We have recently fitted a solar thermal system to our house to provide hot water.  After a fair bit of research and looking at the grants scheme, we decided to do a DIY installation.  Although grants were available for systems installed by accredited companies, very few people had managed to get the grants due to demand.

The system is now up and running and this article will give you a bit of background and tips should you want to do something similar yourself.

A few things helped us decide to go ahead, we had a South facing roof with no over shadowing, traditional boiler & hot water tank setup, I am competent at basic plumbing and no planning permission is needed for solar installations anymore

After much research on the net, it looked like a commercial installation would cost us somewhere in the region of £2,500 to £4,500 or more dependant on panel size etc.  Not being afraid of a little DIY, I decided to look at purchasing all the components myself and fitting it.  The hope was to save at least a £1,000 by doing it ourselves.

There are lots of good reference sources on the net for installations like these so that was my first port of call.  After a lot of reading it was clear that there were numerous options open to us regarding components and hence the overall cost of the system.

The main points for consideration were

1] Flat panel or evacuated tube.  This was an easy decision to make, flat panels are cheaper but not very efficient in colder weather.  They are therefore not ideal for the UK climate and considering that we probably use more hot water in the winter, we opted for an evacuated tube panel.

2] Basic solar controller or one with more advanced features such as the ability to connect a heat dump & variable pump speed control.  These controllers are the heart of the system and do the clever stuff like check that the solar panel is hotter than your hot water cylinder before pumping (otherwise you could cool down your water!).  We opted for a more advanced model, the Resol BS/4.   This is a popular controller and has a lot of options such as,

• Up to 4 temperature sensors (more on why you want this later)
• Variable pump speed control
• Heat dump (auxiliary circuit control)

3] Standard plumbing fittings or solar rated ones.  There are a lot of people out there that have used standard plumbing fittings like the expansion vessel, auto air vents etc.  Standard plumbing fittings are significantly cheaper than ones rated for solar use (half the price for some components) but is there a problem using them or is labeling something for solar use just a way for manufacturers to charge more for some products?

Well it turns out that lots of people who used standard plumbing fittings are now having problems with their installations a couple of years down the line.  Evacuated tube panels can get extremely hot.  So hot (200 C near the panel) that standard plumbing components will start to break down over time.  Solar fittings are usually rated for much higher temperatures and with a little careful planning of your installation they should more than justify the additional cost through longevity of your system.  We opted for solar rated components wherever necessarily, i.e. expansion vessel, air vent etc.

4] Pump station or individual components.  It is possible to buy a number of the components you need already combined into a single unit called a pump-station.  This will usually comprise of the pump, check valve, flow meter, pressure gauge, fill/drain connections, expansion vessel outlet and emergency release valve.  We decided to go for one of these as it simplifies the installation and comes in a thermally insulated enclosure.  The model we selected was the Resol Flowcon A that came with the Resol BS/4 controller integrated into it.  The good bit here is that you can remove the controller from the pump station and situate it somewhere more convenient.  In our case we put the pump-station in the attic and the controller in the airing cupboard.  You just need to extend the power cord from the controler to the pump to do this.

The diagram below shows how an evacuated tube works in principle.  The copper tips on these get extremely hot so watch out if you are installing on a bright day!

The entire circuit looks like this,

You generally have to replace your hot water cylinder for a solar installation as you need an additional coil within it to transfer the heat generated by the solar panel.  It is possible to retrofit a second coil such as ones that replace your immersion heater but these tend to be quite inefficient and a purpose built cylinder is the way to go.

We opted to use a solar cylinder which had the same capacity as our old cylinder (114 litres) and required only minimal modifications to our existing plumbing.  The first job in the solar installation was therefore to swap out our old cylinder.  This went pretty well and we were quickly able to get our hot water back in operation as the solar circuit is entirely seperate and I just left those pipes disconnected for now.  Solar cylinders also come with at least 2 sensor ports for you to insert the temperature sensors from your controller.  Usually one near the top and one about a 1/3 of the way up from the bottom.

Interestingly our old cylinder was absolutely caked in limescale with around 25kg of limescale sitting in the bottom and deposits up to 1cm thick around the coil.  This means our old cylinder was extremely inefficient.  We emptied all this out as we decided to make use of the old cylinder later on.

We purchased the majority of solar components (except the panel & cylinder) from Economical.  They had the best prices I could find and had a lot of really useful information on their website, including a checklist for all the bits you might need.  I got the panel on Ebay as I had a discount voucher.  The panel specifications are attached below but it has 30 1500mm x 47mm tubes and a max output of 155 Litres at 60C.

Choosing your components, specifically your cylinder & panel sizes can be really tricky.  I found there were a number of calculators out there and lots of advice, much of it contradictory.  Many commercial installations are sized so that the panel will on a sunny day in the summer, heat a full tank of hot water and no more.  This is done so that the system does not generate any excess energy which then has nowhere to go.  Unfortunately this means that you are really only getting a useful amount of energy from your panel during the warmer months of the year.

