Central heating and hot water

Yet another thing we have been thinking about is the central heating and hot water. We are keen to have something that uses wood as we have a 'free' supply (the quotes being because it takes a lot of labour, so it doesn't cost money but it does cost time and there is some investment in saws, fuel, oil etc). We are also interested in solar hot water (rather than PV) as it would reduce the amount of wood we needed to provide and it would seem a waste of a perfect south facing roof not to have it.

Our current thinking is that we will have a wood gasification boiler in the garage with an accumulation tank. The solar panel would then also feed the accumulation tank. The idea is that you burn the wood furnace very hot (which is efficient) every day or every other day and heat the water in the tank up to 90C at the top. The water in the accumulation tank is kept stratified (ie hot at the top and cool at the bottom) and the tank is very very well insulated so it can maintain close to that temperature for a couple of days.
Hot water (DHW) for showers etc is taken from the top of the tank via heat exchange (ie there is a coil of pipe in the accumulator through which your mains pressure water is run to heat it up- often there are 2 coils, one low down in the cooler part of the tank and one in the hotter part)
The heating runs using water directly from the tank. Radiators would take water from the top and return it cool to the bottom and under floor heating would take it from the middle (as it runs cooler than radiators) and return it to the bottom. We are thinking underfloor heating downstairs and radiators upstairs (although possibly underfloor in the bathroom upstairs too)

The wood furnace would provide the main amount of hot water and the solar would feed in to this to reduce the amount of wood needed. In the winter the solar wouldn't be doing much and you would need to fire the wood furnace perhaps every day. In the summer the solar would provide a higher percentage but you would still need to fire the furnace every once in a while (every few days???)

It seems that the complex thing in systems like this is keeping the tank properly stratified as you feed hot water in (from your furnace and solar panels) and take hot water out (for heating and DHW) plus feeding cooler water back in (returning from the heating system). To make this work there are also some controls outside the accumulation tank which decide whether to feed water in or not. For example if the water returning from the radiators is above a certain temperature it would be recirculated rather than being fed back in. If water that is too warm goes in to the bottom of the tank it will rise mixing the whole tank up meaning the top of the tank is no longer hot enough to work the heat exchange which is providing your DHW.

I have read a lot on the internet about this and my mind is boggled as there are so many varieties of the different components. We have also spoken (not yet at great length) with the plumber (who specialises in renewable and green heating) - as you can imagine this is a bit of a challenge in French. He is an agent for a company called De Dietrich who do provide solar panels, wood furnaces and accumulation tanks, but as far as I can understand from their very unhelpful website they don't plan on them being configured as we want. In their view the wood furnace is only for use in the winter and only provides hot water for heating not for DHW. You then need an electric immersion heater to provide the DHW when solar is not enough. I think they are a French company and we have noticed the French use electric for heating and hotwater unlike in the UK where electric is considered too expensive and the most common I suppose is gas. We are not keen on this solution, we would have an immersion heater but only for use once in a while when one had failed to run the furnace or something.

It is hard to get a vision of what such a system might cost. The wood burning furnaces are not cheap and nor is a solar hot water system, but the idea is it would then be very cheap to run. Virtually free infact. Once we have the floor plans sorted (if that ever happens) we can speak in more detail to the plumber and get an actual quote (1 million EUR perhaps).

Akvaterm seem most common accumulator in the UK

Alternatives might be to use one of the wood burning stoves in the kitchen or living room to provide hot water, this would be cheaper to install, but would leave us more dependent on electric heating for hotwater in the summer when solar wasn't enough and in the winter if we wanted hot water when we didn't want the stove lit.