat. The heat is transferred to the hot water cylinder by a water-antifreeze mixture that is circulated with a pump.

This process raises the temperature in the hot water cylinder, and means that your boiler will have add less heat and therefore consume less fuel. A typical domestic system will provide most of you hot water in summer, and a small but not unsignificant contribution in winter.

Is there really enough sun in England to make this worthwhile?

Yes. A domestic solar thermal system provides 60-70% of your hot water needs, averaged over an entire year. In the UK alone over 100,000 people have already installed solar water heating systemsand are benefiting from free hot water.

Is my house suitable for solar water heating?

Almost certainly, yes. Solar water heating is suitable for the vast majority of homes in the UK. You will need:

Ideally a south-facing roof – but anything between east and west will work well; in extreme cases we can install an extra collector to compensate if the aspect of your roof is not ideal.
Enough space to fit the panels on your roof (panels are roughly 1m by 2m, and most homes need two);
Space for a solar water cylinder.

The roof should ideally be at 30 and 50 degrees to the horizontal, but there is some flexibility (36º is said to be the optimum for the UK). We can also install collectors on flat roofs using metal framework. Unless you already have a water cylinder with a solar coil, we will need to replace your existing hot water cylinder with a new one that has an additional heat exchanger coil (this new cylinder is included in our prices). Typically the new cylinders are larger than ones we replace, and are preferably tall and thin. We are confident that we can fit solar water heating to almost all buildings. Our technical team have a wealth of practical experience and can often design solar solutions for older or more difficult properties.

Will I need planning permission?

No usually. Planning permission is needed for:

Listed buildings;
Installations on walls (not roofs!) of buildings in Conservation Areas if visible from a public highway;
Installations that protrude above the fidge line or project more than 200 mm above the roof or wall (applies mainly to installations on frames on flat roofs).

For details and confirmation see the page on planning permission for solar on the Government's Planning Portal website.

Where planning permission is needed, we can advise on whether it is likely to be granted. If so we can prepare the application and all its supporting documents for you, for a small additional charge. We have not had one turned down yet!

Designing and building a new house?

See our Fact sheet for Project managers, builders and self builders. This factsheet outlines the various components in a solar system, with their sizes and usual locations, so providing what you need at the design stage. Includes information on thermal stores and mains-pressure water systems.

How much money will I save on my fuel bills?

The average house will save around £300 per year by installing a solar water heating system. This figure could be higher and it could be lower. It is a very difficult estimate to make because it is dependent on so many factors: how much hot water you use, how often and at what times of the day you heat your water, the type of fuel you use to heat water, the efficiency of your boiler and how well it is maintained, for example. Using data from our test site we've done some complex calculations based on recent fuel prices and we think that £300 is about the right order of magnitude for most homes; however it will dependon your circumstances. What we can be certain of is that a well designed solar system will provide you with 60-70% of your hot water needs.

What is the payback period?

If the benefit is from fuel saving alone, the direct payback for most people is around 10 to 12 years. This of course depends on lots of factors: all the issues in the above item, how much you paid for the system in the first place, and what happens to fuel prices in the future. If fuel prices continue to rise as sharply as they have done recently, then this payback could be much shorter.

However, for installations after July 2009, payback will be greatly shortened. Under the Renewable Heat Incentive (RHI), government will pay you a fixed amount each quarter for 20 years, starting from 2012. We do not yet know the exact amount but expect it to be around £200-£300 per year for a typical Soltrac system. The latest announcement is outlined and analysed on our page on the RHI.

Remember too that with interest rates so low, a few thousand pounds in the bank will only return you 1 or 2% interest (100 or 50 years payback!). If instead you invested that money in your property you will get a much more certain return by reducing your fuel bil. You will also increase the value of your property as well. Research has shown that a solar water heating system can make your property more attractive to buyers and increase the value of your home by around £3500. So it is worthwhile considering solar from a number of angles and not just to look at direct payback periods.

I've heard that you don't need direct sunlight, and that you can get hot water even on cloudy days?

There are lots of exaggerated claims about the performance of solar water heating. Because of the data we collect at our research facility we can be confident in the claims we make. On a cloudy day in the summer, the system should heat up and you will get hot water – maybe not right up to 60°C, but hot enough for a bath or shower. However, on a dark day in the depths of winter the system will do very little unless the sun shines.

The vacuum tubes perform better, don't they?

