Solar soluation and 1200 litre cylinder

Again: get your energy demand calculated.If you have no low temperature heating like UFH or wall heating then it makes no financial or enviromental sense to install a solar thermal heating system for providing heat to the space heating demand. It can be done technically but not economically or ecologically justified.If you're installing a large storage tank and have a large output from the collector then go ahead, install a wall heating system (there are ready made wal-heating panels available, dry building system) extra to the radioators and use the radiators as a back-up.The old ball park figure for domestic hot water demand is 50 liters per head. The newer rule of thumb is 30 liters per head.Half of the stored heat will be lost by storage losses within two days- if not used.So plan 100 liters of storage capacity per head living in the household and with the installation of evacuated tubes and a well insulated and installed storage tank you'll meet more then 60% of your anual DHW demand.Once you have 21mm insulated supply pipes installed (from and to the collector field) you can easily ad more collectors to an already existing collector field. The solar control station will be the same for a larger domestic installation as for a smaller one.And a second tank can be integrated with the old installation.Provided there is space somewhere.Always connect the washing machine and the dishwasher to the installation. Replacing electric heating energy with solar heating energy makes sense.There are two major systems to make the system "summerproof":first the standard method of installing an expansion vat where the steam can go without exploding the system. Secondly the drain-back system where the water from the collector goes into a storage tank once a certain temperature in the collector is reached (about 95 degrees Celsius). And being pumped back once the temperature in the collector drops below that pre-set limit.The drain-back system is the better one for several reasons: it's risk free since pressure isn't build up at first, it costs very little and it avoids the frequent renewal of the collector liquid (the water/glycol mixture).Ask your installers for details and try to get some literature on the issue. It is possible to use a PV panel to run the solar pump , I'm doing that with my own home installation. This saves money in several forms: the electricity bill will be lower ( costly day time tarifs will be introduced soon together with "intelligent metering"),it saves the temperature sensors and it it easy to install, no cables to the socket(no socket!), no fuse to be blown.And you can use the smallest pump available on the market as well. "EU21" has patented the system, "AMK" from Switzerland is selling it here in Ireland. There are also a few newer enterpreneurs in this particular field, but I haven't seen their installations yet and so can't comment about.But you can build your own as well, using available technology. The PV panel can be very small, I use a CIS PV panel from "Wuerth", about 20cmx70cm.Most pump manufacturers are selling now solar pumps as well, not the old heating pumps but smaller ones, A-rated, running on very little (solar)energy.Perfectly suited to the task. The company "Laing" has the longest experience with PV pumps as far as I know.But "Grundfoss" and "Wilo" have jumped onto the wagon as well, it makes sense. This controll method (using PV to run a solar thermal collector pump) is called " Global Radiation Control".Check the www for it.It's simple, it's beautiful. And cheap.

See what is available and then check it out with the aid of the estif home page. You'll find there not only the individual test results of the manufacturer's reports but also a calculater, an Excel sheet. This you can use to type in your data (see how much these differ at different outside temperatures due to bad insulation and other technical details!) and the Excell sheet will tell you if your chosen collector is better or worse then average.http://estif.org/(The site might be under construction, try again)The evacuated tubes have indeed a better thermal insulation factor compared to most flat panel collectors. But check this out on a case to case base.Not only the (solar radiation collecting) tube- the glass body- has to be looked at but the entire ensemble incl. the horizontal collector tube has to be seen. This is the weak point with tube collector's thermal conductivity, the vacum itself insulates very well but the rest of the insulation is usually kept slim for aesthetical reasons. And this isn't good for the wallet, the efficiency.

Independent- You asked 3 questions and I will try be as basic as I can.
Q1. Will solar benefit rads – rads require above 60C – solar will not give these temps on many occasions over the year so not very helpful.
Q2. Sizing the system – you are buying a system which has a life expectancy of up to 30 years – how many persons in your house is probably the least relevant question – if you are only going to use to support your DHW – estimate your DHW requirement and assume a 60% to 70% energy saving.
Q3. Yes you can, but solve Q2 first.
In general you are looking at the next 30 years and today we tend to look at energy costs alone, but as we migrate from fossils fuels and deal with the challenges of climate change we need to consider these aspect against a new order of priorities