There are many ways to heat a house -the most common in the UK being gas boilers and electric heaters. But what if gas and electricity became prohibitively expensive? Or maybe you want to reduce your carbon foot print? In the UK, 43% of carbon emissions are apparently released in the process of heating buildings. Burning wood, contrary to common beliefs, is a carbon neutral method of heating a house. Unlike fossil fuels, it doesn’t add carbon from tens of millions of years ago, but only releases what was recently captured. I am also currently in rural France where woodland is plentiful and with a small acreage, you can supply your annual household demands indefinitely by taking out only as much as it is naturally regrown.
I’ve seen hundreds of wood burning systems in my lifetime, from minuscule van burners made to burn twigs to a grandiose fire place that I could comfortably live in. But despite my absolute love for the heart warming heat, hypnotic flames and crackling wood, I’ve never been convinced by the designs that seem to ignore efficiency.
Take log cabin fire places for example: Yes they look beautiful and I can see how making them out of stones would allow the heat to be absorbed into the mass, later to be released slowly. But why anyone would let half of the stones to be exposed to the outside (see above) is a complete and utter mystery to me. It’s as if half of the house’s radiators were built on the exterior of the house. Why not build the stone fireplace and chimney in the centre of the cabin where all the heat can dissipate within the building?
I’ve also found it a bit daft that the conventional wood burner requires the chimney to be insulated. It requires the smoke and other gasses released in the burn to remain hot when it exits the chimney so that it creates enough draw, which then feeds fresh oxygen to the fire. I can see the need for air to be drawn but 300 to 600 degrees heat escaping out of the chimney is pure energy loss. Before I get into the nitty gritty of burning wood efficiently, lets look at the many important stages before lighting that match stick.
Reduce the Need
We now have chainsaws and hydraulic wood splitters that can prepare stupendous amounts of fire wood with very little effort and time. But again, it’s a good idea to assume that prices of fossil fuels will most likely go up, as easily accessible deposits become depleted. As this blog is all about sustainability, let’s consider our options. Would we have to hand saw every piece of wood?
I think the smartest first move is to reduce the need for heating in the first place. You can;
- Build with straw bales to hyper insulate your house to R-30 to R-50 with what is essentially a biodegradable waste product.
- Build small and compact and reduce the amount of heating needed.
- Make the house passive solar with plenty of South facing windows and thermal mass where the sunlight hits to store that heat.
- Use double (or triple if you can afford it) glazed windows with wooden shutters on the outside and thick thermal curtains on the inside to trap the captured heat after dark.
We are currently staying in a house built with these concepts in mind and in sunnier winter climates of South France, the energy stored and released is so plentiful that we hardly need to light a fire at all. Still, with stranger and unpredictable weather patterns becoming more frequent, it’s a good idea to have a back up heat source. In the UK, where you may have many weeks of continuously overcast and drizzly weather, it’s a slightly different story but you can still cut about a third of the energy bill by utilising the above tricks.
These last few months, I’ve been working with chainsaws of various types for many different purposes and I have been pondering over the most efficient way of preparing firewood. After many weeks of considering my options, I think I would probably still buy a petrol chainsaw. The time efficiency compared to hand sawing is so vast that it would take a monstrous price hike in fuel price to consider doing it manually. However, what I envision myself doing is to use the petrol version for mobile purposes such as felling a tree in a forest, but using an electric chainsaw powered by solar to do smaller chops closer to home. There are battery operated chainsaws available but until I see some significant development in material science that doubles the capacity and halves the environmental impact of batteries, I’m not ready to dive in quite yet.
I have also had the pleasure of using this Fiskars axe recently. The perfectly balanced, lightweight, ultra-sharp and double hardened axes have made the task of splitting the wood almost effortless to the point that it became absolutely pleasurable. It was remarkable how much of a difference a well made tool can make and I can now quite easily see myself split a fair amount of wood every year and actually look forward to the task. However, I can also see a possible scenario where a few of us share an electric hydraulic log splitter powered by solar, especially in older age.
Having seen multiple methods of storing and drying firewood, I am currently leaning towards splitting it green and leaving it undercover, aerated and off the ground for two years. I am currently volunteering at a place where they cut the trees to size and leave them stacked in the forests for the first year which will cut the weight by nearly 40 to 50% due to evaporation of water – a smart move if the forest is far from home and you’re relying on human power to carry them up or down the hill. But the exposure to moisture from rain and ground contact, plus the fungal invasion will probably mean that a fairly large percentage will turn out to be sub-optimum firewood. A well kept, two year dried wood has the highest potential to burn efficiently and cleanly – a vital factor in a rocket mass heating solution.
As a side note, since this seems to be a more widely known knowledge, different tree species have different stored energy per unit of weight. And they grow at different paces. Oak will take 120 years to mature and have BTU of 36 whereas coppiced willow may only take 3 to 6 years but have BTU of 14. You may need to burn double the amount of willow to gain the same amount of energy but you only require 5% of time (=space=land=savings) to harvest.