Another approach is to install a much larger panel and you can then get some useful energy even through the colder months.  The problem here though is that during a very hot day you will raise the temperature of your hot water tank to the maximum you set (60C-90C) and there could still be more energy being generated.  At this point, the majority of systems installed will shut down the pump to avoid heating your tank to dangerous temperatures.  This is called 'stagnation' and means that the solar circuit is no longer in operation and the system will now get very hot.

This is where it is really key for Solar rated components to be used and only compression fittings as pipework near the collector can get hot enough to melt soldered joints.  You need to ensure that you scale a system to stagnate safely which in practical terms means having an expansion vessel large enough to accomodate the increase in volume (an 18 litre expansion vessel will nearly always be sufficient) and to make sure that components like your pump station have at least 2 metres  of pipework between then and the collector to help them avoid the highest temperatures.

Allowing your system to 'stagnate' safely is important but ideally you want to have somewhere for that extra heat to go and therefore avoid cooking your system.  Good system installers will therefore fit a 'heat-dump' circuit.  This is a secondary circuit, shown in blue on the diagram above in which the solar circuit switches from heating your hot water tank and instead heats something like a radiator installed in your attic.  If you have something else you can use the excess heat for like warming a swimming pool or hot tub then even better.  Your solar controller manages this process with a sensor near the top of your hot water tank that detects when your store reaches the maximum temperature that you set.

Many commercial installers will not fit a heat dump circuit and instead you will have a system sized just for the hottest days of the year.  However, I would really recommend considering fitting one as it a) allows you to gain more energy throughout the year, b) improves the life of your system by preventing 'stagnation' and c) will pay for itself in terms of extra energy gained, especially if you can find a second hand radiator to use.

The main components of the heat-dump circuit are

• Radiator or secondary water store etc.
• 3-port motorised valve
• Solar controller with a heat-dump function (such as a Resol BS3 or BS4)

We decided to make use of our old water cylinder (now emptied of limescale) and put this in the attic.  The idea was that this would double the thermal capacity of our system which should be enough for even the hottest days and with the insulation taken off this cylinder, it would radiate its heat very effectively.  A couple of gate valves were fitted near the cylinder connections just in case we wanted to swap this out for a radiator instead.

The next stage of the install was to spend a weekend doing all the plumbing in the attic and running pipes to the cylinder.  You can see what this looks like in the here.  One thing to consider is the pressure of your expansion vessel.  These will usually come pre-charged (pressurised to a set amount) and you need to check that this is the value you want.  The instructions for my collector indicated that the cold system pressure should be 2bar but the expansion vessel was pre-charged to 3bar.  Its easy enough the change, there is usually a car tyre valve on the expansion vessel (might be hidden under a cover on the bottom) and you just release or incresse the pressure accordingly and check it with a normal car pressure gauge.  You can use a bike pump to increase the pressure if needed.

The pitch of our roof is fairly steep so we had to rent an aluminium tower to make things a little easier.  The solar collector comprises of a manifold at the top through which you run the Propylene Glycol (antifreeze solution) and it extracts the heat from the evacuated tubes.  You fit the manifold and frame first and connect up the plumbing, the fitting of the evacuated tubes is the very last thing to do.

There are two ways to mount the collector on a pitched roof.  The first is to use straps that tuck under your tiles and hook over the roofing batons.  The second is the option we went for which was to drill through the tiles and use large purpose made bolts.  This was fairly easy to do and a maisontry drill bit (not using hammer action) went through the tiles cleanly.  We lined up one side of the manifold so the holes would meet up with a roofing joist and on the other side we had to add some packing to the nearest joist to have something to drill into.  You then wind in the roofing bolts and you can then bolt your manifold to the exposed heads of these 4 bolts.

To drill the holes for the pipes to come through I used a 17mm Diamond Core Drill bit from B&Q which cost about £6.  This was perfect and again, not using hammer action it went through the tiles cleanly.  The manifold has 22mm pipe coming out the end and I took this down to 15mm with reducing compression fittings before coming through the tiles.  Often the auto air vent is fitted at one end of the manifold here at the highest point of the circuit but given the difficult access to our roof, I opted to fit this in the attic instead.

You can buy lead flashing with a silicon grommet to help seal the pipework as it comes through your tiles but I opted to seal around both the pipe and roofing bolts with high temperature silicon.  If you block the hole from inside the attic you can force the silicon around the pipe very well and we have had no issues with any rain getting through.  Make sure you clean the dust from the hole first so the silicon can get a good grip.

Once the collector is mounted you can connect up the pipework in the attic and you then have a complete solar circuit.  The next stage is to fill your system and test for leaks.  You a pump of some sort and I opted for the cheap solution recommended by Economical of using a good quality pump-up garden sprayer which came supplied by Economical with some fittings that connect to the pump-station.

The system is filled with an antifreeze solution of Propylene Glycol.  This has good thermal properties (transfers and absorbs heat well) and will protect your system in the winter from freezing.  It comes pre-mixed and 10 litres should be enough for most systems, even with a heat-dump circuit fitted.

