Thank you to fuelexplorer for this article: 

I did my own solar thermal panel installation, on the slate roof of an old Victorian house, with not the most accessible roof in the world!  Am I pleased with the result? Yes. Could I have designed the system better? Yes.

I did quite a bit of research before installing my solar water heating panels. Yes I did speak to various vendors, and very good some of them were too.  I am sure some panels are much of a muchness, and I spoke to a lot of techies, before making my mind up.  Did I make any mistakes?  Yes. That's why I'm scribbling this blog, so that others can learn from (and avoid) them.

I checked the most obvious things like choosing a suitable unshaded south facing site, sizing, insulating all the pipes etc.  Though an effective installation goes much further.  I did decide to purchase my units directly from a distributor and install the system myself, as I was in the throws of replacing my very old oil fired boiler, and steel single pipe balanced central heating system.  That was my starting point.  Others have actually fabricated their own units, I decided to buy, as I had enough on my plate to begin with.

A couple of my thought processes were ...
1) Which are most efficient solar hot water devices, flat face or tubular systems?
2) A tube system ...  which design; double evacuated or single walled?
3) Should the sun heat my stored water directly or indirectly?
4) How should I size my solar unit for the hot water cylinder size I want?
5) If I was to use an indirect system, how was I to get the most efficient heat transfer from the system into my cylinder water?

Now I don't claim to be the Oracle or to have made all the right decisions, though these are mine for what they are worth.  Get 3 professors in a room and try to get them to agree to a decision ... it's impossible although they are all experts, so a compromise is the most likely. These are the answers correct or otherwise that resulted from my research.

1) I thought that a tube system was most probably the most effective in temperate climes, though the flat plate design I was advised is more efficient with the sun overhead.  How often does that happen in the UK?

2) The evacuated double tube, takes longer to warm up and cool, than the single wall equivalent. Though I thought the double greenhouse effect of the evacuated tube would be better, as once warmed, it would offer a more even temperature gradient, than perhaps a stop-start of the more reactive single walled design.  I'm on shaky ground here as I didn't manage to get any figures to support my decision!

3) I chose the indirect method of heating water for the 2 reasons I thought were logical. (By indirect I mean that my system didn't directly heat the cylinder water in the tubes, but heated continuously recyled water). The tubes were discretely modular and any tube could be replaced in the header manifold very simply, without affecting any other part of the system.  It seemed a neat solution, though maybe not quite as efficient as water running though the tubes themselves. The next reason was that I tried to keep the recycled water volume to a minimum to reduce latent heat losses.

4) We have 2 showers, a bath, 2 lavatory handbasins, a kitchen sink, a utility room double sink, washing machine and dishwasher, central and underfloor heating.  I decided to have a 300 litre cylinder (heatstore), into which the solar panels would contribute.  I chose a 2 x 20 pipe system, which in my case was, in the hot period this May, raising 300 litres at 3°C per hour.

5) Most hot water cylinders that are solar compatible pass the heated water through an inner additional coil.  I thought a heat exchanger would be more efficient, and take up less space in the cylinder, and went with that. I heat my hot water by passing mains cold through a heat exchanger on the heat store.  Any additional heat required for hot water can be boosted to 65°C via my new gas condensing boiler!

Lessons learned. 

A) Scaffolding was required to locate the two 20-pipe units on my slate roof. As I was repairing the roof at the time I did save a little on the scaffolding costs.  Shop around!

B) Keep the pipe run from the solar units to your hot water cylinder as short as possible.  Although all my pipes are lagged, I still think I'm losing about 300w/ph on the long run I have.

C) The heat exchanger for the solar heat/cylinder needs to have the cylinder water pumped though the heat exchanger to make it efficient.  My temperature flow and return delta is about 20°C at 70°C flow.

I would like to add another two panels!  The newer ones operate at 15% better efficiency too! The cost of the 2x20 tube system with management controller & pump was £1250.

If you have a question about anything in the above blog, please ask it in the comments section below.