The Front End: Rocket Stove
The biggest difference from a normal wood stove is that the combustion chamber is heavily insulated. Normally as the wood burns, volatile compounds such as smoke, soot or creosote is released and blown out of the chimney. In a rocket stove, the insulated burn chamber can reach temperatures of 1300C, inducing a complete combustion of the gasses and releasing even more heat energy as a result. This clean burn extracts the maximum stored energy of the wood, while only releasing a fraction of the carbon dioxide and a little bit of steam as a byproduct.
It can burn any tiny bits of dried plant matter from twigs and sticks and it can be made from basic recycled materials in a few hours. The ease of building it and the efficiency that it achieves have had massive impacts on the livelihood of some of the poorest people and refugees where fuel may not be readily available. The more recent design update called ‘batch box’ rocket stove, with wider horizontal fuel feed and a funnel effect into a burn chamber is worth checking out too. Batch boxes are still very much at the forefront of innovation and it feels like more experimentation and modifications are needed to perfect it, but I find these DIY cutting edge technology seriously exciting. A few interesting threads here and here for more info.
The Back End: Mass
So here is a stove that can burn hyper efficiently at high temperatures, burning small sticks and producing essentially no smoke. It was developed to be used for cooking and I have seen it work wonders as a cook stove and also as an oven. But for purposes of heating a house, the idea is to extract and store the most amount of heat energy as possible and release it slowly over the next couple of days so you have a nice even temperature. This is where the mass aspect comes into play.
The vertical burn chamber creates the draw and the push required to keep the system working in one direction, but also gives a fast radiant heat to warm the space initially. The exhaust duct runs through a thermal mass which extracts the heat and acts as a thermal battery. The exhaust temperature of a functioning rocket mass stove is somewhere in the range of 60 to 90C. Compare that with 400 to 600C of a normal wood stove, you can see how rocket mass stove uses as little as 1/10th of wood of an inefficient stove. If you currently use 8m² of wood per winter, you could reduce it to 0.8m² which is an absolutely mind boggling efficiency gain.
If you have to cut, drag, chop, split, dry and store every piece of wood you burn, it’s a no brainer to opt for a high efficiency mass heater that could reduce your workload by 90%. But more importantly, it reduces the amount of forest you need to be self-sufficient in firewood by a tenth (possibly more since sub-firewood materials such as twigs and bushes can be used equally well).
This might allow for quite a large saving for someone potentially looking for a plot of land to begin a more sustainable way of life. 1 cord/acre or 9m³/hectare seems to be the generally accepted amount that you can extract out of a woodland sustainably on an annual basis. It seems like a rather large amount in my opinion and it’ll vary greatly on the fertility of the land, type of wood grown and whether it’s coppiced. But if you only needed 1m³ or 1/3 cord per year, theoretically, you only need to purchase 1/3 of an acre or 1/9th of a hectare to cover your annual heating. I would also like to dig deep and learn more about hazel coppicing as a means of sourcing firewood in the near future since it seems like the most efficient and ecological method to supply wood from a limited space.
What are the Negatives?
- My first reaction, when I came across this idea, was ‘how do you clean the chimney?’ since it’s buried in thermal mass and even the cleanest combustion will create some smoke at the start and end of a burn. The answer was simple T pipe fittings where you can insert a brush and clean out any buildup of soot. So there’s still cleaning to do but hopefully a lot less than an ordinary wood burner.
- You generally only need about a couple of hours of intense burn to charge the thermal mass for a slow release over a day or two, but those couple of hours have to be closely watched and tended, making sure the fire is well fed and burning at its optimum. It’s a good idea to have the fuel feed positioned close to where it’s easy to keep an eye on.
- They take up a lot of space. In a tiny house or a small house, space is premium. It can be argued that they replace a wood stove and a sofa but I am leaning more and more towards using it for underfloor heating.
- Rocket Mass Stoves are still very much a fringe movement with a large percentage of these stoves built by novices based on information and advice on forums. It will probably take a few more years and a lot more start up companies to get the official approval and public recognition that they deserve.
Once I get to a point of building a house, a rocket mass heater/floor heater is definitely going to be an integral part of the build. Most likely as an underfloor heating system since my current intention is to build within a floor surface of 20m² to bypass local planning permission laws.
With a passive solar straw bale build in a warmer temperate regions of Southern France, I would probably only need a tiny bit of firewood. So much so that I may be able to do it all easily with hand tools which would reduce the complexity, risks of failure and costs. It is however, a pretty time consuming task to go through many forums, collect different ideas, verify the design, build and test but fear not, I will be sharing all the research, detailed calculations and building processes with notes once I get to that stage. Subscribe and stay curious.