No, we do not believe so. As a company we started by installing only vacuum tubes but soon the evidence from our test site was that flat plates perform better for the temperature range needed for domestic hot water. The benefit of the tubes kicks in when the water has to be heated hotter than 60ºC. Typically temeratures above 60º are only needed for industrial applications.

At Soltrac we've invested a huge amount of time doing our research to make sure that we are installing panels of the very highest specification. Experience from our repair work, on systems not installed by us, is that the tubes are much more prone to breakdown, dependent as they are on the vacuum for their performance. Perhaps the most interesting statistic we uncovered on this is that in Germany, which can be considered a mature market for solar, some 80% of domestic installations are done with flat plate collectors, and only some 20% with evacuated tube collectors. However, we have selected the best of both technology and are happy to install either system. See our Products pages for more information.

Can solar also heat my house as well?

We do not think this is usually a good option. The amount of energy you'll get from the sun in winter (when you want the radiators on) is simply too low for solar to make a meaningful contribution. The average solar energy on a December day is one tenth of that on an average July day. When sun levels drop and October comes, you switch on the central heating, but by then the solar is struggling to heat the water to full temperature, let alone having spare capacity to heat the house as well!

Systems that attempt to heat the radiators as well as the water can even reduce the overall performance of the solar panels. The flow temperature for domestic radiators is around 70º. We recently were called out to inspect a system with a thermal store that was heated by a conventional boiler and by solar panels; the radiators were fed from the thermal store. To keep the house warm, the thermal store was heated entirely to 70º all the time. In winter the solar panels will often heat water from say 10 to 40º, but will rarely if ever contribute anything if the water is already above 60º. Thus the so-called "heating assist" function installed for this customer has entirely prevented the solar panels doing any useful work outside summer.

The only way that solar panels can effectively contribute to space heating is if you have an under-floor heating system. The water in these runs at 30–40°C, which is closer to solar can achieve on a sunny winter's day. A separate thermal store would be used with a solar heating coil at the bottom and this would enable the solar to make a contribution to the space heating.

Unless the house is new and built to comply with or even exceed the thermal insulation levels stipulated by current Building Regulations, we recommend that the solar panels heat just the domestic hot water in a twin-coil cylinder. Insulation and draught-proofing are the best ways to reduce space heating costs, not solar panels.

Can I use solar to heat my swimming pool?

Yes! Using solar to heat your swimming pool is an excellent application of the technology. For indoor pools we use a closed pressurized system in which the water heated by the solar panels heats the swimming pool water through a heat exchanger. It can either replace or work in conjunction with an existing pool boiler, and makes use of the existing pool pump. We have done a number of pool installations. One of our clients kindly recorded some data on the performance of their swimming pool system, which you can view by clicking here. You will see that for almost all the swimming season, the solar system heated to pool to over 25°C (a good swimming temperature). Combined systems that heat both the domestic hot water and the pool are excellent as it means that the solar system can be 'sized up', giving better solar contributions, as the pool can take any excess heat in summer. Solar heating is the greenest option for heating a pool, but because of the large number of panels needed – typically with an absorber area about half that of the pool's surface area – it can at times be quite expensive.

For un-heated outdoor pools, we recommend the much cheaper Öku system from Germany. This uses plastic panels, through which the swimming pool water can be pumped directly. The system drains down automatically when the pumps is not running thereby preventing frost damage in winter. Again panels extending to about half the surface area of the pool are needed. Please ask for details.

Heated outdoor pools with a target tempeature above 25°C are less suitable for solar. A much larger area (in excess of 50% of the pool area) would be required to make a significant contribution.

How much electricity does the system use?

Very little. The pump is uses around 30 watts (less than a conventional light bulb), and this pump is only switched on when the panel is hotter than the water in the cylinder. We do not have exact figures for the controller but their energy use is very low, too (less than 10 watts).

What happens if there is a power cut?

The system can cope with this. If it is dark or the sun is not shining, there is no problem at all. If the sun is shining bright, then soon after the pump stops, the water in the collector will vapourize. The volume increases, and is taken up by the expansion vessel included in the solar circuit. In fact the volume of water in the collector at any time is small. The vapour remains in the collector, not moving round the circuit, until nightfall when it condenses back to water. The system then rights itself without user intervention and, if the power has been restored, starts again as normal the next morning. If this happens occasionally, it is not a problem; if it happens too often, then the antifreeze may start to congeal and should be replaced.

Can I use a photovoltaic (PV) solar panel to power the pump and controller?