To fill the circuit, you need to follow the instructions that come with your pump station.  Its quite a long process of opening and closing valves, refilling your water pump but once you have the air removed, you raise the pressure of the system as high as your pump will allow or to about 5-6 bar and let it sit there for a few hours to see if the pressure drops.  Your pump station will probably have a pressure gauge which makes this easy.  Some air will probably come out and you leave your auto air vent open so help drive any out so expect a little drop initially but check all your joints for any weeping and tighten if needed.

I did not fit any of the pipe insulation at this stage to make it easier to see any leaks.  A quick note about pipe insulation is that you really do need solar rated high temperature insulation.  Normal pipe insulation will melt, especially close to the collector or withina few metres of it.  Also make sure you insulate any exposed pipework, the system will be much more efficient if you can prevent unnecessary heat losses.

Part of the filling process is to run the pump at full speed to also drive out air, this released a lot in my installation so expect to top up your system during the filling and checking process.

Once you are happy that everything is ok it is time to install the evacuated tubes into the manifold.  You should have a small tube of thermal transfer paste come with your tubes, this helps to make a good tranfer of heat between the tip of the tube and the manifold.  You will need to smear a tiny amount of this around the tip before pushing it into the manifold.  One tube will be plenty!

There is usually a rubber bung on the manifold that seals around the evacuated tube as you push it in.  Your tube will go in much more smoothly if you put a little soapy water around the top of the tube.  Insert the tubes one by one making sure they all go in all the way.  My collector then had little plastic cups at the bottom to secure the tubes but other makes may be different.

I chose to fit the tubes in the late afternoon when the sun had gone down as they can get extremely hot, wear gloves and have your pump running if you are installing in bright daylight.  Even in a coudy day the tubes will warm up quickly.  Keeping them covered until you are ready to use them is sensible.

That is then about it.  I waited for a sunny day to check again for leaks before fitting the insulation but once the tubes are in, you set your controller to automatic and you should hopefully have many years of trouble free life.  The auto air vent will bleed out any remaining air in the system so keep an eye on the pressure initially and top up as required.  After a week or two you should be able to close off the valve on the auto air vent as it will no longer be needed.

The image at the top of this page shows our completed installation.  We fitted our collector in September 2009 and had quite a few days already in September where our boiler was not needed at all to heat our water.  We have set the boiler controller to only heat our hot water in the evening so if the solar panel has not had enough sunlight to heat the tank fully, the boiler then tops it up.  This makes sure we always have hot water to use but also get the most our of the solar. Overall it was a really fun project and the DIY option is certainly within the capabilities of a competent DIYer.

I hope this helps if you are considering a solar thermal installation.  We spent approximitely £1,500 so kept to budget.  I will attach all the details about the parts we used below when I can get them together.  Use the comments area below to post any questions, it would also be great to hear abut anyone elses experience with solar thermal.

UPDATE

Based on some comments below, I wanted to provide an update on this installation.  

The old hot water cylinder is serving well as a heat dump (minus its insulation) but I am only using it as that for the moment, it isn't serving in a pre-heat capacity for the main tank so no additional capacity.  I was hoping to do something like that but there was no easy way to get a feed into the second tank as the only space I had was in the attic and so it is as the same height as the cold water tank so no head available.  

If I had the space, I would have fitted either a larger primary cylinder or a second cylinder next to the primary so there wouldn't have been any issues with pressure then.  The other option would be to use mains pressure cylinders but that would have required some major changes and cost.  

In reality my setup works well.  The solar panel generates all the hot water we need in the summer so the boiler typically doesn't run at all during days when there is even just a few hours of sun.  During the winter the heat dump doesn't get any use at all so no issue there.  

I have set a maximum temperature of my hot water tank to be 60 degrees C and at that point the heat dump becomes active.  I am probably going to change this system though by increasing the maximum to 80 degrees C before the heat dump kicks in but I will be fitting a thermostatic mixing valve on the output of the cylinder.  This just mixes a feed from your cold water tank with the hot water and you can set it to say 60 degrees C.  In effect this will increase the capacity of my hot water cylinder.  

If you have a gravity fed (low pressure vented system) such as mine then you do run the risk of the mixing valve reducing your flow to the shower etc.  However, you can buy thermostatic valves designed for low pressure systems and the company I used for most of my bits http://www.eco-nomical.co.uk/index.htm has these in stock.  

So, after living with the system for quite a while now I would suggest the following, 

1] Fit a large primary cylinder

2] Fit a thermostatic mixing valve to the output of your tank and set a maximum temperature of 80-85 degrees before starting to dump heat

3] Definitely have some sort of heat dump in your circuit.  This means your installation can be large enough to generate useful heat in the winter but you have somewhere for that excess heat to go in the summer.

4] Make sure your hot water heating controller can be programmed to only come on once at a pre-determined time each day.  We set ours to come on in the evening after dusk so that it only tops up the tank temperature when needed.  Should additional water every be needed then we just press the boost button which gives us enough for a bath full.    Our water tank thermostat (linked to the boiler) is set to 50 degrees C which provides water at around 60 degrees C at the top of the cylinder and enough water for the family to all have showers etc.  

During the winter months we were able to get our tank up to 45 degrees C on clear days and at least 30 Degrees C even on overcast days so a lot less work for our boiler to do.