It is possible but takes time working it all out from first principles as there are no websites, books or training courses that I know of that cover this type of system. I wouldn't worry about turning on the UFH at certain times just turn it one when the thermal store has sufficient stored energy that you are now saving for the DHW. I appreciate that your drawing is not to scale but the first thing I would do is drop the UFH flow to mid-tank to ensure that you do not run out of DHW. To get the UFH on when the tank has enough energy you need a thermostat with a 15 - 20 degreee gap between it turning on and off (or off and on). Then link the sensor in the tank to the UFH pump so that it comes on when the temperature of the tank at midpoint is about 50 degrees; the pump can then run on demand from the UFH until the tank falls to 30 degrees at which point the pump is starved of electricity and stops. Works fine for me.

The problem I see is that the solar will have trouble heating the upper part of the tank as you are relying solely on stratification.

Look at the Akvaterm Solar Plus tanks - http://www.akvaterm.fi/eng/Accumulators/AKVA_SOLAR.41.html I really do think these have the best solution. UFH would normally be taken off the bottom half, but if needed can be topped up off the top. Solar will charge the top when hot enough, else the bottom. Use the boiler to heat the top only (as UFH will take it off there if necessary).

The only addition is a blending valve to mix the boiler input so it can be set to the ideal temperature for max efficiency.

You do not say how well insulated/airtigh the property is - ideally the UFH should be run as cool as possible, for as long as possible to keep a steady temperature.

BTW that is an enormous cylinder. Just got a 750L delivered and wow it seems huge!

Solar heats the top of my Solar Plus tank (750l) just fine. Have 8 sq m of flat thermal solar panels.

I'm not a heating system designer but I'm a bit concerned about the horizontal (orange?) connection from the boiler to the heating circuit!

The role of the system boiler is to put heat into the thermal store either: indirectly via a coil (probably no need for an external expansion tank); or directly where the fluid is shared with the boiler circuit, thermal store and heating circuit with a large external expansion tank required if the thermal store is unvented. I have the latter with an unvented 350l thermal store.

Domestic hot water is taken from the tank-in-tank and, depending on the temperature of the stored water may need to be blended with cold water to achieve a safe temperature. As this has a large quantity of stored water you will probably need a controller that ensures that this DHW achieves a temperature to control Legionella (my system produces DHW via a heat exchanger which avoids this problem).

A heating circuit takes heating fluid from one or more levels of the thermal store and blends it to achieve the target temperature. In my case, I use a bivalent (4-port) valve to blend between top; middle and return. A simpler option would be to blend from a single level and return. You might want to use separate heating circuits for UFH and radiators as they typically have different temperature requirements.

If you want an easy life I would suggest taking a serious look at a Viessmann system boiler with a Vitotrol 200 or 300 series controller. These controllers can be extended to handle two separate heating circuits with mixers and also a solar thermal system. The programming options include support for thermal stores (some boiler controllers don't). For various reasons, I went with a Vitotrol 200A and a Resol solar thermal controller. Getting the two to play nicely together was, to say the least, an interesting exercise!

However, my system is quite modest with a couple of solar collectors, 350l and nine radiators on one heating circuit.

The system you outline is of industrial proportions and you definitely don't want to be trying to sort out problems after it is installed! There are lots of things to consider in a complete system such as building regulations; expansion vessels; non-return valves; system cleaners; sensor positions; pipe insulation; smart pumps; thermal breaks and so on. A system of your size might want additional equipment such as low loss headers or other stuff.

Wish I could recommend a good source of information! In the end, I figured it out myself with the aid of manufacturer's technical documentation, an excellent plumber and a good electrician. I'm happy with the results but wouldn't want to go through that again!

Posted By: hairydudeI've been getting conflicting advice on thermal store size - some saying stick around 350l as anything bigger would swamp input from ST whilst others say the bigger the better

I think it is asking the wrong question.
What you need to ask is 'What can I reasonably expect the sun to supply'. You need weather data for that rather than the generic kWh.m^-2.year^-1, that tells you very little except how far north you are.
I think most people will agree that ST works best when sunny, so how often is it sunny every week? and when it is what is the intensity?

Go to this page and start downloading the text files, or ask the neighbour if you can get all his data as a text file:
http://www.wunderground.com/weatherstation/WXDailyHistory.asp?ID=IANGUSDU2&wuSelect=PWS
Then you can easily chart each day. Cloud cover is part of a chaotic system, but from the data you can work out the probability of when it will be sunny enough for any given week (very hard to to at the daily level, let the MET office do that for you on the 3 day forecast)
So if we assume that below 400 W/m^2 there is no useful contribution, then you need to count up the times it is above 400 in the given sampling time period (every 10 minutes or 0.167h).
Find the average intensity at those times.
Then multiply the two to get your Wh.
Yesterday you would have got 2820 Wh or 2.82 kWh.
There are some big assumptions in there though.
This one is yesterday up your way

Thanks guys,

Interesting, our store will serve both domestic hot water and UFH so the advice I've been given that 350litres would suffice seems to be erroneous or certainly conflicting. Pmusgrove, how is your system working in practice (how often is the electric backup required and is it an immersion or a coil from an electric boiler you use?). I assume you mean the PHPP software when you say passivhaus?

The downside of a large store is that you can easily end up with a lot of luke warm water. If your need is mainly for domestic hot water then better to match the store size to your daily DHW requirements.

There is a fundamental conflict when designing a solar thermal system for both underfloor heating and domestic hot water. For UFH you want to start the circulation pump as soon as the panels are above about 25 degrees C as this is all you need UFH, you will collect a lot more heat on most days & lose less to the environment by re-radiation. For DHW you want to start the circulation pump when the panels are above the temperature in the storage tank. This may take a lot longer to be achieved & on some days may never be achieved.

You can prioritise DHW by waiting to see if the panel temperature gets up to DHW temperatures. However, this may never be achieved & you could be missing out on the chance to provide UFH.

You can have multiple solar thermal coils in the thermal store or some kind of stratified loading system, but you still need to decide on the strategy as DHW temperatures will never be achieved if you send the solar thermal flow to the UFH.

One approach could be to have a summer/winter switch which prioritises DHW in summer & UFH in winter, but I've not seen this implemented on standard solar controllers.

David

Hi hairydude my thermal store was from a McDonald engineers in Scotland it was put installed in 2006 when my Solar thermal system was put in afterwards I realised I could have had modifications made to it.
But by then it was too late and in anycase insulating my house has been a bigger priority, we are a superhome now.

Arnyj

I note the thoughts about the woodburners but mrs hairydude insists on the woodburners from a visual / character perspective so that is an argument I wont win. What I need to do is find a woodburner with max output to store as opposed to room.
The UFH is in the structural slab rather than a screed so will work at lower temps with high thermal mass and in-slab temp sensors. This should fit with borpin's suggestion of a well stratified tank with UFH taken from bottom of tank. I'll discuss with Mcdonald.