This is not something we recommend. If run off the mains, the pump uses 30-40 watts when on and is likely to be running for 1000-2000 hours per year. The controller is on all the time but uses only a trivial amount of power. So replacing this power use by PV is effectively like buying a PV cell to power half a light-bulb elsewhere in the house. It can be done by using a 12-volt DV pump and simply providing it with power when the sun is shining. In our view this has two snags: we do not believe 12-volt pumps are as reliable as the tried and tested 240-volt ones. Secondly, one sacrifices all the information provided by the controller. There may also be times on dull days when the light is bright enough to provide heat in the panel but not enough to drive the pump. So, while a nice idea, powering the solar thermal system from a small dedicated PV panel is not really such a good idea

Could the system get too hot in summer?

Not in a way which is dangerous to you or to the integrity of the system. The panels will get very hot in the summer, especially on long periods in summer of full sun and if no water is being used. Because the panels we use are made to such high standards, these high temperatures will not damage the system. Additionally, because the solar controller controls the solar transfer fluid, it will prevent these high panel temperatures from ever raising the cylinder above 60°C. If you feel this is still too high, we can install a Themostatic Mixing Valve at the outlet of the hot water cylinder which will ensure that the temperature of the water at the taps is never above a set level.

Can you install solar onto a thatch roof?

No, not onto thatch. With panel temperatures up to 250°C not unheard of in summer we think this would be tempting fate. However, if you do have a thatched property, but have a suitable un-thatched outbuilding, a safe solution might be possible.

What about solar for my business?

Solar heating is a perfect technology and a sound investment for businesses that use a lot of hot water. For example: restaurants, hotels, breweries, food processing plants and dairies could all make significant savings. Soltrac have successfully installed systems for the Keystone Brewery in Berwick St Leonard, a dairy farm in Tisbury and for a McDonalds Drive-Thru restaurant in Southampton.

How much disruption will the installation cause?

Very little. Almost all our standard domestic installations are completed in two days. When our installers arrive on the first morning, your hot water system will be drained and disconnected while the new cylinder is installed, but you should have hot water available again by the same evening. Much of the work happens in the loft and on the roof so the disruption should be minimal.

If the installation is part of a new building or extension, the solar work will typically be done in two or more sessions. Our installers will first come to your site once the roof has been felted and battened, we'll then put the panels on the roof before the roof covering is added. If you are having other plumbing works done we will normally supply your plumber or heating engineer with our solar hot water cylinder (which is better for you because you only pay 5% VAT by getting it from us). Your plumber will then install our cylinder as part of the new pipework. We then return after the cylinder has been installed and the electrics are done, to connect up the solar circuit.

Do you need to do anything to my boiler?

Not normally, no. In most cases we are only dealing with the hot water cylinder and so do not need to do anything to your boiler circuit. We sometimes recommend minor modifications to the controls of older systems to enable the solar to give you the best savings.

I've got a combi boiler, will solar work with it?

Yes, solar can be made to work with a combi boiler. The first thing you will have to consider is that you are going to need to find space for a hot water cylinder (you can't do solar without a cylinder). Once we've found this space, interfacing the solar to the combi requires some additional parts and plumbing so these systems do cost slightly more than our standard systems. Please feel free to contact us with questions.

If your combi boiler is more than 5 years old, we recommend that you consider replacing it with a normal boiler. Most combi boilers have a life expectancy of 10 years, and it would not make financial sense to install a more costly solar thermal system to accommodate the combi boiler when it's not got much life left.

Can you give me a rough price on a system?

Yes, certainly. We like to do things differently at Soltrac and so we publish our prices up-front. Please take a look at our Price List to get an idea and then give us a call if you've got any questions.

How quickly can you give me a quote?

We will need to visit your home to conduct a site survey. This is NOT a sales call and we are not going to spend hours trying to sell you a system – it is a technical survey and should take no longer than 45 minutes. Once we've collected the information we designed a personalised solar solution for your property and send you a quotation within a working week.

If I place an order how soon can you come round and install the system?

Typically we have a lead-time of around 6 to 8 weeks from the date of contract to the date of starting the installation.

Can I get a Government grant?

Sadly no! The previous £400 grant under the Low Carbon Buildings Programme ended on 24 May 2010. Instead all installations after July 2009 by qualified installers will receive quarterly payments from the Government for 20 years - see our page on the Renewable Heat Incentive for details.

If you've anyother questions of a general nature, please email them to us and we will consider adding the question and answer to this page. Alternatively please don't hesitate to give us a call.