This is a post for the technophiles out there who may be slightly interested in the heating and hot water system we installed in the Moulin. We suggest those who aren't into this to put the kettle on!
With the diminishing world supply of oil and gas controlled by distasteful companies and the prices climbing steadily into the stratosphere, we realised from the start that wood-fired heating with some solar input would be the way forward and once the capital investment was completed it would mean the only running cost would be our man-hours to cut logs. This would be especially eco-friendly if the wood used was cut from the land around the property and luckily we have plenty of decent wood to burn so we started doing some more in- depth research into the subject.
Initially, we hoped to cook, wash and heat the house using a wood-fired cooking range with back boiler (for a radiator system) but we struggled to find any of these appliances that could get anywhere near the 25kW requirement to heat the house (this figure was established by our plumber Klaus by calculating all the heat loss from the various materials used in each room in the house) so we decided to put a wood-fired boiler in the barn which is very close to the house.
This also had three knock-on advantages in that the site of the proposed solar panels was around 10m closer to this position (meaning shorter tubes to connect them and hence a smaller and cheaper pump), secondly that the serious mess of 10 cubic metres of logs being burnt every winter would not come into our kitchen but stay in the barn and finally the dedicated boilers available are simply far more efficient than a hybrid machine that tries to achieve cooking/hot water heating/heating in one and such a machine would need to be burnt at 100% of its capacity to heat the house which would make the kitchen unbearably hot.
Having decided to go for a wood boiler (also known as biomass boiler) our plumber recommended a German company called Buderus (part of the BOSCH group) and in particular a new technology call wood gasification. Effectively the wood is turned to gas at quite a high temperature as air is forced down onto the wood. The flame is pushed down through a slot under the logs and the gasses burn again in a lower chamber reaching a temperature up to 1100 degrees C. The boiler achieves 85% efficiency and very few cinders are produced (removal once a week is sufficient). Here is a link to the sales docs in French (not available in English unfortunately)
http://www.buderus.fr/files/200904271112190.Logano%20s121%20_%2066%20100%20100.02-08%20pdf.pdf
After researching the subject in depth, we learnt that in order to maximise the benefits of these boilers and to increase the ease of use, it is very useful to feed the heat produced into a massive water tank known as a heat store (we chose 1250L) which takes the massive amount of energy produced in the relatively quick burn period and stores it to be then used for the heating or hot water throughout the day. Often the space heating and hot water in the house is required in the morning and evening whereas the boiler might only be lit once or twice a day. Without a heat store, you have to keep the boiler burning slowly 24 hours a day which ruins the efficiency and quickly fills the chimney with soot and tar. Heat stores are also fantastic for solar systems as they take heat in all day and make it available for your use in the evening or even 3 days later if you have a cloudy period.
The decision to add a large set of solar thermal panels (not to produce electricity just heat for the heat store) was influenced by three reasons - firstly in summer it is pretty disagreeable to have to light a wood boiler (it just gets super hot in the boiler room), the panels are large enough to contribute to winter heating by preheating the water in the tank and most importantly the French government gives grants to install solar systems (which includes the heat store tank which reduces the cost of that by 45%) which brings the total cost of the installations down by around 30% in total. Rather than put them on the roof of the house which we found pretty ugly, we placed them on a concrete slab close to our car port.
The radiators circuit is supplied directly from the water in the tank itself in a closed loop. This is pumped by a standard central heating pump controlled by a digital thermostat in the house (we were lucky enough to have spare cabling going from the fuse board to the boiler room during the rewire of the boiler room so these were used for the thermostat controller).
For the domestic hot water production we had the choice of two systems - firstly a tank in tank system where the hot water of the large heat store tank heats a second tank within the large tank or secondly a seperate heat exchanger outside the tank where when a tap is opened, a pump will take hot water from the heat store and run it past cold flowing water from the mains supply which instantaneously heats the cold supply and sends it into the hot taps in the house. We chose the second solution because it is safer for potential legionella problems (large quantities of domestic hot water are not stored for long periods hence the push towards combi boilers that heat water instantaneously in the uk) and because the second solution does not hinder stratification of the water in the heat store.
Stratification is all about having very hot water at the top of the heat store (which is where you need it to supply the hot water or heating production) while the bottom of the tank might be cold or luke warm. What you are trying to avoid is heating all the tank at the same time and waiting a long time while the whole 1250L goes from cold through luke warm to moderately warm water which will not really give you a good shower or warm the house up much.
This is the control panel on the instantaneous hot water exchange module;
And the position of the hot water module and central heating pump;
All of the equipment except the wood burning boiler was supplied by Klaus the super efficient German plumber who we found on an internet forum and we are grateful for his exceptionally neat work, honest approach to delivering a top quality system and generally being a top man. All the equipment was ordered from Germany and the quality is excellent. Klaus prefers to order equipment that he knows he can fit with confidence that it won't fail and let the clients down. Prior to the the current cold temperatures we are experiencing we thought we had over engineered the system but we now appreciate why Klaus suggested such a big boiler and a big heat store as we are enjoying living in a toasty warm house and with a system that is simple to use, efficient and eco-friendly.
In terms of radiators, Klaus originally proposed some modern efficient radiators but this time, we decided that form would govern function and Al found that Acova the French company that makes towel rails also made their retro style radiators in central heating models so we increased the budget slightly and went for the lovely cast iron look.
We are still in our first few months of using the new system (although we had the solar hot water working since mid September which was very efficient) and thus far it has proved very easy to use and the house is very warm. Old stone houses like these are known for being draughty and cold but we have installed double glazing throughout the living rooms and kitchen and added thick curtains to all the single glazed bedrooms. While we treated the roof beams in the loft, we also added 250mm of rock wool (essential for summer cooling as well as winter heat loss). In the autumn, we used purely solar until quite late into the season and then went through a period of lighting once a day and burning one batch of logs and then letting it go out. This was a bit painful as you go through tons of kindling and newspaper. As the temperatures dropped and are now below zero at night (during the last 7 days) we are burning 24/7 and the boiler needs loading perhaps 3-4 times a day. It is going through logs at a fair rate - we estimate approximately 10 cubic metres a winter which is a lot but we believe the trees on our land can cope with that production. We hoped the solar would produce more heat in the winter but we intend to change the angle on the panels (currently set to maximise year round outputs) to maximise winter output and we hope this will improve further the contribution made by or solar system.
As we are refusing to purchase logs for this current winter, we are obliged to burn the log pile that the previous owner left behind which is of variable quality. We are supplementing this with freshly cut Ash as this is the only tree that can be burnt green and luckily we have enough Ash to keep us going this winter.
We are gradually preparing wood stores for the next two winters and thinning out Alder copses we have growing on the wetter areas of the land. Alder being part of the beech family should be a decent firewood once dried and handily, none of the trees are large enough to warrant splitting which saves man hours. We also have plenty of dead or sick wood to clear including a massive Poplar that needs felling before it drops on nearby power cables and plenty of Horse Chestnuts, Acacia, Cherry, and so on. A couple of hours work every few days soon cuts and stacks a lot of wood and luckily we have the tractor and trailer to move the wood back to the drying area on the end of the barn (under direct cover but open to the wind).
We would also like to thank Rob Gwillim at the Centre for Alternative Technologies in Wales who advised us on the planning of the system in a professional, knowledgeable and independent manner.
